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Trapped Ion Quantum Information
Novel Atom Sources and Ultra-fast ElectronicSwitches for Trapped-Ion Quantum-Information
Experiments
Master Thesis
Author:
Roland Hablutzel
Supervisor:
Dr. Joseba Alonso
Prof. Dr. Jonathan Home
Trapped Ion Quantum Information group (TIQI)
Institut fur Quantenelektronik
ETH Zurich
Abstract
The TIQI group is preparing two Paul traps: a segmented linear one at room temperature and a
planar one in a cryogenic environment.
The first part of this master thesis is about atom ovens, which are used to load the ion traps with
atoms. Two heating methods are presented: the electric one and the novel laser one. While the further
is widely used in the ion-trap community and will be set up in the linear trap, the latter showed to be
a better option for the cryogenic environment due to its lower power dissipation: for the calcium ovens
this power is reduced in a factor 13.
The second part of this work deals with “ultra-fast” electronic switches immersed in liquid helium.
These electronic components are able to switch between voltages in the range 0-10 V in less than 5
ns at 4 K. The ultra-fast transport allows the ion to be displaced in the trap faster than one secular
oscillation of it. This new technique can be used also for motional-state squeezing and entanglement
generation. Switches based on GaAs field-effect transistors (FETs) are known to work at 4 K, while
integrated circuits (ICs) using the CMOS technology are preferred because of its low static-power
consumption and high noise immunity, but there are few references about this type of ICs working at
such low temperatures. The selected switch to be incorporated to the cryogenic environment resulted
to be a digital CMOS with rise and fall time of approximately 4 ns in each case within the mentioned
voltage range.
Keywords: Quantum information; Ion trap; Laser-heated atom ovens; Ultra-fast switches
i
Acknowledgements
I would like to thank Prof. Dr. Jonathan Home for giving me the opportunity to let me do my master
thesis in the TIQI group. I would like to thank also Dr. Joseba Alonso for his time and dedication along
my learning of the experimental techniques in the group and for his advices in the personal ambit. It
is for me an honour to had worked with them, Dr. Home and Dr. Alonso, for sharing with me their
experiences in the research of the quantum information in the ion-trap community, opening me the doors
of this field around the world.
I am glad of all the help that I received from the members of the TIQI group, and of all the people
that made possible this achievement: my master work.
ii
Contents
I Introduction 1
1.1 Background and Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2 Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.3 Structure of the Thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
II Atom Sources 7
2 Theory Background 8
2.1 Thermal Atoms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.1.1 Vapour Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.1.2 Outgassing Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.2 Electric Heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.3 Laser Heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3 Experimental Set-up 12
3.1 Vacuum System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.2 Vacuum Chamber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.3 Electric-Heating Set-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3.4 Collimator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.5 Laser-Heating Set-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.6 Beryllium Winder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.7 Measurement Plan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
4 Experimental Methods and Results 19
4.1 Electric Heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
4.1.1 Calcium Ovens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
4.1.1.1 Testing the single and the shielded Calcium Oven . . . . . . . . . . . . . 20
4.1.1.1.1 Current up to 2 A . . . . . . . . . . . . . . . . . . . . . . . . . . 21
4.1.1.1.2 Current up to 3 A . . . . . . . . . . . . . . . . . . . . . . . . . . 21
4.1.1.2 Testing the Collimator with the Calcium Oven . . . . . . . . . . . . . . . 22
4.1.2 Beryllium Oven . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
4.1.2.1 Functionality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
iii
4.1.2.2 Collimator Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
4.1.3 Conclusions on Electric Heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
4.2 Laser Heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
4.2.1 Calcium Oven . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
4.2.1.1 Calcium Block . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
4.2.1.2 Calcium Granules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
4.2.1.3 Calcium Granule . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
4.2.2 Beryllium Oven . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
4.2.2.1 Large Piece of Beryllium Foil . . . . . . . . . . . . . . . . . . . . . . . . . 32
4.2.2.2 Small Piece of Beryllium Foil . . . . . . . . . . . . . . . . . . . . . . . . . 33
4.2.3 Conclusions on Laser Heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
5 Electric vs. Laser Heating 36
III Fast Switches in Cryogenics 38
6 Theory Background 39
6.1 SPST and SPDT Switches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
6.2 Cryogenic Semiconductor Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
7 Experimental Set-up 41
7.1 Test Boards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
7.2 Cable Tree . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
8 Experimental Methods and Results 45
8.1 Characteristics of a Pulse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
8.2 Selection of the fastest Switch in LHe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
8.3 Characterization of the selected Switch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
8.3.1 Power Dissipation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
8.3.2 Switching Behaviour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
8.3.3 Introduced Cable-Length Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
IV Conclusions 55
9 Summary and Outlook 56
iv
V Appendix 58
A Design and Construction of a Base for a Turbo-Molecular Pump 59
B Instructions to Build Current-Heated Atom Ovens 60
B.1 Calcium Oven . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
B.2 Beryllium Oven . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
C Developing and setting up PCBs 67
C.1 Impractical Spot Welding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
D Resistance in parallel to the Oscilloscope 69
E Coupling of Switched Voltages in AC Mode 71
F Precautions working with some Materials 73
F.1 Calcium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
F.2 Beryllium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
F.3 Stainless Steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
F.4 Acetone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
F.5 Isopropanol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
F.6 Latex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
F.7 Nitrile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
G Miscellaneous Pictures 78
G.1 Tantalum Wires glowing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
G.2 Burning Calcium Oven on Collimator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
G.3 Calcium Spots under the Light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
G.4 Beryllium Oven glowing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
G.5 Frozen Moth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
v
List of Figures
1 Examples of Paul traps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2 Schema of ultra-fast switching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3 Microscopic picture to explain the vapour pressure . . . . . . . . . . . . . . . . . . . . . . 8
4 Outgassing rates of the atom sources and of the used wires . . . . . . . . . . . . . . . . . 9
5 Mean velocities of calcium and beryllium as function of the temperature . . . . . . . . . . 10
6 Schema of the focusing of a Gaussian beam . . . . . . . . . . . . . . . . . . . . . . . . . . 11
7 Test mount . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
8 Electric-heating set-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
9 Collimator for the room-temperature trap . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
10 Improved PCB for the collimator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
11 Laser-heating set-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
12 Coil winder to build the beryllium oven . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
13 Beryllium coiled around the tungsten wire . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
14 Calcium oven for electric heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
15 Calcium-ovens set-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
16 Shielded calcium oven . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
17 Mirror-like calcium layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
18 Oxidation of the calcium layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
19 Temperature and resistance of the calcium ovens in dependence on their power dissipation 23
20 Calcium oven on the collimator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
21 Calcium spot of the collimator test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
22 Beryllium oven for the electric heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
23 Electric-heated beryllium-oven set-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
24 Beryllium layer on glass slide after electric heating . . . . . . . . . . . . . . . . . . . . . . 26
25 Beryllium spot after the collimator test . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
26 Beryllium completely evaporated from the oven . . . . . . . . . . . . . . . . . . . . . . . . 27
27 Oven temperature vs. dissipated power during the electric heating of calcium and beryllium
atom sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
28 Set-up for the laser-heating test with the calcium-block . . . . . . . . . . . . . . . . . . . 29
29 Layers of calcium on glass slides from the laser-heated calcium-block test . . . . . . . . . 29
30 Two different calcium ovens for the laser-heating tests using calcium granules . . . . . . . 30
31 Calcium spots from the laser-heated oven filled up with calcium granules . . . . . . . . . . 30
vi
32 Calcium spots from the laser heating of the oven with one calcium granule . . . . . . . . . 31
33 Laser-heating set-up with the beryllium foil . . . . . . . . . . . . . . . . . . . . . . . . . . 32
34 Laser-heating set-up with a small piece of beryllium foil . . . . . . . . . . . . . . . . . . . 33
35 Layers of beryllium on glass slides after laser heating of the small beryllium foil . . . . . . 34
36 Oven temperature vs. laser power during the laser heating of calcium and beryllium . . . 35
37 SPST and SPDT switches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
38 Series configuration of two SPST switches . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
39 Test board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
40 Schematics of the switch test boards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
41 Cable tree and pin board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
42 Characteristics of a pulse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
43 Cryogenic functionality-test set-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
44 Rise times of the SPST CMOS switches in dependence on the temperature . . . . . . . . 48
45 Rise-time-pulse example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
46 Characterization set-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
47 Schema of the current-measurement set-up to estimate the power dissipation . . . . . . . 50
48 Dissipated power in dependence on the switching amplitude . . . . . . . . . . . . . . . . . 51
49 Switching-amplitude deviation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
50 Rise and fall time in dependence on the input switch amplitude . . . . . . . . . . . . . . . 52
51 Rising pulse of the chosen CMOS IC switch between 0 V and 10 V . . . . . . . . . . . . . 53
52 Inventor drawings of the base for the Turbo-Molecular Pump . . . . . . . . . . . . . . . . 59
53 Calcium-oven construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
54 Loop of the beryllium wire around the left-out screw . . . . . . . . . . . . . . . . . . . . . 64
55 Weight on the beryllium wire . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
56 Improved beryllium coil winder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
57 Schema of the transversal cut of the beryllium coiled around the tungsten wire and of the
coiled beryllium-tungsten wire . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
58 Original PCB for the collimator of the room-temperature set-up . . . . . . . . . . . . . . 67
59 PCB after spot-weld try . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
60 Signal without the discharging resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
61 Resistance in parallel to the Oscilloscope . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
62 Voltage coupling due to the oscilloscope capacitance . . . . . . . . . . . . . . . . . . . . . 72
63 Tantalum wires glowing in calcium-ovens set-up . . . . . . . . . . . . . . . . . . . . . . . . 78
vii
64 Burned parts after the over-heated test of the calcium oven on the collimator . . . . . . . 78
65 Calcium spots under reflection of lamp light . . . . . . . . . . . . . . . . . . . . . . . . . . 79
66 Beryllium ovens glowing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
67 Frozen moth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
List of Tables
1 Coefficients for the vapour pressure of the elements of our interest . . . . . . . . . . . . . 9
2 Test of the calcium ovens in the electric-heating set-up up to 2 A . . . . . . . . . . . . . . 21
3 Test of the calcium ovens in the electric-heating set-up . . . . . . . . . . . . . . . . . . . . 22
4 Test of the calcium oven in the collimator . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
5 Test of the beryllium oven in the collimator . . . . . . . . . . . . . . . . . . . . . . . . . . 27
6 Resumed results on the laser-heated oven filled with one calcium granule . . . . . . . . . . 31
7 Results on the laser-heated beryllium foil (large) . . . . . . . . . . . . . . . . . . . . . . . 32
8 Resumed results on the laser-heated small piece of beryllium foil . . . . . . . . . . . . . . 34
9 Electric- and laser-heating comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
10 Switches models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
11 Voltage configuration for the switch-functionality tests . . . . . . . . . . . . . . . . . . . . 46
12 Rise times of the tested switches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
13 Pulse shapes for different BNC cable lengths . . . . . . . . . . . . . . . . . . . . . . . . . 54
14 Stainless-steel-tube sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
viii
Part I
Introduction
1.1 Background and Motivation
The Trapped Ion Quantum Information (TIQI) Group started its function at the ETH on Sept. 1,
2010. Focused on the research about quantum applications, the group has been developing and testing
techniques to generate and ionize atoms for their trapping and further manipulation.
Ion traps have gained popularity in this area because they have shown a relative high fidelity in the
field of quantum logic gates [Bro11, Ben08] and qubits readout [Bur10]. There are two main set-ups being
prepared in the TIQI group, each with a specific kind of Paul (radio-frequency or RF) trap: a linear
segmented one at room temperature and a planar one in a cryogenic environment.
A Paul trap (figure 1) consists of a spatial configuration of electrodes, each one driven either by a DC
or by an AC RF voltage for axial and radial confinement respectively, such that charged particles can be
trapped in the pseudo-potential minimum, i.e., the particle feels an attractive force (averaged in time)
towards a determined point [Maj05].
(a) Linear Paul trap (b) Planar Paul trap
Figure 1: Electrode configuration in two examples of Paul traps: red electrodes are driven by AC voltages,
while the green and blue ones are DC driven (AC grounded).
This thesis is about two main projects: atom sources to load the ion traps and ultra-fast electronic
switches for the transport and the manipulation of the ions in the cryogenic system.
1. ATOM OVENS: prior to loading the traps with ions, the atoms have to be produced from an atomic
source, which from now on will be called an oven.
The traps at TIQI will make use of 40Ca+ and 9Be+ ions, which have been used by various groups
from the same community [Gul01, Ben08, Tur99, Bro11] due to their simple electronic configura-
tion resulting from their having a single valence electron. Moreover, both species of ions will be
simultaneously trapped for sympathetic cooling and the simulation of open quantum systems.
2
Electric-heated ovens are commonly used in room-temperature traps [Gul01], but the relatively
high power dissipation of such sources becomes a problem in a cryogenic environment. On the other
hand, while laser heating has been done previously in other areas of physics [Pro90], it has been
never used in the ion-trap community.
The fact to use laser heating arises from three ideas:
A new heating method that introduces less power into the system than the electric one.
The local heating of a large sample and so reducing its power dissipation. However, this is
valid for metals with relative small heat conductivities.
Laser heating does not require any electrical connection. In the case of electric heating, the
wires would affect the thermal isolation of the oven in the cold environment and, moreover,
they would generate heat themselves once an electric current starts flowing through them.
The present work describes the construction and presents the successful results of both electric-
heated and the novel laser-heated (calcium and beryllium) ovens, the latter proving to be a better
alternative for a cryogenic set-up.
2. ULTRA-FAST VOLTAGE SWITCHING: two concepts motivate the “instantaneous” switching of
the voltage on the DC trap electrodes, which nobody has achieved until now in the ion-trap com-
munity:
Ion transport in trap arrays (figure 2) over distances many orders of magnitude larger than the
size of the particle wavefunction.
