Miniaturisation of Quantum Technology:Atomic clocks, grating magneto-optical

traps and magnetometry

Erling Riis

University of Strathclyde, Glasgow, UKExperimental Quantum Optics and Photonics Group

http://photonics.phys.strath.ac.uk/

Outline

• (Motivation not required)

• UK landscape

• Grating MOT

• Coherent population trapping for atomic clocks

• Optically pumped atomic magnetometers FPGA

Control

UK Quantum Technology landscape

£350M investment:• 4 QT Hubs - 20 Universities and 170

companies• PhD training • Research Fellowships • Innovate UK joint academic/industry

projects• ~40% to Strathclyde collaborators

£36M MoD programme in QT

£338M of investment proposed for Phase 2 of UK-NQTP.

• Four funded hubs led by:• Birmingham

• Quantum sensors and metrology

• Glasgow• Quantum imaging

• York• Quantum communication

• Oxford• Quantum networks

• NPL closely involved in most

• University of Strathclyde only university taking part in all four

• Led by University of Birmingham

• ~£10M Strathclyde involvement

• Aim: advance cold atom technology for sensors – cf. 20th century electronics revolution

QT Hub for Sensors and Metrology

Our approach to portable atomic clock

• Drop cold atoms• Shielded from environment

• Only weak interatomic interactions

Accuracy

• Favourable scaling for compact clocks:

• Halve interaction time:

Size of trajectory down by factor of four

Linewidth doubles

Data rate doubles } loose 𝟐

Compact measurement device?

• Vision: • Make small (cm-scale) vacuum system with:

• Integrated optics for MOT beams

• Integrated pumping

• Properties:• Large optical access

• Simple single beam alignment

• Micro-fabricated

• Good atom number (~108)

• Low temperature (<10 µK)

©NPL

Opt. Express 17, 13601 (2009)

Mirrors

Opt. Lett. 35, 3453 (2010)

Gratings

Nature Nanotech, 8(5): 323-324,2013

GMOT

Grating chip for cold atom experiments

(3) CSAC performance 3x10-11@100s

(1) Sr lattice 1.6x10-18@5000s

Vo

lum

e

1/(relative instability)

Region of interest

Performance(Allan Deviation)

87Rb88Sr

(2) Atomic fountain

4x10-16 @ 10000s

f0 = 430 THz

Δfa = 1 mHz

Q=4x1017

f0 = 6.8 GHz

Δfa = 10 Hz

Q=6.8x108

N=107

Tc=100 ms

σy (100)=2x10-14(2) T. Heavner et. al. Metrologia 51, 174 (2014)

(3) S. Knappe et. al., Applied Phys. Lett. 85, 1460-1462 (2004)

(1) N Hinkley et. al. Science, 341, 1215-1218 (2013)

Compact measurement device?

Precision measurement – clockLocal Oscillator Counter /

Output

Feedback

Atoms

A microwave clockLocal Oscillator

Feedback

Atoms

The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of caesium 133 atoms (1967)

ȁ1

ȁ2

𝜔0 =𝐸2 − 𝐸1

ℏ

Optical excitation: the basic CPT model

𝜔0 𝛿𝜔

Coherent population trapping: Characteristic reduction in absorption when laser frequency difference δωmatches atomic transition ω0

On resonance: Destructive interference of quantum mechanical amplitudes for light absorption

𝜓 =1

2ȁ1 − ȁ2

-1 0 1

-1 0 1

• Optical pumping poor signal• High contrast schemes:

Lin LinLin || Linσ+/σ—

Push-Pull Optical PumpingPolarization Modulation

The basic CPT model

𝜔0 𝛿𝜔

CPT implementation (87Rb D1)

Raman-Ramsey Technique (Ramsey with a bit of a twist)

Pulse 1 – drives atoms

into a ‘dark’ state:

𝜓 𝑡 =1

2ȁ1 − ȁ2

Pulse 2 – Maps

phase difference

into absorption

1

2ȁ1 − ȁ2

1

2ȁ1 + ȁ2

𝜓 𝑡 =1

2ȁ1 − 𝑒−𝑖 ∆𝐸21𝑡/ℏ ȁ2

Τ∆𝐸21 ℎ =6,834,682,610.904 290 90 𝐻𝑧

During T – evolution:

Ramsey interrogation

∆𝐸21

T

Linewidth: 1/T

Recapture efficiency vs T Atom number vs T and TcModel for equilibrium atom number with recapture from Rakholia et al.

A. V. Rakholia, et al. Phys. Rev. Appl., 2014

𝑁𝑒𝑞 =𝑇𝐿𝑇𝑐

𝑁∞

1 +1 − 𝑟𝛾𝑇𝑐

Atom recapture

• Basic idea: for sufficiently short expansion time atoms remain in the overlap volume and are recaptured readily

Stability optimisation

• Trade-off between drop time and data rate• Increased drop time

Narrower line

Fewer atoms recaptured

Slower data rate

• Estimated realistic limit:

Allan deviation

Assuming, atom shot noise limited detection, TR=10 ms, Tc=100 ms, Nat=107

Clock stability just now

• Currently ~ Τ10−11 𝜏

• Limitations:• Magnetic noise

• Light shifts

• Temperature drifts

• Laser noise

• Doppler shifts

• ….

• Experimental re-build under way

Atomic magnetometry

UnshieldedShielded

• Alkali vapour cells• Optimised geometry & fill• Manufacturable

• Low-noise electronics• VCSEL driver• Polarimeter

• Firmware demodulation• FPGA platform• Additional functionality

Unshielded sensor• 200 µT dynamic range• 1 kHz bandwidth• ~1 pT.Hz-1/2 sensitivity

• 20 ppb @ 50 µT

The Physics

Step 1 Step 2 Step 3

Optical pump

Create an atomic dipole

Monitor precession Initial unpolarised atomic sample

Oscillation frequency in Earth magnetic field ~175 kHz

The journey out of the lab

0.001

0.01

0.1

1

0.1 1 10 100 1000

NEP

(n

T.H

z-1/2

)

Frequency (Hz)

28 mm

On the banks of Loch Lomond

FPGA

Analogue

MEMS vapour cell development• MEMS vapour cell

• Si etched via KOH or DRIE

• Anodically bonded glass/Si/glass

• Cavity contains alkali metal and buffer gas

• Alternative geometries investigated

• Optimisation path for Strathclyde Magnetometry

• Scalable manufacture

• Large and clear optical path

• Controlled/process tuneable buffer gas pressure

• Optional on-board rf/bias coils, heater, cold spot control structures

• Technology development towards

miniaturised magnetometers and

cold atom clocks

– CPT (Raman-Ramsay)

– Microfabricated gratings

• still a lot of work to do with lasers and

vacuum!

– MEMS cell production

– Field-tested magnetometers

Conclusions

Collaborators & Sponsors:

Aidan ArnoldPaul Griffin ER

Jeremy WardGreg HothRachel ElvinMichael WrightBen LewisCarolyn O’DwyerIain Chalmers

Dominic HunterMatt VangeleynChidi NshiiOliver BurrowJames McGilliganStuart InglebyTerry Dyer

Leo Hollberg

http://photonics.phys.strath.ac.uk/

The Team