Motional-state squeezing and entanglement generation [Ser09].
Ions are confined in a trap by an electromagnetic field and thus their transport involves, in a gen-
eral sense, changes in the voltages applied to the trap electrodes. Current ion-transport techniques for
room-temperature traps incorporates “slow”, adiabatic (using “smooth”, low-pass filtered control volt-
ages) [Bla11] or “fast”, non-adiabatic (leading to excited states) [Hub08] displacements of the ions. Recent
results show the fast shuttling of cooled ions preserving their stored quantum information [Wal12]. More-
over, fast transport takes several axial oscillations, while the ion-transport runtime described in figure 2
takes only one half of it.
The pseudo potential originated by a Paul trap can be approximated by a harmonic one. Under this
approach, the ion oscillates around the potential minimum with a defined axial period Tax = 1ωax
. The
position of this minimum can be set using the DC voltages applied to sets of electrodes. An ultra-fast
3
(a) t < 0: the ion is confined to the left side due to a positive
bias potential applied at the right part of the trap.
(b) t = 0→ t = Tax2
: the voltage from the right side is switched
off. The ion feels a force towards the new minimum in poten-
tial, and then it reaches the right side due to its own inertial
motion.
(c) t > Tax2
: the positive bias is now turned on at the left side
to confine the ion to its new position.
Figure 2: Schematic description of a fast switching on the axial axis of a trap.
(as defined in this work) switch means that the voltage switching of a set of the mentioned electrodes
should take place faster than one hundredth of a cycle time in the axial direction, i.e., the switching time
tsw should be smaller than Tax100 , such that the sudden approximation 1
ωax= Tax >> tsw holds. For Calcium
(ωax ≈ 2π· (1 MHz)) one obtains tsw ≈ 10 ns. Slower switches (for which this approach is no longer valid)
not only introduces more complications when realizing the transport (by the use of waveform generators
for the control voltages), but also it could also excite the ions due to couplings between motional and
electronic excitations and thus change its quantum state, leading to a loss of information, being this a
drawback in quantum-information experiments.
The DC voltages of the planar trap in cryogenics are not controlled from large distances (e.g. a switch
at room temperature connected to the cryogenic set-up) because this would introduce high noise that
could destroy the measurements. One possibility is to filter out that noise near the trap electrodes, but
this would also get rid of the ultra-fast components of the voltage switching. Therefore, the electronic
4
switch should be placed as close as possible to the trap, i.e., in the cryogenic environment. The tested
voltage regime was 0-10 V according to the future projects with the cryogenic trap.
Some kinds of doped semiconductors able to perform electric functions at room temperature could
stop working at low ones due to the “carrier-freeze-out” effect: charge carriers (from the dopants) do
not have enough thermal energy in order to end up in the conduction band [Sta98]. For this reason, the
candidates for the ultra-fast switching have to be tested in liquid helium (LHe) for our purposes. Well
known semiconductor integrated circuits (ICs) that work at temperatures of 4 K are based on GaAs, InP
and SiC [Wil11]. Nevertheless, CMOS-technology IC switches are in general preferred because of their
simple control and low heat dissipation, but there are in general few references of ICs from this technology
working at low temperatures; hence, such electronic switches were tested in liquid helium in the present
thesis. The fastest one resulted to be in fact a CMOS IC with ≈ 5 ns switching time between 0 V and 10
V.
1.2 Goals
The aims of the mentioned projects are listed below:
Main goals:
– To design, build, and test atomic calcium and beryllium sources.
– To test and characterize ultra-fast electronic switches in cryogenics.
Specific goals:
– The construction and optimization of calcium and beryllium atom sources for electric- and
laser-driven heating methods.
– To characterize the oven performance for each heating set-up.
– To determine the heating method to be used in cryogenics.
– To test and optimize the room-temperature mount (collimator) using the electric-heated ovens.
– To test the functionality of CMOS and GaAs ultra-fast switches (10 ns or lower switching time)
at 4 Kelvin by switching voltages in the range between 0 and 10 V.
– To select the fastest switch for the cryogenic set-up and to characterize it in detail.
1.3 Structure of the Thesis
This thesis is divided according to the two projects presented above:
5
Part II is about the atom sources. Chapter 2 introduces the theory necessary in order to under-
stand the vapour emission from solid samples, as well as the principles of electric and laser heating.
Chapter 3 explains the vacuum configuration and the heating set-ups with a short excursion to
the beryllium coil winder. In chapter 4 the methods and results from the tests on electric- and
laser-heated ovens are described and in chapter 5 both heating principles are compared.
Part III deals with ultra-fast switches at low temperatures (close to 4 K). Chapter 6 recalls the
definitions of SPST and SPDT switches and introduces possible semiconductor technologies for the
ultra-fast switching in cryogenics. Chapter 7 describes the circuitry of the PCBs on which the
electronic switches are mounted and the connection set-up from the cold to the room temperature
system. In chapter 8 the functionality in the cold environment of selected commercial switches is
presented, and then the fastest one is characterized by measuring its power dissipation and its rise
and fall time in dependence on the switching amplitude.
The appendices at the final part of this thesis deal with side projects and/or issues occurred during
my experience in the lab. Appendix A presents the motivation and drawings of the construction of a
metallic base for the turbo-molecular pump used for testing the ovens. Appendix B describes step by
step the construction of the electric-heated ovens. Appendix C contains a short description on how to
develop circuit boards and points out the difficulty of spot welding the wires onto it. Appendix D refers
to a resistance adapted in parallel to an oscilloscope in order to be able to measure rise times of SPST
switches. Appendix E explains the coupling of the switched voltages occurred during the tests in AC
mode on the electronic switches and how to get rid of it. Appendix F summarizes the handling and the
data sheets of many materials of importance for our experiments. Finally, appendix G contains a series
of pictures that contribute to the knowledge from a general point of view.
6
Part II
Atom Sources
2 Theory Background
2.1 Thermal Atoms
The ovens are sources of atoms, which are emitted from the surface of a material whose temperature is
increased. In this sense, the outgassing rate is the property of the atom ovens that was exploited, which
relies on the definition of the vapour pressure. Both theoretical principles are explained below.
2.1.1 Vapour Pressure
The vapour pressure pvp is the pressure of a substance vapour at which it coexists in equilibrium with
its solid (or liquid) state in a closed system (figure 3) [ILPI].
Figure 3: Microscopic representation of the surface of a liquid in equilibrium with its gas phase. Picture
taken from [Chem].
The vapour pressure of most metallic elements is fitted by the formula [Alc84]
log pvp(atm) = A+B · T−1 + C · log T +D · T · 10−3, (1)
where T is the absolute temperature and A, B, C, and D are empirical coefficients. These can be found
in table 1 for the elements used for the ovens in this thesis.
2.1.2 Outgassing Rate
Consider now that a fraction η < 1 of all the atoms from the vapour that reach the surface material
condense onto it (the rest is reflected). At equilibrium, the same amount of atoms will sublimate from
the surface into the vapour. The outgassing rate Rgas is defined as the number of atoms that sublimate
per unit area per unit time and is given by [Lan13]
Rgas(T ) = ηpvp(T )√2πmkBT
, (2)
8
A B C D
Calcium 10.127 -9517 -1.4030 ≈ 0
Beryllium 8.042 -17020 -0.4440 ≈ 0
Tantalum 16.807 -41346 -3.2152 0.7437
Tungsten 2.945 -44094 1.3677 ≈ 0
Table 1: Set of the coefficients for the vapour pressure formula of the elements of interest in this thesis in
the temperature range 298-2500 K [Alc84].
where m, kB, and NA are the molar mass, the Boltzmann constant, and the Avogadro constant respec-
tively.
Tantalum and tungsten wires are used in the electric-heated ovens in order to conduct electricity
because their outgassing rates can be neglected with respect to the ones from calcium and beryllium
(figure 4).
Out
gass
ing
(a) Calcium and beryllium outgassing rates
2 0 0 4 0 0 6 0 0 8 0 0 1 0 0 0 1 2 0 0 1 4 0 01 E - 1 2 91 E - 1 1 91 E - 1 0 91 E - 9 91 E - 8 91 E - 7 91 E - 6 91 E - 5 91 E - 4 91 E - 3 91 E - 2 91 E - 1 9
T a n t a l u m T u n g s t e n
Outga
ssing
[Atom
s/m2 s]
T e m p e r a t u r e [ K ]
(b) Tantalum and tungsten outgassing rates
Figure 4: Outgassing rates of the atom sources and of the used wires. Note that the respective rates of
the wires are more than 20 orders of magnitude lower than the ones of the atom sources. In all plots η =
0.1 is assumed [Dou54]. Molar masses taken from [MOLAR].
The average velocity of the gas particles can be calculated from V = NA
√8kBTmπ . The result for calcium
and beryllium is represented in figure 5. It can be seen that, in order for an atom to fly 5 mm (typical
distance at which the glass detector is placed), the time required is ≈ 10−5 s. Thus, gravity effects can
be neglected.
The situation presented above was used in order to explain the outgassing rate at equilibrium between
the solid surface and the gas. Now, considering that the atoms from this surface escape from it because
9
2 0 0 4 0 0 6 0 0 8 0 0 1 0 0 0 1 2 0 0 1 4 0 0
4 0 0
6 0 0
8 0 0
1 0 0 0
1 2 0 0
1 4 0 0
1 6 0 0
1 8 0 0
C a l c i u m B e r y l l i u m
Mean
veloc
ity [m
/s]T e m p e r a t u r e [ K ]
Figure 5: Mean velocities of calcium and beryllium in dependence on the temperature.
they have enough thermal energy to do so, one can conclude that the outgassing rate depends only on
the temperature of the solid, and not on whenever the atoms in the gas phase strike the surface. Thus,
the obtained expression is still relevant in our experimental set-up, albeit it is an open system (the pump
is evacuating constantly the vacuum chamber).
2.2 Electric Heating
One method to heat up conductors (and therefore the ovens) is to apply an electric current of intensity
I to flow through them. The equation of the electric heating is given by Joule’s law [Cre08]
∆Q = I2 ·R(T ) ·∆t, (3)
where ∆Q, R(T ), and ∆t are the heat flow, conductor resistance, and heating time respectively.
Due to the heating up of the conductors, it is necessary to consider the dependence of the resistance
on the temperature T , which can be approximated by [AAC]
R(T ) = R0(α(T − T0) + 1), (4)
with R0 the resistance at T0 and α the temperature coefficient.
The heating process will reach a steady state because of the thermal energy dissipation, which has
to be minimized in cryogenics. In vacuum, the sample loses its thermal energy in form of radiation or
conduction, e.g. through the wires.
2.3 Laser Heating
Another method to heat up materials is the laser heating, which can make use of a continuous-wave (cw)
laser beam1 of power P focused on a sample [Pro90].
1in contrast to laser ablation, where a pulsed laser is used in order to heat the sample locally over its boiling point
10
Roughly speaking, the heat gain of the sample due to the laser is given by ∆Q∆t = P · A, where A is
the fraction of absorbed power.
As in the case of electric-heated ovens, the loss of energy happens both via thermal conduction and
grey-body radiation.
Waist of a Gaussian Beam after a Thin Lens
As the idea of the laser heating is to achieve a local heating of the sample, it is outstanding to calculate
the area irradiated by the laser on the sample, which sits at the focal point.
fdi
ωi ωout
Figure 6: Schema of the focusing of a Gaussian beam (not to scale). The collimated incoming beam has
a waist of ωi and propagates a distance di. The thin lens has a focal length f and the beam propagates
to the focal point having a new waist of ωout.
The laser used is, to a good approximation, a collimated Gaussian beam (curvature radius R ≈ ∞) of
wavelength λ = 1050.98 nm and ωi = 1.4 mm waist. It propagated di = 1 m until it reached a thin lens
of focal length f = 200 mm. The beam was then focused (R =∞) on the sample, reaching a beam waist
of ωout ≈ 47 µm (figure 6) calculated with matrix optics [Sal91]:
q
1
= k
Free propagation f︷ ︸︸ ︷ 1 f
0 1
Thin lens︷ ︸︸ ︷ 1 0
− 1f 1
Free propagation di︷ ︸︸ ︷ 1 di
0 1
qi
1
, (5)
with the beam parameters qi =iπω2
iλ and q =
iπω2outλ for the initial and final waist respectively.
11
3 Experimental Set-up
3.1 Vacuum System
Ion-trap set-ups (and therefore also the atom sources) are operated at ultra-high-vacuum (UHV) pressures
in order to reduce impurities and thus increase the lifetime of the trapped ions. For this reason, the oven
tests should be run inside a vacuum chamber. However, a high-vacuum system is easier to achieve and
adequate to test the ovens.
A vacuum system driven by a turbo-molecular pump (TMP) and a pre pump is used to reach pressures
down to 10−7 mbar at room temperature. The following parts formed the vacuum set-up:
TMP: Pfeiffer HiPace 80 (DN 63 CF-F)
(Oil-)Pre pump: Pfeiffer DUO 5 M rotary vane pump
DCU: Pfeiffer display and operating unit TC 110
Air cooling: Pfeiffer PM Z01 300, controlled by DCU
Gauge: Pfeiffer Compact FullRangeGauge, FPM sealed PKR 251
Oil mist filter ONF 16: Pfeiffer, it recycles oil from exhaust gas
Oil-return filter: Visi® Trap 4.5” Sump from Mass-Vac, Inc.
The main operating parameters from the vacuum and the devices are displayed in the DCU, e.g. the
pressure (measured by the gauge) and the rotation speed of the TMP rotor.
3.2 Vacuum Chamber
The vacuum chamber is a cross-piece connected to the vacuum system (figure 8), with two KF50
viewports at opposite sides and a feedthrough KF 40 D-sub 9. The viewports helped to check that
everything was correctly placed, and through them the deposition of material on the detector glass could
be actually seen; and the feedthrough allowed for electrical connections between the high-vacuum and
atmospheric-pressure sides, i.e., to connect the current-supply wires as well as the thermocouple pairs
from the vacuum to the corresponding measuring devices.
The test mount is a system inserted in the centre of the cross-piece and used to detect the emission
of atoms by the ovens inside the vacuum. It consisted of the slide holder with one or two glass slides
(figure 7) facing against the sample. This glass-slide holder is a rectangular metal piece (45 mm x 25 mm
12
x 8.4 mm) with four grooves for the glass slides. Each groove was 1.1 mm long and 3.5 mm deep, with
8.9 mm spacing between two of them.
The object slides are 76x26x1 mm3 Thermo scientific from Menzel-Glaser made of soda-lime glass.
Because the object slides are too long, they were cut ≈20 mm long such that they fit in the KF40 tube2.
Since the laser beam passes through one glass slide in the laser-heating set-up, their transmitivity at the
used wavelength (λ = 1050.98 nm) was measured and found to be 92 %.
Figure 7: Test mount: the glass-slide holder with a glass slide.
Thermocouples are used to measure the temperature of the samples. The alumel-chromel junction is
standard when measuring in the range -200 C to 1200 C [TMCP]. The temperature was measured by
previously calibrated multimeters UNI-T UT39C connected to the thermocouples via the feedthrough.
3.3 Electric-Heating Set-up
In this set-up a DC voltage supply is connected to the oven in the vacuum via feedthrough in order to
have an electric-current flow through it and thus heat it up (eq. (3)). This will lead to an increase of the
outgassing rate of the sample, according to eq. (2).
The vacuum chamber and the gauge were connected to the pump by a T-piece (figure 8).
Some considerations to bear in mind during the setting up of the experiment are:
The wires inside the vacuum could touch the metallic case. This causes that some current flow
through the case (which is supposed to be grounded) instead of the oven (in case of the tantalum or
tungsten wires) or that the reading of the temperature to be wrong (in case of the thermocouple).
The resistance between the output of each feedthrough pin and the metallic structure were measured
with a multimeter (UNI-T UT61-B) right after closing the system.
2Mark a line with the glass cutter and then press by hand on it. Cover the glass with paper for safety reasons
13
Gauge
Fee
dth
rou
gh
Vacuum chamber
Ovens
Glass slide
T-p
iece
(a) (b)
Figure 8: Set-up of the electric heating (prepared in this picture for the tests with calcium ovens). Behind
the glass slide is the gauge in order to have a meaningful measurement of the pressure near the oven.
The alumel-chromel junction is polar. The reading of the temperature will be wrong if the wires
are connected the other way around to the multimeter.
The temperature readout increases rapidly as a response to the applied current. This occurs when
the alumel or the chromel wire is touching one of the electric-current-supply wires.
3.4 Collimator
The collimator (figure 9), as the name suggests, collimates the atom gas expelled from the oven. This
piece3 will be used in the room-temperature trap (with electric-heated ovens) to ensure that the atoms
fly in a very precise direction in order to be ionized and finally trapped, and also to avoid the atoms from
hitting the electrodes of the trap.
The collimator has two ducts (with openings at the end of ≈ 1.7 mm x 1.5 mm), one for the calcium
and the other for the beryllium gas. However, each oven was tested separately, starting with the calcium
ones, because otherwise the vacuum pieces with a beryllium layer on them could potentially be hazardous
for us in further tests (see app. F.2).
A printed circuit board (PCB) with three electrodes (figure 10) enables the electric contact to the
atom ovens in the room-temperature trap, such that the current through each oven can be controlled
independently. The (wires of the) ovens are fixed to the PCB and to the power-supply wires by screws
(like in figure 9b). The PCB was cut at the corners to fit in the KF 40 tube, and the copper electrode
touching the screw fixing the PCB to the collimator was filed in order to isolate both from each other
(figure 20), otherwise, there would be a short circuit between the oven and the metallic case.
3It is open on the sides for technical reasons.
14
Aperture
Macor piece
Ovens
PCB
(a) Collimator for room-temperature trap with the original
PCB (see appendix C)
(b) Detail at the mechanical junction with the improved
PCB
Figure 9: Collimator for the room-temperature trap. Each oven (red) emits gas that reaches a small
aperture at the other end (a small aperture in the lower face). The white piece is made of macor, which
is a good thermal isolator and serves as a shield between the two ovens in order to keep the atom gases
separated from each other. The gases will be collimated at the mentioned rear apertures.
Electrodes
GND electrode
Screw holesfor fixing the wires
Figure 10: Improved PCB for the collimator. The wires are connected mechanically by screws at the
sides of the board.
3.5 Laser-Heating Set-up
The laser-heating set-up uses a focused continuous wave laser in order to heat up the ovens. This laser
passes through one of the viewports of the vacuum chamber placed on the optical table.
It was necessary to place the vacuum chamber on the optical table for safety reasons (figure 11). For
this, a 1 m long flexible hose (KF40) was used in order to connect the vacuum chamber and the gauge
to the TMP. The hose slowed the function of the pumping system and reduced the vacuum quality to ≈
10−6 mbar.
The use of a UV-laser could lead to possible excitations or ionizations of the atoms in the gas; therefore,
15
1m
hos
e
Laser beam
Figure 11: Laser-heating set-up.
an infrared laser was used with a wavelength λ = 1050.98 nm (high-power fibre laser, which was available
in the lab).
The sample was placed between two glass slides and, in general, the laser beam was focused near the
thermocouple such that the beam neither impacts on the junction (it is otherwise heated by the beam
such that a much higher temperature is measured than the one of the oven) nor too far away from it
(because the measured temperature would be much lower than what it actually is at the heated spot).
The set-up was covered by black aluminium plates in order to absorb scattered laser light, since metals
(like calcium and beryllium) have a high reflectivity and it is expected powers up to 10 W to be used.
3.6 Beryllium Winder
In order to build the electric-heated beryllium ovens (sec. 4.1.2), the coil winder is introduced (figure 12).
This device rotates two axes simultaneously, between which a tungsten wire is tensed. Besides that, one
end of the beryllium wire is fixed at one of the axis, while a mass is attached at the other end of this wire,
such that it hangs down. The machine coils the beryllium wire around the tungsten one automatically
(with a heat gun pointing to the coiling beryllium to make it more ductile). In order to get the tightest
winding (figure 13), the base, on which the set-up relies, can be tilted with respect to a horizontal plate
(see app. B.2).
3.7 Measurement Plan
As already mentioned in sec. (3.4), but only for the collimator, all the tests with the calcium sources
were executed before any test on the beryllium ovens, since the contact with beryllium layers deposited
on the test mount and inside the vacuum chamber should be minimized.
The measurements were realized in the following order:
16
Ber
ylliu
mw
ire
(a) Side view. The coil winder is tilted forward to prevent
the beryllium wire from passing by the edge of the basis.
Tungsten wire
Ber
ylliu
mw
ire
(b) Front view. The rotating set-up is tilted to the left such
that the beryllium is coiled tight all the way up the tungsten
wire.
Figure 12: Coil winder during coiling of the beryllium wire around the tungsten wire. The structure was
tilted to the front side such that the beryllium wire hangs freely down. The tight coiling is possible due
to the mass pulling down the beryllium wire and the fact that the rotating axis is slightly inclined such
that the beryllium “climbs” on the tungsten during the coiling.
(a) Length ≈ 2 mm
(b) Length ≈ 3 mm
Figure 13: Beryllium coiled tightly around the tungsten wire.
Calcium-oven tests in the electric-heating set-up in order to estimate the temperature at which a
calcium spot is visible on the glass slide (sec. 4.1.1.1).
Electric-heated calcium-oven test on the collimator for the room-temperature trap to estimate the
power dissipation (sec. 4.1.1.2).
Laser-heated calcium-ovens tests to estimate the incident laser power required to reach the temper-
ature already measured for the electric-heated one, at which a calcium spot on the glass slides was
observed (sec. 4.2.1).
Laser-heated beryllium-ovens tests in order to estimate the temperature at which a beryllium layer
is visible on the glass slides and to estimate also the corresponding incident laser power (sec. 4.2.2).
17
Electric-heated beryllium-ovens tests on the collimator to estimate the dissipated powers at the
point when this oven reaches the already estimated temperature for the laser-heated one (at which
the glass slide was coated with beryllium). Also a longer run was carried out to measure the power
dissipated in order to get a spot behind the collimator (sec. 4.1.2).
18
4 Experimental Methods and Results
The following sections present the results of the heating tests of the atom ovens in a deductive way, i.e.,
each result led to an improvement of the oven.
4.1 Electric Heating
The intensity of the electric current through each oven was controlled by means of the voltage generator.
The temperature, the voltage, and the pressure were also measured in a way described as follows: some
seconds after increasing the electric current, an increase (in the temperature and therefore) in the pressure
can be expected due to the higher outgassing rate of the sample, which then reaches a maximum (Pmax)
and decreases due to the diffusion of the gases in the vacuum and the constant suction of the TMP. When
the temperature reached a stable value, it was noted down together with the voltage and the pressure.
4.1.1 Calcium Ovens
The (“single”, see below) calcium oven tested under the electric heating (figure 14) consisted of a 316
stainless-steel4 tube filled with calcium granules and connected at its ends to tantalum wires, which
enable the flow of electric current. The tube had an opening in its centre to allow the emission of
calcium; therefore, calcium spots are expected on the glass in front of the opening. A thermocouple
(alumel-chromel) was spot welded to the tube (appendix B.1). Calcium ovens of this kind have been used
previously in many ion-trap experiments [Gul01, Luc11, Bla11].
Alumel-chromelthermocouple
Tantalum wires
Stainless-steel tubeOpening
Calcium
Figure 14: Calcium oven for electric heating.
The calcium granules had in average 1 mm diameter (mesh -16) and the smallest ones were selected and
inserted in the tube. These granules were taken out from its container and placed inside the vacuum system
4316 stainless steel is typically used in high vacuum systems due to its low outgassing rate [Dan] and the fact that it is
not magnetic.
19
after building the ovens in less than 2 hours. Thus, it can be safely assumed the calcium-oxide surface of
the granules is cleared off when the temperature (and therefore their outgassing rate) is increased.
4.1.1.1 Testing the single and the shielded Calcium Oven
In order to test the functionality of electric-heated calcium ovens, two different designs were used (figure
15). The stainless-steel tube from each oven had a length of 14 mm. They were placed horizontally with
their openings facing against a glass slide.
II
I
Glass slide
Tantalum wire
Glass-slide mount
Alumel-chromel thermocouple
Figure 15: Set-up of the calcium ovens inside the vacuum chamber. The glass slide on the glass holder is
in front of the openings of the two calcium ovens.
Oven I (single): using the biggest available diameter tube: ID = 1.60 mm, OD = 1.83 mm.
Oven II (shielded): a medium sized diameter stainless-steel tube (ID = 1.27 mm, OD = 1.47 mm)
placed inside another one of the biggest diameter (figure 16). The idea is to thermally shield the
inner oven.
Calcium Inner tubeOuter tube
Figure 16: Detail of the “shielded” calcium oven (oven II, in focus). The alumel-chromel junction is spot
welded to the outer tube as well as the tantalum wires at its ends.
20
These ovens were heated at the same current steps at the same time (II = III = I) using power
supplies, from which the corresponding voltage was also measured.
4.1.1.1.1 Current up to 2 A
In a first run, a current of 2 A was set for around 100 minutes. No calcium deposition was observed on
the glass slide (table 2).
time (min) I(A) VI(V) TI(C) VII(V) TII(
C) P (mbar) Pmax (mbar)
0 0 0 20 0 20 1.4 · 10−6 -...
73 2 1.06 368 1.08 369 1.4 · 10−6 1.6 · 10−6
176 2 1.05 363 1.05 355 1.2 · 10−6 -
Table 2: Test of the calcium ovens in the electric-heating set-up up to 2 A using a TTi EL302RT power
generator.
4.1.1.1.2 Current up to 3 A
Table 3 summarizes a run up to 3 A showing that for a current I = 2.7 A the ovens had a temperature
T ≈ 433 C and a calcium layer was visible on the glass (like a mirror, see figure 17). Increases in the
pressure during the run proved the emission of atom gas.
Figure 17: Mirror-like calcium layer on the glass reflecting the calcium ovens in the vacuum.
The calcium layer oxidized within 1 hour from the moment, at which it was taken out of the vacuum
chamber (figure 18).
21
time (min) I(A) VI(V) TI(C) VII(V) TII(
C) P (mbar) Pmax (mbar)
0 0 0 22 0 22 7.4 · 10−7 -
14 0.5 0.15 76 0.2 77 7.4 · 10−7 1.0 · 10−6
24a 1.0 0.37 196 0.5 197 7.4 · 10−7 7.4 · 10−7
35 1.5 0.67 295 0.7 296 1.2 · 10−6 d1.2 · 10−5
45 2.0 1.02 363 1.0 367 1.1 · 10−6 2.5 · 10−6
55b 2.4 1.33 403 1.3 411 5.4 · 10−7 3.2 · 10−6
59 2.5 1.41 412 1.3 421 4.6 · 10−7 4.6 · 10−7
63 2.6 1.50 420 1.4 430 3.4 · 10−7 4.6 · 10−7
70c 2.7 1.58 428 1.5 438 3.4 · 10−7 3.4 · 10−7
74 2.8 1.68 436 1.5 447 3.4 · 10−7 3.4 · 10−7
76 2.9 1.77 444 1.6 456 4.0 · 10−7 4.0 · 10−7
79 3.0 1.84 452 1.7 465 4.6 · 10−7 4.6 · 10−7
a Some visible particles on the glass slide
b Ta wires slightly glowing orange
c Calcium spot appeared
d Getting rid of CaO2 layer on the granules
Table 3: Test of the calcium ovens in the electric-heating set-up. Once the current was set to a value, the
maximal pressure Pmax was recorded, and then the temperature reached a stable value T , measuring also
at this point the voltage V and the pressure P .
A new glass was placed in front of the ovens to show their reusability, obtaining the same result as
for the run above.
The resulting temperature T of the ovens (which is directly related to the outgassing rate) as well as
their resistances R are plotted in figure 19 in dependence on their dissipated power P . The second oven
(II), in contrast to the first one (I), had a “thermal shield”; in principle, the resistance of the inner tube
in parallel with the outer has a negative effect on the current required to heat up this oven, however, this
effect proved to be negligible, since the curves in both plots follow the same behaviour.
4.1.1.2 Testing the Collimator with the Calcium Oven
A third calcium oven (approx. 8 mm long similar to oven I) was built in order to fit in the collimator
(figure 20), which was placed on the test mount with its open end facing against a glass slide.
22
(a) 0 min (b) 1 min (c) 6 min (d) 20 min
(e) 22 min (f) 32 min (g) 40 min (h) 53 min
Figure 18: Oxidation of the calcium layer deposited on the glass during the first hour out of the vacuum.
The two calcium spots, each emitted by one oven, are superposed on the glass, but are still differentiable:
the lower one from oven I and the upper one from oven II.
0 1 2 3 4 50
100
200
300
400
500
T Oven IT Oven II
Power [W]
Te
mp
era
ture
[oC
]
(a) Equilibrium temperature (T was changing slower
than 1 degree per minute) in dependence of the dissi-
pated power of the ovens.
0 1 2 3 4 5 6
RP Oven IRP Oven II
Power [W]
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0.55
0.60
0.65
Re
sist
an
ce [Ω
]
(b) Resistance of the ovens in dependence on their power
dissipated.
Figure 19: Measured temperature and resistance of the calcium ovens as the dissipated power was in-
creased (by increasing the electric-current intensity). Oven II is the shielded one. Only minimal differences
were observed.
In order to see a spot on the glass behind the collimator (figure 21), a longer run was carried out
(table 4). The calcium spot showed the same shape as the opening of the collimator, proving that the
23
Calcium oven
Filed electrode
Figure 20: Testing the calcium oven on the collimator.
divergence of the atomic beam is small.
time (min) I(A) V (V) T (C) P (mbar)
0 0 0 24 1.9 · 10−5
244 4.6 2.2 508 4.0 · 10−6
Table 4: Test of the calcium oven in the collimator
(a) The collimated calcium beam is evidenced on the
glass slide. The shape matches that of the open end of
the collimator projected on the glass.
(b) To the left of the collimated beam another calcium
spot is visible that comes from the opening at one side
of the collimator.
Figure 21: Calcium spot of the collimator test.
24
4.1.2 Beryllium Oven
A beryllium oven for the electric-heating set-up consists of a wire of beryllium wrapped around a wire of
tungsten of approx. 3 cm long (600 turns of beryllium wire) (figure 13), coiled itself using a 1 cm drill
bit. It takes a spring-like form (figure 22b) of approx. 5 mm. Such ovens have been used in [Kin99]. The
alumel-chromel junction was placed touching the oven, ensuring again that there were no short circuits
between the thermocouple wires and the metallic case of the vacuum chamber.
(a) Beryllium oven coiled around a
1 mm drill bit. Note that the bit
is inserted backwards to avoid the
grooves.
(b) Beryllium oven for the electric
heating.
Figure 22: Beryllium oven for the electric heating. It was coiled using a 1 mm drill bit in 5 - 7 turns.
This (rather complicated) oven is more efficient than the calcium one (higher temperature vs. dissi-
pated power). The reason is that the temperature of this oven must be increased to higher values than
for the calcium one in order to get an acceptable amount of beryllium (i.e. a visible layer on the glass),
because the outgassing rate of beryllium is lower than the one of calcium (figure 4).
Since the beryllium oven emits particles in all directions, it was tested both for its own functionality
and on the collimator at the same time by placing one glass behind the oven (held by the alumel and
chromel wires) and another glass behind the collimator (figure 23). In contrast to the calcium oven, a
beryllium layer is expected on the glass slide behind it (rather than a spot).
The oven with 600 wrappings (of beryllium wire around the tungsten wire, BeW) broke while being
manipulated (at room temperature outside the vacuum) after an unsuccessfully run.
4.1.2.1 Functionality
A run with an oven of 300 wrappings BeW in the same set-up was carried out. A visible beryllium layer
is expected on the glass near the oven at a temperature above 507 C (table 8). At this point and for a
current of I = 0.8 A the power dissipated was P = 1.9 W. After more than 1 hour and increasing the
current until 1.4 A (T = 790 C) the run was stopped. The glass behind the oven got fully coated with
beryllium (figure 24), while the slide behind the collimator was still transparent.
25
Glass slide
Beryllium oven
Figure 23: Electric-heated beryllium-oven set-up. The glass behind the oven is visible (left) and the wires
go around it. The collimator is on the test mount, which holds a glass slide in the last groove (not visible).
Figure 24: Beryllium layer on glass slide behind the oven after electric heating. An uncoated line is visible
where the alumel-chromel wires covered the glass.
4.1.2.2 Collimator Test
This second oven also broke during the manipulation outside the vacuum and a new one of about 400
wrappings BeW was placed instead and tested for a longer time interval. The glass behind the oven
was removed since the functionality was already shown.
The parameters measured are displayed in table 5 and the beryllium spot is visible on the glass (figure
25). It also has the same shape as the opening at the collimator, i.e. the gas beam was collimated.
After approx. 5 hours the resistance and the temperature fell down. There was also a short circuit
between the alumel-chromel wires and the metallic case of the vacuum chamber, caused by the contact
between the thermocouple, the oven, and the collimator, after all the beryllium had completely out-gassed.
The remaining oven was just the coiled and now fragile tungsten wire (figure 26), which also broke off the
collimator after taking it out from the vacuum chamber.
26
time (min) I(A) V (V) T (C) P (mbar)
0 0 0 20 3.4 · 10−6
...
207 1.4 7.3 575 2.2 · 10−5
...
302 1.4 1.3 303 8.6 · 10−7
Table 5: Test of the beryllium oven in the collimator. The system after approx. 5 hours showed a short
circuit between the pins of the feedthrough connected to the thermocouple and the metallic case.
Figure 25: Beryllium spot after the collimator test.
Figure 26: Beryllium completely evaporated from the oven.
4.1.3 Conclusions on Electric Heating
Electric-heated calcium and beryllium sources are compared in figure 27. The beryllium oven showed a
mayor efficiency as it reaches a higher temperature for a given dissipated power.
4.2 Laser Heating
In the laser-heating tests the power of the laser was scanned and the temperature in steady state was the
relevant variable. The sample was slightly heated up already when the focused beam point was calibrated,
and then the runs were started when the oven was cooled down again to a temperature of 30 C.
For each setting of the power the laser light was interrupted during the measurement by the power
meter, causing a fall in the sample temperature.
27
0 2 4 6 8 10
0
200
400
600
800
CalciumBeryllium
[oC
]
Power [W]
Tem
pera
ture X
X
Figure 27: Oven temperature vs. dissipated power during the electric heating of calcium and beryllium
atom sources. The temperature increasing in dependence on the power V · I is plotted. The marks stand
out the estimated temperature at which a visible evidence of the emitted gas is expected on the glass.
As already mentioned in sec. 3.5, all the ovens were placed between two glass slides. The laser light
is coming to the oven from the front slide, while the back one will test particle emission in the opposite
side.
4.2.1 Calcium Oven
4.2.1.1 Calcium Block
The first calcium oven to be tested under the laser heating was an 18 mm × 15 mm × 7 mm calcium piece
that was cut from an ingot. One plane surface was shaped on the block by filing it in order to minimize
the random scattered light (i.e., the laser beam was back reflected), since in the cryogenic set-up any
scattering of this laser light onto the trap could damage it. This planar face was irradiated by the laser
focused near the thermocouple junction (figure 28). The entire block surface was also filed, because it
was already oxidized and thus it would have worsened the vacuum. However, the surface was so rough
that some (deep) calcium oxide spots were left. The remaining calcium ingot was enclosed in a nitrogen
atmosphere.
After shining a point on the plane surface of the calcium block with a power up to 8.5 W, the front
glass slide showed a circular calcium layer, while calcium and calcium oxide in an irregular mixture were
adhered to the back slide (figure 29), because the back surface was not carefully polished and had oxidized
28
Figure 28: Set-up for the laser-heating test with the calcium-block. The laser light is focused on its planar
face near the thermocouple (approx. 1 mm away) and the reflected spot went out of the vacuum chamber.
spots, as stated above. This result means that the whole calcium block, and not only its front, was heated
up.
Figure 29: Layers of calcium on glass slides from the laser-heated calcium-block test. The front slide
(right) exhibits a nearly circular calcium layer while the back slide (left) contains an irregular deposition
of calcium and calcium oxide.
4.2.1.2 Calcium Granules
The calcium block showed to be not as efficient as the corresponding electric-heated oven (sec. 4.1.1)
because of its size. Therefore, another oven was fabricated using a 7 mm stainless-steel tube, which was
spot welded to an alumel-chromel junction (in order to measure its temperature and also to hold the
tube) and filled with calcium granules (figure 30). The tube was slightly squeezed on the back opening
in order to keep the granules inside, whereas the front side was open round such that the calcium could
be irradiated by the laser.
29
Calcium
(a) Front view of the oven filled with cal-
cium granules. The beam is focused on
the calcium inside the tube.
Calcium granule
(b) Rear view of the laser-heated calcium
oven with only one granule. The beam
enters the tube from the front side (not
visible) and is focused onto the calcium.
Figure 30: Two different calcium ovens for the laser-heating tests using calcium granules. The rear part
of the tube is squeezed in order to keep inside the calcium granule(s).
The reason to fill up the tube is to keep inside it the random reflections of the laser light on the rough
surface of the granules (figure 30a). This yields a localized spot in the front glass slide (figure 31) already
with 800 mW.
(a) First run (b) Second run
Figure 31: Calcium spots on front glass slide after two different runs of the laser-heated oven filled up
with calcium granules.
The filled tube makes the emission of calcium in the back side of the tube more difficult, since the
emitted atoms on the inner front part of the tube would be deposited on the granules behind. This was
ascertained by the fact that the back glass was still transparent after each run. Moreover, the temperature
inside the tube was not homogeneous due to the space among the granules, i.e., the calcium granules in
the back part of the tube will be colder than the frontal ones (on which the laser is irradiated). Therefore,
these granules at the back will not reach the needed temperature in order to generate a visible calcium
spot on the glass.
30
4.2.1.3 Calcium Granule
The tube was emptied, and for further runs only one calcium granule was fixed in its squeezed back (figure
30b). The behaviour of this oven is listed in table 6, and the heating of the oven yielded two calcium
spots (figure 32), one on each glass slide.
The temperature at which the calcium spot appeared in the electric heating was around 438 C. Such
temperature was reached with the laser heating at a power of 0.3 W (table 3).
1 calcium granule
time (min) T (C) Pw. (mW) P (mbar) Pmax (mbar)
0 30 0 2.5 · 10−6
4 284 100 5.4 · 10−6 5.4 · 10−6
10 388 200 3.4 · 10−6 a1.0 · 10−5
13 450 300 3 · 10−6 1.0 · 10−5
......
28 510 800 1.4 · 10−6 1.9 · 10−6
a Getting rid of CaO2 layer on the granule
Table 6: Resumed results on the stainless-steel tube filled with one calcium granule (figure 36). A first
drop in the pressure is observed after the outgassing of the CaO2 on the granule and a second drop as
the oven is heated up to 800 mW.
(a) Front glass slide (b) Rear glass slide
Figure 32: Calcium spots on glass slides after laser heating of the oven with one calcium granule.
31
4.2.2 Beryllium Oven
4.2.2.1 Large Piece of Beryllium Foil
The starting point of the tests on laser-heated beryllium ovens was a 25 mm × 25 mm × 0.5 mm foil
made of this material placed in one of the test-mount grooves between two glass slides (figure 33). The
thermocouple was placed in contact with the foil and the laser was focused on a point close to it.
(a) Front view (b) Side view
Figure 33: Laser-heating set-up with the beryllium foil.
This oven showed a temperature of 321 C at approx. 5 W showing an increase of only 3.33CW ; at
that point the run was stopped (table 7). Furthermore, the whole foil was emitting atoms, what could be
hazardous. The layer was too big and the beryllium is a good heat conductor, i.e. it was not possible to
achieve a local heating.
time (min) T (C) Power (W) P (mbar) Pmax (mbar)
0 24 0 2.2 · 10−5
......
24 312 2.7 1.2 · 10−5 1.2 · 10−5
34 318 3.6 8.6 · 10−6 1.2 · 10−5
40 321 4.5 7.4 · 10−6 8.6 · 10−6
Table 7: Laser-heated large beryllium foil. The drop in pressure at the final point (40 min) shows that
the sample was emitting particles (possibly beryllium oxide), but no gas deposition was still observed on
the slides. The run was stopped because the temperature increment at each increased power step reduced
to a low value: 3.33CW .
32
4.2.2.2 Small Piece of Beryllium Foil
As discussed in section 4.2.2, the heating of beryllium proved to be non-local. Thus, a smaller piece of
area 1 mm × 5 mm was cut from the original foil and placed between two tungsten coils, which are fixed
by their own feet in the grooves of the test mount. The glass slides were mounted as for the previous case
(figure 34).
Tungsten coils
Beryllium foil
Las
erbe
am
(a) Beryllium foil and set-up (b) Beryllium foil in contact with the thermocou-
ple
Figure 34: Laser-heating set-up with a small piece of beryllium foil.
In a first run the thermocouple was placed above the foil (no direct contact), and in a second run
behind the foil, in contact also with the springs and therefore in electric contact with the metallic case of
the vacuum chamber (applying a voltage to this case showed an increase in the temperature read out).
The glass slides were controlled for the presence of visible spots at each setting of the power value in
order to estimate the temperature at which a beryllium layer was visible on the glass slides (since the
laser heating of the beryllium was carried out before the electric heating of its respective oven). The
small piece started glowing for 1.6 W, and both glass slides had visible beryllium layers for 1.8 W at a
temperature of 507 C (table 8). After this, the run was stopped (figure 35).
4.2.3 Conclusions on Laser Heating
The temperatures vs. the incoming laser-beam power for the calcium oven with one granule and for the
beryllium small foil are compared in figure 36.
A step in the increasing behaviour in the temperature was observed for both ovens. This could be
attributed to the (calcium- or beryllium-) gas that scatters the incoming light, and in this way slows the
33
time (min) T (C) Power (mW) P (mbar) Pmax (mbar)
0 30a 0 1.0 · 10−6
3 84 100 1.0 · 10−6 1.0 · 10−6
......
54 463 1400 7.4 · 10−7 1.2 · 10−6
60b 488 1600 4.0 · 10−7 1.0 · 10−6
66c 507 1800 4.0 · 10−7 8.6 · 10−7
a Sample was heated during the alignment of the laser
b Beryllium piece glowing
c Beryllium coating on glass
Table 8: Resumed results on the laser-heated small piece of beryllium foil. A drop in the pressure can be
observed at the time the beryllium layer appears on the glass.
(a) Front glass slide (b) Back glass slide
Figure 35: Layers of beryllium on glass slides after laser heating of the small beryllium foil.
heat-up process.
A small circle in the centre of the atom coats on the respective front glasses is caused by the laser that
re-heats the deposited atoms as it passes through it. This means that the set-up can be reusable without
opening the vacuum.
34
Figure 36: Oven temperature vs. laser power during the laser heating of calcium and beryllium ovens.
The marks stand out the estimated temperature at which a visible layer of deposited gas on the glass is
expected.
35
5 Electric vs. Laser Heating
Table 9 shows the power dissipated inside the vacuum chamber during the functionality tests of the
electric- and laser-heated ovens. Recall that these powers are estimated for the detection of a visible
coating on the glass slide(s) and serve as a comparison between the ovens and the heating methods;
therefore, these powers are higher than the ones, which will be used in the ion traps (only a few ions are
needed for quantum-computation experiments).
In the case of the calcium ovens: a reduction in the power of a factor of 13 is observed by using the
laser-heated oven.
Regarding the beryllium ovens:
The power dissipated by this type of current-heated oven that was measured for the comparison
(sec. 4.1.2.1) turns out to be an upper bound because it was done during a run while this oven
was mounted on the collimator. This construction generates extra power due to the inefficient
mechanical junctions (by screws) of the wires on the PCB. Since this piece won’t be part of the
cryogenic trap, the power dissipated by the bare oven (in the cryogenic set-up) would be lower than
this measured value.
Only a fraction of the incoming laser power in the corresponding heating method is absorbed by
the oven (see heat gain in sec. 2.3), the rest can be back-reflected by aligning the surface of the
foil adequately. From [BeMi], an absorption of approx. 25% of the incoming beam intensity (table
8) can be assumed. Thus, the effective dissipated power inside the cryogenic environment from the
laser-heating can be estimated to be Plaser = 1.8 W ·14 = 0.45 W. This power is comparable with
the one from the laser-heated calcium oven, and so this oven can be considered for the cryogenic
trap.
T C Pelectric (W) Plaser (W)
Calcium 438 4.1 0.3
Beryllium 507 < 1.9 0.5
Table 9: Comparison of the dissipated powers from electric Pelectric and laser Plaser heating at the tem-
perature T , at which the deposition of gas on the slides was observed.
From the reasons explained above, laser-heated ovens are ideal for the cryogenic trap.
36
Tests of long duration on the collimator
Comparing the runs of the electric-heated ovens for which the collimated spot was observed, the power
dissipated by the calcium and by the beryllium oven was obtained to be 10.12 W (from table 4) and 10.22
W (from table 5) respectively, concluding that these values are comparable the same.
For the room-temperature trap, proper thermal isolation suffices to carry out the experiments. Indeed,
this approach will be taken for this set-up, since electric-heated ovens require a simpler configuration
(electric current is much more readily available than high-power laser light).
37
Part III
Fast Switches in Cryogenics
6 Theory Background
6.1 SPST and SPDT Switches
Single-Pole Single-Throw (SPST) switches (figure 37a) are electronic circuits that adopt one of two states
between an input and an output: low impedance (“ON”) or high impedance (“OFF”). On the other
hand, Single-Pole Double-Throw (SPDT) switches (figure 37b) connect one output to one of two possible
input voltages [SPSw].
IN OUT
Control
(a) SPST switch
IN1
IN2
OUT
Control
(b) SPDT switch
Figure 37: Schemas of SPST and SPDT switches.
Two SPST switches in series driven by complementary signals reproduce a SPDT switch (figure 38).
V1
C1 C2
OutV2
Figure 38: Series configuration of two SPST switches giving rise to a SPDT switch with C2=NOT(C1).
6.2 Cryogenic Semiconductor Technologies
Not all Field-Effect Transistors (FETs) can operate at low temperatures due to the carrier-freeze-out
effect [Sta98] (see sec. 1.1). Semiconductor compounds for FETs that are known to work in cryogenic
environments are Galium-Arsenide (GaAs) [Ric84], Indium-Phosphide (InP) [Sch12] and Silicon-Carbide
(SiC) [Kim10]. However, a candidate technology that could be suitable for our switching requirements is
CMOS (Complementary Metal-Oxide-Semiconductor). The characteristics and issues of the mentioned
FETs are detailed below.
GaAs: It is often found in integrated circuits due to its broad use in microwave communications,
and thus it is typically designed for low-voltage applications. The commercialization of this kind
of ICs have made possible the miniaturization and development of fast switches. Thus, the GaAs
technology was tested in the scope of this project.
39
InP and SiC: Although they work at higher voltages than GaAs, it is hard to find commercial
switches made of these semiconductors. Anyway, these devices are not small, thus, they occupy
great volumes, being this a disadvantage at the moment of inserting such electronic components in
a cryogenic space.
CMOS IC Switches: Another option for ultra-fast switching is the CMOS technology. Such ICs
are very convenient due to its purely digital control (as opposed to GaAs, which would require an
analogue control pulses to suit our purposes [Wil11]). Commercial CMOS ICs contain millions of
transistors (MOSFETs: Metal-oxide-semiconductor FETs) providing high immunity against noise
and low static-power consumption [CMOS2], making them ideal for our aim. The disadvantage of
CMOS ICs is that there is very little literature about its performance at cryogenic temperatures
[Ock09, Cre09], so, a good part of the work presented here went to testing if any commercial CMOS
switch would be suitable for ultra-fast voltage switching at low temperatures (4 K).
40
7 Experimental Set-up
7.1 Test Boards
The test boards are PCBs that serve as interface between the pins of the IC switch and the cable tree.
Rapid thermal changes, voltage and current peaks, and other effects could damage bare ICs; therefore,
resistors and capacitors are placed for its protection as shown in figure 40 and are detailed below. Addi-
tionally, a metallic wire of approximately 10 cm extends straight over the PCB in order to thermalise it
(figure 39), avoiding a temperature shock.
Figure 39: Test board plugged in the connector of the cable tree. The outgoing metallic wire (left) is used
for a smooth thermalisation of the board. Courtesy of S. Stahl.
Each “quad” CMOS IC consists of four SPST switches (figures 40a and 40b), two of them are used
in series (the other two being unused) and are controlled by complementary signals, thus having a SPDT
switch with input voltages V1 and V2 (figure 38). The GaAs switches (figure 40c), in contrast to the
CMOS ICs (each SPST CMOS contains many MOSFETs), have commonly a simply design: they have
two GaAs FETs in series as in figure 38, which are also controlled by complementary signals, although
this control includes more complications.
The resistors and capacitors on the boards have the following functions:
The 47 Ω resistors at V1 and V2 are brake-before-make-failure protections.
The 10 nF || 100 nF capacitors at V1 and V2 are the actual switching voltages.
The 10 MΩ resistors at C1, C2 and Vout connected to ground are anti-charge-up protections.
The 560 Ω resistors in the CMOS PCBs are short-cut protections.
The 220 Ω resistors at C1 and C2 are additional protection resistors.
Each connection in the figure 40 is labelled corresponding to its function in the IC. These labels are
explained in the next section.
41
(a) Analogue/digital CMOS board (VDD = V+). (b) Analogue CMOS board. In contrast to the digital
one, this is externally controlled by only one signal: C1.
However, it needs a logic input voltage.
(c) GaAs-switch board.
Figure 40: Schematics of the switch test boards. The connections are represented on the green dashed
line. Courtesy of S. Stahl.
42
7.2 Cable Tree
The cable tree (figure 41) consists of six wires and three coaxial cables for cryogenic connected to the
plug for the test boards.
The wires are welded to pins on a pin board (figure 41b) and are denoted by:
GND: ground
V1: first switched voltage
V2: second switched voltage
V-: negative supply-voltage input
V+: positive supply-voltage input
VL: logic supply voltage
The 3 coaxial cables for cryogenic (Nexans VMTX) adapted to BNC connectors are used for the
transmission of the signals. They are denoted by C1, C2 (control signals) and Cout (output signal).
43
(a) Cable tree. Six wires and three cryogenic coaxial cables approx. 160 cm
long connect the pin board to the switch-PCB plug. Courtesy of S. Stahl.
(b) Pin board and coaxial cables with their labels.
Figure 41: Cable tree and pin board.
44
8 Experimental Methods and Results
8.1 Characteristics of a Pulse
The pulse-shape characteristics of the switching between two voltages are widely used in the next sections.
The rise trise and fall tfall times as well as the output amplitude V1,sw − V2,sw were studied (figure 42).
The rise time (analogously the fall time) is defined as the time elapsed between the 10% and the 90%
of the pulse amplitude.
VC1 ≡OFF
VC1 ≡ON
V1,sw
V2,sw
Delay
trise
90%
10%
Figure 42: Characteristics of a pulse (violet) originated at the output of a SPDT switch after a change in
the control signal (green C1 : OFF → ON, this implies C2 : ON → OFF). The rise time (analogously
the fall time) is defined as the time elapsed between the 10% and the 90% of the pulse amplitude.
8.2 Selection of the fastest Switch in LHe
As already mentioned in section 6.2, the carrier-freeze-out effect represents an adverse event for our aims.
For this reason, the functionality of the switches in table 10 was tested at 300 K (room temperature),
ensuring they work according their data sheets, at 77 K (in LN2), to possibly discard some of them, and
at 4 K (in LHe). The set-up is shown in figure 43. A discharging resistance was adapted in parallel to
the oscilloscope to enable us to carry out the measurements (see app. D).
A CMOS SPDT switch is composed by two SPST switches in series. Each of the two used SPST
CMOS switches from each IC was tested separately as described in table 11. In order to test cold-start
capability, the switching was started only after the test board was submerged into the liquid and therefore
ensuring thermal equilibrium with it.
The rise times of the SPST CMOS switches were measured and plotted in dependence on the temper-
ature (figure 44). All CMOS electronic switches performed as expected at room temperature as well as
45
Technology Model Fabricant Ref. number
74VHC 4066 M FAIRCHILD 1
CMOS analogue/digital 74HC 4066 D NXP 2
CD 74HC 4066M Texas Instruments 3
DG 413 DY Maxim 1
CMOS analogue ADG 413 BRZ Analog Devices 2
DG 413 HSDY Maxim 3
GaAs MASWSS0179 M/A-COM 1
1AS179-92LF Skyworks 2
Table 10: Switches tested at 300 K, 77 K and 4 K.
Technology Label V1 (V) V2 (V) V− (V) VL (V) V+ (V) C1 (V) C2 (V)
CMOS V1/C1 9 – – – 9 $0 to 8 –
analogue/digital V2/C2 – 9 – – 9 – $0 to 8
CMOS V1/C1 4.5 – -9 5 9 $0 to 5 –
analogue V2/C1 – 4.5 -9 5 9 $0 to 5 –
GaAs V1/C1C2 4.5 – – – – $0 to 5 $0 to 5
V2/C1C2 – 4.5 – – – $0 to 5 $0 to 5
Table 11: Detailed voltage configuration used for testing the switches. Each square ($) control signal
(C1 and C2) oscillated at a frequency of 100 kHz and was driven by a SRS DS345 connected to the switch
and to the oscilloscope. The ±9 V and 4.5 V potentials were driven by batteries, while the 5 V by a
voltage source. The SPDT GaAs switches were driven by complementary control signals using a TTL
NOT gate (denoted by an over-line). The symbol “–” stands for an open terminal.
in liquid nitrogen (according the rise and fall times on the data sheet), but some of them did not respond
to the control signal when cooled with liquid helium; these were tested again at room temperature and
all of them started switching again, meaning that they were not cold-start capable at 4 K (table 12).
The increasing in the switching time of the tested analogue CMOS as the temperature is lowered can
be explained by the decrease in free charge carriers in the semiconductors, while the faster switching in
the tested digital ones as the temperature goes down could be a result from the reduction in the load
impedance of the electric components in cryogenics.
Once the CMOS switches were proven to work, the GaAs candidates were ruled out, even though
46
Oscillosope
Voltage generator
Signal generator
Battery
LN2 Dewar
Figure 43: Cryogenic functionality-test set-up. A hollow tube with the test board and part of the cable
tree in its inner was inserted in the dewar. Devices and measurement configuration are described in table
11.
their switching speed is not supposed to be hindered by low temperatures. The reason is that in these
ICs there is internal circuitry other than that purely dedicated to switching. It is theoretically possible to
overcome this problem by adapting the control pulses to the voltages which one wants to switch, but that
poses many experimental complications. However, minimal tests were carried out with these switches,
although the used NOT gate showed to be too slow for our purpose because it introduced a high delay
between the complementary control signals, thus increasing the rise time.
The ultra-fast switching demands an IC with the shortest rise time (the fall time is supposed to be
nearly the same in each case). The CMOS switch CD 74HC 4066M from Texas Instruments will be
used in the cryogenic trap, since it showed the fastest switching for which both integrated SPST switches
worked. As required, the rise time lies below 10 ns. The rising pulse shape of the first SPST switch
(V1/C1) from this IC is depicted in figure 45. Note that in this picture the rise time of the control signal
can be estimated to be smaller than the one of the switch, i.e., this signal is “slower” than the switching
itself, what entails that the switch reacts only to a determined threshold voltage and does not depend on
the shape of the signal used to control it.
47
0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 03
4
5
6
7
8
9
1 0 # 3 V 1 / C 1 # 3 V 2 / C 2 # 2 V 1 / C 1 # 2 V 2 / C 2 # 1 V 1 / C 1 # 1 V 2 / C 2
Rise t
ime [
ns]
T e m p e r a t u r e [ K ]
(a) analogue/digital CMOS
0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 00
2 04 06 08 0
1 0 01 2 01 4 01 6 01 8 02 0 0 # 1 V 1 / C 1
# 1 V 2 / C 1 # 2 V 1 / C 1 # 2 V 2 / C 1 # 3 V 1 / C 1 # 3 V 2 / C 1
Rise t
ime [
ns]
T e m p e r a t u r e [ K ]
(b) analogue CMOS
Figure 44: Rise times of the SPST CMOS switches in dependence on the temperature. The labelling in
the legend is according to tables 10 and 11.
CMOS trise (s): V1/C1 trise (s): V2/C2
74VHC 4066 M 4 -
74HC 4066 D 4.7 4.7
CD 74HC 4066M 4.6 4.6
DG 413 DY - -
ADG 413 BRZ 10.6 10.6
DG 413 HSDY 180 141
GaAs trise (s): V1/C1C2 trise (s): V2/C1C2
MASWSS0179 x x
AS179-92LF x 7.4
Table 12: Rise times of the tested switches. Some switches were unable to cold-start when submerged
in liquid helium (denoted by “-”). Only minimal tests were carried out on GaAs switches and their
measurements were discarded (x), since their performance does not suit our future experimental needs.
8.3 Characterization of the selected Switch
As described in the previous section, the CMOS IC with the fastest switching, the CD 74HC 4066M from
Texas Instruments, was chosen for the cryogenic set-up. This switch was characterized in detail to study
its behaviour in liquid helium. Characterizing a switch refers to measure its power dissipation, as well as
the amplitude, rise and fall time of the output signal and the introduced cable-length noise. The set-up
48
Figure 45: Example of a typical rising pulse. CD 74HC 4066M from Texas Instruments at 4 K.
for the characterization of this electronic switch is represented in figure 46.
As already described in previous sections, the control signals C1 and C2 are complementary, i.e., one
is inverted with respect to the other. Since the two SPST switches are in series (i.e., two input voltages
V1 and V2, and common output), the switch is strictly speaking a SPDT switch (figure 38). The signal
generators are coupled by a signal of frequency 1.25 MHz, being this also the frequency of C1 and C2.
This frequency also corresponds to the one that will be used in the experiments on ion transport in the
cryogenic trap.
The DC voltage generators are used in order to scan the behaviour of the switch over different values
for the switched voltages V1 and V2. These input voltages were chosen in the set of integer values 0 V,
... , 7 V.
Since the voltage at the output of the switch is always defined by one of the two input voltages, the
resistance in parallel at the oscilloscope was removed (it was introduced in the last section, because of a
high-impedance state in the switch).
AC coupling
There was a coupling of the switched voltages (appendix E) in the AC driven mode: the lower voltage
was limited by the higher one. In order to be able to measure the behaviour of the switch at higher
amplitudes, the lower potential was connected either directly (0 V) or over a potentiometer (between 0
V and the minimal coupling voltage) to ground.
8.3.1 Power Dissipation
The power dissipated is measured in order to estimate the repercussion of the circuit on the environment.
This power was found by measuring the introduced (DC or time-averaged) currents and the respective
voltages as depicted in figure 47.
49
4 K / 77 K
Switch
V1 V2
Cou
ple
dcl
ock
s
Oscilloscope
C1
C2
V+
Figure 46: Set-up for the characterization of the electronic switch. The complementary control signals
are generated by coupled signal generators and the DC voltages by their corresponding generators. The
output can be read at the oscilloscope.
V1
C1 =ON
V2
C2 =OFF
Oscilloscope
1 MΩ
15 pF
I2
I1
IC2
IC1
I+
V+
Figure 47: Schema of the current-measurement set-up to estimate the power dissipation. The colours
correspond to the ones of the wires in the pin board (figure 41b).
From the precision of the measuring devices it can be concluded that the currents flowing through the
switch were less than 10−6 A in each case in static mode (C1 and C2 constant signals). The switch will
50
stay also the most of the time in this state when unused.
- 8 - 6 - 4 - 2 0 2 4 6 88 . 1
8 . 2
8 . 3
0 1 2 3 4 5 6 7
Pstat
ic [mW
]
V 1 - V 2 [ V ]
V m i n
(a) Dissipated static power.
- 8 - 6 - 4 - 2 0 2 4 6 8
1 0
2 0
3 0
0 1 2 3 4 5 6 7
Pcon
t [mW]
V 1 - V 2 [ V ]
V m i n
(b) Total power dissipated by the switch.
Figure 48: Dissipated power in dependence on the switching amplitude. We observe a symmetric be-
haviour around ∆V = 0.
In the AC-driven switching the static power consumption Pstatic = I+V+ is plotted in figure 48a in
dependence on the switching amplitude |V2 − V1| for many Vmin = min (V1, V2). Since the measurements
were carried out with the output cable connected to the oscilloscope, despite the fact that in the final
configuration by the trap the output will be an electrode, the current flowing through this cable was not
measured, and, moreover, this and other currents are fictitious (as stated below); then, the static power
consumed by the switch will be its dominating dissipation mechanism in the final design.
It can be seen that this power increases for strictly positive and increasing Vmin. An increase in the
static power is also observed as the switching amplitude is increased. From the symmetry of this plot it
can be stated that this power depends only on the absolute value of the amplitude |V1 − V2|. It can be
estimated to be between 8.1 mW and 8.4 mW.
The continuous power consumed by the switch is found by adding the absolute values of the powers
dissipated by each input Pcont = |I+V+| + |IC1VC1 | + |I1V1| + |IC2VC2 | + |I2V2|. Remember that some
of these currents are fictitious, as they originate due to the capacitance of the oscilloscope, in the same
manner as explained in appendix E.
The continuous power consumption is plotted in figure 48b in dependence on the switched amplitude
for many Vmin. This power shows to be symmetric around ∆V = 0 and increases for higher voltages
amplitudes and voltage minimum. The maximum power up to ∆V = 7 V can be estimated to be 30 mW.
51
8.3.2 Switching Behaviour
The behaviour of the switching describes the quality of the pulse in two terms: the rise and fall time, and
the output amplitude ∆Vsw = V1,sw − V2,sw in dependence on the input amplitude ∆V = V1 − V2 (set up
at the signal generators).
- 8 - 6 - 4 - 2 0 2 4 6 8
- 0 . 0 2
- 0 . 0 1
0 . 0 0
0 . 0 1
0 . 0 2
0 . 0 3
% D i f f
% Dif
f [-]
V 1 - V 2 [ V ]
Figure 49: Switching-amplitude deviation in dependence on the switching amplitude. This deviation is
lower than 3 % in all the cases.
The percentage deviation of the output amplitude ∆Vsw−∆V∆V was measured with respect to ∆V and is
plotted in figure 49. This deviation can be bounded by∣∣∆Vsw−∆V
∆V
∣∣ ≤ 0.03 ≡ 3 %.
The rise time and fall time of the switch pulses is plotted in dependence on the amplitude of the
switched voltages ∆V = V1 − V2 (figure 50). Both times are below 5 ns for |∆V | > 2 V. In the ion-trap
experiments this does not represent any problem, since the goal is to switch large voltage amplitudes.
-8 -6 -4 -2 0 2 4 6 8
4
5
6
7
8
9
10 0123456
tris
e [n
s]
V1 - V2 [V]
Vmin
(a) Rise time
-8 -6 -4 -2 0 2 4 6 8
2
3
0123456
tfall
[ns]
V1 - V2 [V]
Vmin
(b) Fall time
Figure 50: Rise and fall time in dependence on the input switch amplitude.
52
Based on one of the goals of this project, the pulse shapes were measured for a switching amplitude
of 10 V and represented in figure 51.
(a) Rising pulse for V1 = 10 V, V2 = 0 V (b) Rising pulse for V1 = 0 V, V2 = 10 V
Figure 51: Rising pulse of the chosen CMOS IC switch between 0 V and 10 V.
8.3.3 Introduced Cable-Length Noise
Another interesting feature to study is the influence of the cable which connects the output from the
switches to the oscilloscope. In the future use of this switch, the output will be directly connected to
the electrode of the trap, which can be pictured as a small capacitance (approx. 1 pF) to ground. The
mere fact of having a cable at the output and of connecting it to the scope dramatically increases the
capacitance. The effect of this increase is that high-frequency components are filtered partially (washed
out), leading to slower switching times, but also possibly to glitches, noise and ripples in the pulses.
Therefore, the shape of the output pulses was recorded while continuously switching between 0 and 7
V (1.25 MHz) for the cable lengths of 0.5, 1, 5 and 10 m. In addition to this, the approx. 1.6 m coaxial
cable for cryogenics (table 13) has to be considered also into account.
There seems to be no major dependence on the cable length, which suggests that the capacitance
of the oscilloscope (15 pF) and that of the cryocoaxial cable (approx. 50 pF @ 50 Ω) already limit the
behaviour of the switch. Indeed, 50 Ω × 50 pF gives a time constant of 2.5 ns, which is in the range of
what it is measured.
An asymmetry between the right and the left case in table 13 can be observed. This is a result of the
internal circuitry of each of the two CMOS SPST switch.
53
V1 = 7 V, V2 = 0 V1 = 0 V, V2 = 7
0.5 m
1 m
5 m
10 m
Table 13: Pulse shapes for different BNC cable lengths.
54
Part IV
Conclusions
9 Summary and Outlook
The greatest success from this work is that the results obtained both regarding atomic ovens and ultra-fast
switches will be implemented into the trap set-ups at the TIQI group within the next months. The main
achievements are:
The electric-heated atom ovens worked properly in the collimator for the room-temperature trap.
Finished ovens were built and kept in an argon atmosphere in order to be used in the future set-up
trap.
Due to the low power consumption of the laser-heated ovens, they showed to be the best alternative
to be used in the cryogenic trap.
The CMOS switch CD 74HC 4066M from Texas Instruments was chosen for the ultra-fast switching
in the cryogenic environment. Its rise and fall time are below 5 ns between 0 V and 10 V. The
switch can be started after it is inserted in the cryostat (cold-start capable).
There are other (less critical but still important) achievements which have been attained in the scope
of this work:
It was shown that shielding a calcium oven with a stainless-steel tube does not increase its temper-
ature (as function of the power dissipated) with respect to the single one.
The beryllium coil winder was improved such that the beryllium wire hangs down freely without
turning the entire device ahead. The coiling process runs automatically.
The PCB of the collimator was improved in order to connect the ovens to the current sources by
screws.
Several switching candidates have been tested and more than one have been proved to work at 77
K and 4 K.
A few interesting experiments remain, which can be carried out based on this work, like estimating
the amount of calcium and beryllium obtained from the ovens, e.g. by measuring the weight of the glass
plates before and after the runs. But especially important steps to take before setting up the final trap
experiments are:
The mount of the atom ovens in the cryogenic set-up needs to be designed. The laser will be
conducted to the oven via an optical fibre. Pinholes can be used in order to collimate the atom gas
56
from the laser-heated sources. The low power of the laser beam should not represent a big problem,
a simple mechanism in order to dissipate its produced heat to the outside can be installed without
worsen the cold environment inside the cryostat. The sources emit atom gas in both front and back
directions, so the scattering of the laser light can be even absorbed in the walls of the mount.
The PCB for the selected switch in the cryogenic set-up needs to be designed. Essentially low pass
filters will be placed in front of the inputs of the switch and the output will be connected directly
to the electrode of the trap.
57
Part V
Appendix
A Design and Construction of a Base for a Turbo-Molecular Pump
(a) Base for TMP. Screw holes for the Turbopump (centre),
hand pulls (opposite sides) and for the feet (corners) are visible.
Roland Hablützel 10.10.2011
Designer DatumGruppeKontakt
Prof. J. Home
Auftrag
TMP holder
Material Anzahl Einheit
±
BaseTMP.idw
Toleranz
Dateiname
Aluminium 1 mm 1%
J. Alonso (32329)
No
Expressauftrag?
1.0
Version
M
4
x
0
.
7
-
6
H
M6x1 - 6H
12,00
R
4
3
,
0
0
M5x0.8 - 6H
20,00
4
3
,
0
0
°
3
3
,
0
0
°
6,50
270,00
15,00
270,0015,00 15,00
15,00
80,00 140,00
80,00
20,00
260,00
20,00
15,00
(b) Projections of the base on the three main planes, including dimensions.
Figure 52: Inventor drawings of the base for the Turbo-Molecular Pump.
An aluminium base was designed, constructed and adapted to the bottom of the TMP (figure 52).
The need for such a base arose from the fact that the electric-heating set-up (figure 8) was a vertical
structure which required to be stabilized. If the TMP turns and falls while it is on, the rotor at a velocity
of 90000 rpm could break off and cause serious injuries.
The hand pulls were purchased at the D-PHYS-Shop and the rubber (neoprene) feet at Distrelec.
With this design, the tube connecting the TMP with the pre pump goes below one of the hand pulls.
59
B Instructions to Build Current-Heated Atom Ovens
The steps followed to build the calcium (sec. 4.1.1) and beryllium (sec. 4.1.2) atom ovens for the electric
heating are described in this section.
B.1 Calcium Oven
One oven was built for each steel-tube size (table 14).
Materials and tools:
Calcium granulate: -16 mesh, 99.5 %
Stainless-steel tubes
Tantalum wire: 0.5 mm, annealed, 99.95 %
Acetone and isopropanol
Chromel and alumel wires (0.25 mm)
Sharp and (needle-nose) stork pliers
File with a sharp edge
Spot-weld machine
Crimps EK-SUBD-F-CLG10, 15 A, 1 mm, copper alloy - gold plated (BarUvac/VACOM)
Crimping tool
We worked with three kinds of tubes with different inner (ID) and outer (OD) diameters (table 14).
All were purchased from McMaster-Carr.
Gauge ID (mm) ID (in) OD (mm) OD (in)
18XT 1.07 0.042 1.27 0.050
17XT 1.27 0.050 1.47 0.058
15XTS 1.60 0.063 1.83 0.072
Table 14: 316 stainless-steel-tube sizes. This type of steel is non magnetic both at room temperature and
at 4 K.
60
The resistance at each temperature is given by the geometry of the sample [Cre08]. In the case of a
hollow cylinder: R = ρ Lπ(r2ext−r2int)
, where ρ, L, rext and rint (= 0 for a wire) are the resistivity, the length,
and the external and the internal radius respectively.
These tubes are made of stainless steel, since this material is widely used in vacuum systems. It has
a relatively high resistivity, transforming in an efficient way electric current into heat. It has also a good
heat conductivity, giving the oven a homogeneous temperature. Another advantage from stainless steel
is that it does not become superconducting at 4 K, so it is also suitable for the cryogenic set-up.
We use tantalum wires to connect the oven to the electric-current source, because this is a good
electric conductor, it has a relative low outgassing rate (figure 4), and has a small thermal conductivity
(compared to the stainless steel).
The calcium is from Alfa Aesar and comes in granular form of approx. 1 mm thickness (mesh -16, the
finest available).
Chromel and Alumel are standard materials in order to measure the temperature, which is related to
the voltage difference at the junction between them. The thinnest available were chosen, since thicker
wires broke off repeatedly after being spot-welded.
(a) Tube is cut. (b) Hole is filed in the
middle and the ends are
opened.
(c) Tantalum wire is fixed
at one end.
(d) The tube is filled with
calcium and the other end
is closed holding a tanta-
lum wire.
Figure 53: Calcium-oven construction.
Clean the tools with acetone and isopropanol. One may use commercial towels or paper on items
that will not go inside the vacuum5.
Cut gently the tube with sharp pliers in order to get a piece with a length of approx. 1 - 1.5
cm (figure 53a). The thicker the tube is, the longer it should be. This length pattern makes the
diameter difference more visible and has the advantage, that a longer (and thus thicker) oven will
need approximately the same current as a smaller one in order to be heated to the same temperature
(due to the homogeneity of the resistivity). Be aware that this piece will spring away when cut;
therefore, cover the biggest space angle around it with the hand in order to prevent injuries. Note
5For the pieces that are going inside the vacuum use optical paper, as the commercial one could leave shavings.
61
also that if the ovens are too short, one will not get enough calcium inside for the experiment. If
they are too long, calcium will be wasted.
Open both ends with stork pliers, since they will be squeezed after the cut.
Cut the tantalum wire in pieces of approx. 90 mm.
File the tubes at their middle with a (triangular) file (figure 53b). Be sure that the hole is not
too small, otherwise one will not get enough calcium gas, and not too big, such that the calcium
granules cannot escape. Clean from the inside with one of the tantalum wires the zone around the
hole in order to remove shavings. An opening width of approx. 0.5 mm is ideal.
All the components are cleaned with acetone (remove organic impurities) and then isopropanol (to
clean the acetone before it dries) in an ultrasonic bath approx. 3 minutes each. Shake ovens in
order to take out possible trapped drops from the chemicals used.
A tantalum wire is introduced in each oven about 2 mm through an open end, which is then
pressed-closed (figure 53c)6.
Fill the ovens selecting the calcium granules. Do it as fast as possible, since the calcium oxidises.
Start with the smallest ovens, as it is very hard to fill it up, otherwise one could also insert small
granules in bigger ovens, remaining at the end big granules for smaller ovens.
Insert another tantalum wire on the open end of the ovens and press-close (figure 53d).
Spot weld the ends of the ovens with two pulses at 7% and 14% of the maximal energy (230 V)7.
Spot weld near one end (at the opposite side of the hole on the tube) an alumel-chromel junction
in order to be able to measure the temperature.
Insert the ends of each wire (2 from the tantalum wires of the oven, one Chromel and one Alumel)
into the crimps and close them with the respective pliers.
B.2 Beryllium Oven
A beryllium oven is a coil of beryllium-wrapped tungsten. The current flow through the tungsten will
heat up the beryllium.
Materials and tools:
6Do not press too fast or too hard since the lateral ends of the now squeezed hole could break on the edges.7Do not weld with tin, because it melts at temperatures which we are going to reach. Spot welding ensures a better
electric contact between the components.
62
Beryllium wire: 0.05 mm, annealed, 99.7 % purity
Tungsten wire: 0.1 mm, 99.95 % purity
Beryllium coil winder (figure 12)
Voltage source: Blanko DF1730SB3A
Heat gun: WELDY PRO 1800 W from Leister Process Technologies
Small weight (nut M2)
Drill with a 1 mm bit
Sharp pliers
Tungsten wire is used (higher resistivity than tantalum) since beryllium needs more heat in order to
produce an adequate amount of vapour atoms (figure 2). The instructions to wrap the beryllium wire
around the tungsten follows:
Tie the small weight to the beryllium wire (still in the reel)8.
Cut approximately 35 cm of each tungsten and beryllium wire.
Tense the tungsten wire between the two rotors. They rotate at the same velocity and are set in
motion by a motor, which is controlled by a voltage source.
– Pass the wire through the tap A (rotating the wire with the fingers will do it easier) and tie
it to a nut in order to fix the wire. Insert the tap in its respective hole (near the motor) and
screw.
– Pass the wire through the tap B and through a 10 cm long hollow tube. Pass the tube through
the corresponding hole in order to bring the wire to the other side. Take out the tube (holding
the tungsten wire) and then press with one hand the rotor in order to compress the spring
while with the other hand two windings around the outer screw are done. Screw and release
in order to tense the tungsten and then fix tap B.
The screw at tap B is long enough such that part of it will stick out (figure 54). Tie the free end
of the beryllium wire to this screw (a small loop is enough in order to hold the beryllium with the
screw).
8Use the heat gun to get smaller loops, do not pull very hard
63
Figure 54: Loop of the beryllium around the left-out screw.
Figure 55: Weight on the beryllium wire (picture rotated 90).
Add more weight to the nut tied to the beryllium wire by passing a cable through it with three
small metal rings (figure 55). The beryllium wire should hang all the way down, i.e., it should not
pass by any edge9. The mass is built as follows:
– Place two metal rings in the centre of the cable and wind once such that the rings are fixed
– Insert the cable through the hole of the nut with the beryllium and wind once below and once
upon the nut
Tilt the set-up about approx. 1.2 to the left10 by turning the corresponding screw. NOTE: the
coil winder was improved such that it is not necessary to turn it ahead in order the beryllium wire
to hang down (figure 56).
Reset the counter before the motor is started. Start with the lowest voltage needed to drive the
rotor, which is approx. 1.2 V. Heat constantly the winding point of the beryllium wire at the
tungsten. The heat gun was set to the maximum heating (around 300 C) and minimum fan speed.
9If so, you prevent the beryllium wire and the mass from rotate freely as it is coiled, and once the mass passes by the
edge, the wire feels a rapidly torque which will end up breaking it in the weakest point10The maximum angle without inserting something below the base
64
Figure 56: Improved beryllium coil winder. The aluminium plate in the basis was cut and the rotating
set-up was moved forwards with respect to the table.
Speed the motor up until voltage values of 7 V. The optimal value is 5 V, that is, around 7 seconds
per turn.
When the oven is finished, cut the exceeding beryllium, release the screw at the end (that tenses
the tungsten wire), open tap A to relax the tungsten and open tap B and in order to take out the
wire.
Once the beryllium is wrapped around the tungsten wire, wind the beryllium-tungsten wire around a
1 mm drill bit by fixing the exceeding tungsten wire together with the bit into the drill (figure 22a) and
holding the other end with the hand.
In order to calculate the amount of wire needed for an oven, the following calculations were carried
out:
Given the length x and the diameter d of the beryllium wire, and the diameter D of the tungsten
wire, the maximum number (tightest) of possible coils is the length of the wire divided by the length of
one loop x(D+d)π . Then the minimal length L of the beryllium-tungsten wire is the number of wrapped
coils times the thickness of the beryllium wire: L = d·x(D+d)π .
The final wire will be coiled around a cylinder with diameter D′ with a spacing of ∆s (figure 57).
Since the diameter of the beryllium-tungsten wire is D + 2d, the length X of the coiled wire is (after an
analogous calculation):
X ≈ (D + 2d+ ∆s)L
(D′ +D + 2d)π=
(D + 2d+ ∆s) · d · x(D + d)(D′ +D + 2d)π2
The values of the different wire diameters are given: d = 0.05 mm, D = 0.1 mm and D = 1 mm. The
spacing is set to σ = 1 mm and the desired coil length is X = 7 mm. The length of the beryllium wire is
calculated to be x ≈ 207 mm.
65
Be
W
D′
D + 2d
Be
WD
d
∆s
Figure 57: Schema of the transversal cut of the beryllium coiled around the tungsten wire and of the
coiled beryllium-tungsten wire.
66
C Developing and setting up PCBs
Figure 58: Original PCB for the collimator of the room-temperature set-up.
PCB is the acronym for printed circuit board. The manufacturing process is:
Print figure in white paper
Check the scale and print the figure in bond paper
The developing room should have yellow lights only
Heat up 2 minutes the UV lamps of the printing machine
Place the paper bond with the printed side on the photosensitive side of the board and both under
a soft vacuum. Turn on 2 minutes the UV light in order to create the impression of the figure on
the board
Take the board into a developer bath for 1 minute
Clean the board with water and place it into the corresponding acid. The board should be stirred
constantly (this was done by fixing the board to a rotating set-up11). To develop again, clean the
acid and repeat from previous step (developer and acid react, therefore clean always with water)
Once it looks well, clean the board using water and sponge
Drill holes if necessary, using the back side to draw their positions.
My plate did not have any photosensitive layer, so instead of developing it under UV light, scotch
tape was cut in the desired form.
11HINT: place the side to develop towards the rotation axis, as this side will be developed faster. This is so, because the
inner face feels more friction with the acid as the board rotates. The difference in the developing time between the inner and
the outer face is about 30 min.
67
C.1 Impractical Spot Welding
The tantalum and tungsten wires should be fixed to the PCB. For this, the wires were tried to be spot
welded onto it without success (figure 59), since copper is a both good thermal and electrical conductor
and it has also a relatively high melting point (for spot welding a local heating is required in order to
melt the metals at the junction).
Figure 59: PCB after trying to spot-weld on it. The copper is deformed due to the welding.
A possible solution is to spot weld the copper to constatan, and then again to the wires.
A new PCB was developed with holes for screws, such that these screws would fix the wires to it, and
it was the one used in the tests with the collimator of section 3.4.
68
D Resistance in parallel to the Oscilloscope
The oscilloscope is schematically considered as a capacitance and a resistance in parallel. In the func-
tionality tests, the output of the switches was connected to the oscilloscope DPO 2014 from Tektronix,
which has an internal resistance of Rosc = 1 MΩ and capacitance Cosc = 15 pF.
In the CMOS technology, an “OFF” SPST switch represents a high-impedance state and can be
considered as an open circuit (in contrast to a grounded wire). When this switch is turned off, the current
flows internally through the oscilloscope as its capacitance is discharged, in analogy to a RC circuit,
governed by the equation
V (t) ∝ e−t
2·π·R·C ,
thus, the potential seen by this device falls exponentially from the high voltage to ground with time
constant τ = Rosc · Cosc = 11.5 µs.
Hence, when a switching signal frequency of 100 kHz is used, the potential at the oscilloscope drops
only to 1− e−10−5
22·π·11.5·10−6 ≈ 7% at the time, when the switch is turned on again (and the potential at the
oscilloscope returns to its high value). Therefore the measurement of the rising time is experimentally
not possible to perform (figure 60).
Figure 60: Signal (violet) without the discharging resistance drops only about 7 % at the time, when the
switch is turned ON again.
A 1 kΩ resistance (figure 61) was connected in parallel at the input of the oscilloscope, in this way
it became the effective resistance of the new schematic RC circuit. In this case the signal drops a factor
e−10−5
22·π·11.5·10−9 ≈ 10−32 of the high voltage value, from the time the switch is turned off until it is turned
on again. Therefore, the measurement of the rise time can be carried out precisely. By doing this, the
amplitude of the new output signal is smaller, but few tests indicated that the difference with respect to
the prior amplitude can be neglected.
69
Figure 61: Resistance in parallel to the Oscilloscope for its faster discharging.
70
E Coupling of Switched Voltages in AC Mode
A CMOS SPDT switch consists of two independently controlled switches with a common output (figure
40a). Each signal defined the state of each switch (ON or OFF). The signals were coupled such that they
were complementary, i.e., one switch was ON while the other was OFF.
An ON switch establishes a low-impedance line from the input to the output, while in the OFF state
the output is floating (high-impedance state). Since the switches were turned on alternately, the output
was always defined to be the voltage of the switch 1 (V1) or switch 2 (V2), which could be controlled by
voltage generators.
We observed that the higher of the two voltages limited the value of the lower one. This issue is
explained in two steps (figure 62):
There is a capacitance at the output of the switch given by the oscilloscope. Without loss of
generality, we may assume V1 > V2. When switch 1 is ON, the capacitance is charged to V1. At the
point where both switches change their states, the capacitance feels a lower potential, therefore it
discharges and part of the current flows from the output channel to the input.
The voltage source does not allow any current flowing in the negative direction (from the negative
(ground) to the positive output). In such a case the source generates automatically a voltage in
order to counteract the negative current.
The result is that the lower voltage was limited by the higher, reducing the voltage amplitude of the
measurements. In order to be able to measure bigger amplitudes, a potentiometer was introduced in the
set-up (see section 7).
This issue will not be present in the final cryogenic set-up since the DC generators will be able to
accept currents in any direction.
71
V1 V2
C2 =OFFC1 =ON
Oscilloscope
V1
(a) t . 0: the capacitance of the oscilloscope is charged to V1.
V1 V2
C2 =ONC1 =OFF
Oscilloscope
V1
I
(b) t = 0: the capacitance of the oscilloscope discharges since
it feels a lower potential V2.
V1 V2
C2 =ONC1 =OFF
Oscilloscope
V2
(c) t & 0: the capacitance of the oscilloscope is charged to V2.
Figure 62: Description of the voltage coupling due to the oscilloscope capacitance.
72
F Precautions working with some Materials
F.1 Calcium
The calcium granules were stored in a closed glass jar. The calcium ingot was closed in a can with a
nitrogen atmosphere.
Contact with calcium produces skin and eyes irritation, as well as respiratory problems when inhaled.
Since we always wore gloves, there was no direct contact the calcium. We avoided the contact of calcium
with water (and in general, with any liquid) due to the risk of generation of hydrogen otherwise.
Some considerations should be taken into account when cutting the calcium (ingot):
When cutting a metal, it can become hot. Do not cool down with water or other possibly reactive
liquid.
We cut it with a saw (the pliers failed, because the ingot was too thick), which became dirty, because
calcium stayed in between the tooth, as well as in the file, when the planar face was filled.
The calcium block was cleaned with acetone and isopropanol.
Abstract of MSDS
Hazard statements [Calcium MSDS, Sciencelab.com]:
Irritating to eyes and skin
Ingestion or inhalation of dust will produce irritation to gastro-intestinal or respiratory tract
Prevention:
Keep under inert atmosphere and dry container
Never add water to this product
Do not breathe dust
Wear suitable protective clothing
Avoid contact with skin and eyes
Keep away from incompatibles such as acid
73
F.2 Beryllium
Beryllium is very poisonous. Thus, we took more safety measures when handling beryllium than by the
construction of the calcium oven. This metal causes serious injuries when it gets in contact with the skin
or the eyes and could be fatal if inhaled for long periods of time. This material also has to be disposed
in a regulated way.
I used gloves and mask with a P3 filter when handling beryllium, since we manipulate and heated it.
The heat gun was never pointing to a person or their clothes for long periods of time when heating the
beryllium wire, since the increase in temperature implies a higher vapour expulsion.
Abstract of MSDS
Hazard statements [MATERIAL SAFETY DATA SHEET, ELECTRONIC SPACE PRODUCTS INTER-
NATIONAL]:
Toxic if swallowed
Causes skin irritation and serious eye irritations
May cause an allergic skin reaction
May cause respiratory irritation, cancer and death if inhaled
Causes damage to organs through prolonged or repeated exposure
Very toxic to aquatic life
Prevention:
Obtain special instructions before use
Wear protective gloves
Wear eye or face protection
Wear respiratory protection
Avoid release to the environment
Do not breathe dust
74
F.3 Stainless Steel
Abstract of MSDS
Hazard statements [MATERIAL SAFETY DATA SHEET, ThyssenKrupp Materuials NA, Inc.]:
Dust or fume (from welding) may cause irritation to the eyes, nose, or throat and may leave a
metallic taste in the mouth
Prevention:
To avoid contact use appropriate protective gloves or clothing to protect against cutting edges
F.4 Acetone
Acetone was used in order to clean organic impurities in the components, since such rests deteriorate the
quality of the vacuum.
Abstract of MSDS
Hazard statements [SICHERHEITSDATENBLATT, MERCK]:
Hazardous in case of skin contact (irritant), of eye contact (irritant), of ingestion, of inhalation
Slightly hazardous in case of skin contact (permeator)
Prevention:
Provide exhaust ventilation or other engineering controls to keep the airborne concentrations of
vapours below their respective threshold limit value
Splash goggles
Lab coat
Vapour respirator
Gloves
F.5 Isopropanol
This was used in order to clean the residual acetone.
75
Abstract of MSDS
Hazard statements [Isopropyl alcohol MSDS, Sciencelab.com]:
Hazardous in case of eye contact (irritant), of ingestion, of inhalation
Slightly hazardous in case of skin contact (irritant, sensitizer, permeator).
Prevention:
Splash goggles
Lab coat
Vapor respirator
Gloves
F.6 Latex
The use of latex over large periods of time could cause allergies to this material.
Abstract of MSDS
Hazard statements [MATERIAL SAFETY DATA SHEET, Aseptico Inc.]:
Certain individuals may have been sensitized and could react to latex products upon coming into
contact with them. Persons with Spina-bifida are likely to react.
Prevention:
None
F.7 Nitrile
Nitrile gloves are easy to wear on when your hands are dry. Acetone in high quantities eats away the
material.
Abstract of MSDS
Hazard statements [MATERIAL SAFETY DATA SHEET, ADENNA INC.]:
None
76
Prevention:
Do not reuse gloves
They are not intended for use as a chemical barrier.
77
G Miscellaneous Pictures
G.1 Tantalum Wires glowing
The tantalum wires of the electric-heated calcium ovens shone due to the high temperature (figure 63).
Figure 63: Tantalum wires glowing orange during the electric heating of the calcium ovens inside the
vacuum chamber.
G.2 Burning Calcium Oven on Collimator
(a) Glass slide full with cal-
cium
(b) O-ring with its interior
obscured
(c) Collimator with copper
residues
(d) Copper fell out from
PCB
(e) Tantalum wire with the
crimp. The gold surface is
gone
(f) Burned calcium oven
Figure 64: Burned parts after the over-heated test of the calcium oven on the collimator.
The test of the calcium oven on the collimator resulted in an excess of outgassing. Accidentally, we
78
did not see after some time any layer of metal on the glass slide when the current flowing through the
oven was below 3 A (figure 64).
Another voltage source with a bigger range was used and the current was increased up to 7 A. The
pressure showed a value in the order of 10−3 mbar when the run was stopped. After opening the vacuum,
all the parts were coated and the calcium oven was burned (figure 64). After that the gauge could not
measure pressures below 10−4 mbar. The device was brought to the workshop, where it was cleaned and
calibrated.
G.3 Calcium Spots under the Light
The calcium spots from the laser-heated calcium ovens showed to be colourful under the reflection of the
lamp light (figure 65).
(a) First run with filled tube (b) Second run with filled tube (c) Tube with one granule
Figure 65: Calcium spots under reflection of lamp light.
G.4 Beryllium Oven glowing
The beryllium oven was observed to glow in both cases, under electric and laser heating.
The laser-heated foil (figure 66a) can be seen in orange, while the thermocouple and the tungsten coils
are dark.
The electric-heated ovens (figure 66b) showed a bright orange-yellow. In this case the tungsten wire
is also glowing because of the high temperature.
G.5 Frozen Moth
A moth of approx. 2 cm length was trapped in the clamping ring of the liquid nitrogen. It was probably
camouflaged when the ring was installed. Pitifully it died because of the low temperature.
79
(a) Laser-heated beryllium (b) Current-heated beryllium oven
Figure 66: Beryllium ovens glowing.
Figure 67: Big moth trapped in the clamping ring of the liquid nitrogen dewar
80
References
[AAC] All About Circuits, Temperature coefficient of resistance [Online] Viewed: 5th Jan. 2012,
http://www.allaboutcircuits.com/vol 1/chpt 12/6.html.
[Alc84] C. B. Alcock, V. P. Itkin and M. K. Horrigan, Vapour pressure equations for the metallic elements:
298 - 2500 K, Canadian Metallurgical Quarterly, Vol. 23, No.3, pp. 309-313 (1984).
[BeMi] Vladimir Vudler and Peter Richard, Beryllium Mirrors, [Online] Viewed: Aug. 31, 2012,
http://www.hardric.com/images/BeMirrors.pdf
[Ben08] J. Benhelm et al., Towards fault-tolerant quantum computing with trapped ions, Nat. Phys. 4, 463
(2008).
[Bla11] R. B. Blakestad et al., Improved high-fidelity transport of trapped-ion qubits through a multi-
dimensional array, arXiv:1106.5005 (2011).
[Bro11] K. R. Brown et al., Single-qubit-gate error below 10−4 in a trapped ion, Phys. Rev. A 84, 030303
(2011).
[Bur10] A. H. Burrell, High Fidelity Readout of Trapped Ion Qubits, PhD Thesis, Exeter College, Oxford,
2010
[Chem] ChemPRIME, Molecular Interpretation of Vapor Pressure, [Online] Submitted: 16/03/2009 -
15:59, From: Moore, Collins and Davies, Chemistry, McGraw-Hill Companies, (1978),
http://wiki.chemprime.chemeddl.org/index.php/
File:Molecular Interpretation of Vapor Pressure .jpg.
http://en.wikipedia.org/wiki/File:Cmos impurity profile.PNG.
[CMOS2] Universitat Hamburg, CMOS gates demonstration, [Online] Viewed: Jan. 20, 2012,
http://tams-www.informatik.uni-hamburg.de/applets/cmos/
[CMOSanalog] DG411/DG412/DG413 Data sheet, Maxim, 19-4728; Rev 7; 9/08
[Cre08] H. Crew, General physics: an elementary text-book for colleges, The University of Michigan: The
Macmillan Company (1908).
[Cre09] Y. Creten et al., An 8-bit Flash Analog-to-Digital Converter in Standard CMOS Technology Func-
tional From 4.2 K to 300 K, IEEE Journal of Solid-State Circuits 44 (7), 2019 (2009).
81
[Dan] P. Danielson, A Journal of Practical and Useful Vacuum Technology, The Vacuum Lab.
[Dou54] P. E. Douglas, The vapour pressure of calcium: I, Proc. Phys. Soc. B 67 783 (1954).
[Gul01] S. Gulde et al., Simple and efficient photo-ionization loading of ions for precision ion-trapping
experiments, Appl. Phys. B 73, 861-863 (2001).
[Hub08] G. Huber et al., Transport of ions in a segmented linear Paul trap in printed-circuit-board tech-
nology, New J. Phys. 10, 013004 (2008).
[ILPI] Interactive Learning Paradigms Incorporated, The MSDS HyperGlossary: Vapor pressure [Online]
Updated: 9th May 2010,
http://www.ilpi.com/msds/ref/vaporpressure.html.
[Kim10] Hyungtak Kim et al., DC Characteristics of Wide-bandgap Semiconductor Field-effect Transis-
tors at Cryogenic Temperatures, J. Korean Phys. Soc. 56 (5), 1523 (2010).
[Kin99] B. E. King, Quantum State Engineering and Information Processing with Trapped Ions, Ph.D.,
University of Colorado at Boulder, 278 pages (1999).
[Lan13] I. Langmuir, The vapour pressure of metallic tungsten, The Physical Review, Vol. II., No. 5
(1913).
[Luc11] D. Lucas et al., Isotope-selective photoionization for calcium ion trapping, Phys. Rev. A 69,
012711 (2011).
[Maj05] F. G. Major et al., Charged Particle Traps, Springer (2005).
[MOLAR] Lenntech, Chemical elements listed by atomic mass, Viewed: Jan. 30, 2012,
http://www.lenntech.com/periodic/mass/atomic-mass.htm
[Ock09] B. Ockan et al., A cryogenic analog to digital converter operating from 300 K down to 4.4 K,
Rev. Sci. Instrum. 81, 024702 (2010).
[Pro90] A. Prokhorov, Laser heating of metals, A. Hilger (1990).
[Ric84] M. Richards et al., Cryogenic GaAs FET amplifiers and their use in NMR detection, Rev. Sci.
Instrum. 57 (3), 404 (1984).
[Sal91] Bahaa E. A. Saleh, Malvin Carl Teich, Fundamentals of Photonics, John Wiley & Sons, Inc.
(1991).
82
[Sch12] J. Schleeh et al., Ultra-low power cryogenic InP HEMT with minimum noise temperature of 1 K
at 6 GHz, IEEE Electron Device Lett. 33 (5), 664 (2012).
[Ser09] A. Serafini, A. Retzker, M. B. Plenio, Generation of continuous variable squeezing and entangle-
ment of trapped ions in time-varying potentials, arXiv:0904.4258 (2009).
[SPSw] TEDSS Glossary of Switch Parameters, [Online] Viewed: Jan. 20, 2012,
http://www.tedss.com/Switches/#Switch Circuit
[Sta98] S. Stahl, Aufbau eines Experimentes zur Bestimmung elektronischer g-Faktoren einzelner wasser-
stoffahnlicher Ionen, Dissertation, Johannes Gutenberg-Universitat in Mainz (1998).
[TMCP] Pico Technology, Thermocouple Application Note, [Online] Viewed: 19/01/2012,
http://www.picotech.com/applications/thermocouple.html
[Tur99] Q. A. Turchette et al., Heating of trapped ions from the quantum ground state, Phys. Rev. A 61,
063418 (1999).
[Wal12] A. Walther et al., Controlling fast transport of cold trapped ions, arXiv:1206.0364 (2012).
[Wil11] D. Wild, Evaluation of Semiconductor Switches, Summer-student project, TIQI group at the
ETH (2011).
83
