Microwave Spectroscopy of the radio-frequency Cooper Pair

TransistorA. J. Ferguson, N. A. Court & R. G.

Clark

Centre for Quantum Computer Technology, University of New South Wales, Sydney

Summary

1. Engineering the properties of superconducting aluminium

2. The single cooper pair transistor (SCPT)3. Radio frequency operation of the SCPT4. The superconducting transport processes5. Microwave spectroscopy

Aluminium Devices

Y. Nakamura et al Nature 398 786 (1999) I. Chiorescu et al Science 299 1869 (2002)

Superconducting Qubits

Single electron (Cooper-pair) transistors

Aluminium Materials Science

Thin films: dramatic change in superconducting properties

R. Meservey and P. M. Tedrow J. Appl. Phys. 42, 51 (1971)

J. Aumentado et al., PRL 92, 066802 (2004)

An alternative approach to O2 doping

Tc, Bc

d (nm)10 1001 1000

1

0.1

10

B (

T)

d-1 (nm-1)0.30.10.01

2

3

1

Tc

(K)

Pauli-limited Bc: spin effects in superconducting SETs.A. J. Ferguson et al. on cond-mat soon

The thin-film SCPT

7 nm30 nm 30nm

~ 200 V ~ 300 V ~ 200 V

~1K of quasiparticle barrier

Films evaporated onto LN2 cooled stage at 0.1 nms-1

Electrically continuous films to 5 nm possible

7 nm

30 nm 30 nm

7 nm islands used for these devices

Single Cooper pair transistor

2

2

)2(2

cos

2cos)(

gCJ

JgC

nEE

EnEH

In a 2-band model

EJ/EC=0.5

EJ,C1 EJ,C2

Cg

EC=e2/(C1+C2+Cg)

Q

mmsw d

dEeQI

max

2)(, h

Why do it? QP poisoning

Careful filtering required to avoid non-equilibrium qpsThese qps tunnel on and ‘poison’ supercurrent

2

1/2~exp(2-1/kT)A QP barrier reduces poisoning rate

1

The device itself becomes a qp filter

2122

2

22

J. Aumentado et al., Phys. Rev. Lett, 92, 066802 (2004)

rf-SET

1

1

RCj

LjZ

Main idea: LC circuit matches high resistance of SET towards 50 Ohms.

50/

50/

RCL

RCLa

Amplitude of reflected signal (S11), related to resistance (R) of SET.

R. J. Schoelkopf et al., Science 280 1238 (1998)

rf (321MHz)

Reflected signal either diode or mixer detected.

rf-SCPT

Lj

V~I rf

rf

Irf<Isw: R~0

Irf>Isw: R>0

Single shot: QP poisoning events

J. Aumentado et al., cond-mat\0511026

Resistance is now Reff(Irf, Isw), use to find reflection coefficient in the usual way.

Device I: Parameters R = 18 k EJ = 43 eVEc = 77 eVEJ/EC = 0.56

Ima

x

Imin

B=0T Diamonds

Ec=180 eVR=71 kEJ=11 eVEJ/EC=0.06

2e supercurrent enabled by thin-island

21 + 22 = 1.05 meV

2e ‘supercurrent’

JQP DJQP

Mix

er

out

(a.

u.)

Device II: Parameters

0

1

Ima

x

Imin

Resonant CP tunnelling

D. B. Haviland et al., PRL 73, 1541 (1994)

V

E(n+2)-E(n)=0E(n+2)-(E(n)-2eV)=0

Supercurrent occurs when resonance occurs for a CP on both junctions.

ResonantDissipative

V 02

@ A21 3

B

2 3 4

A

Vds)C(CgVgC

ne

2C

(ne)E(n) 1

ΣΣ

2

DJQP resonance: QPs involved

Resonant Dissipative

V02

@ B

Resonant Resonant

02

Microwave Spectroscopy

D. J. Flees et al., Phys. Rev. Lett., 78, 4817 (1997)Y. Nakamura et al., Czech. J. Phys., 46, 2301 (1996) Y. Nakamura et al., Phys. Rev. Lett., 12, 799 (1997)

40GHz

-25 dBm -19 dBm

Suppression of supercurrent

Frequency dependent sidebands on supercurrent

Frequency dependent sidebands on resonant CPT

No -waves

PAT + resonant CPT

0

2

012

N

irf

Ni NhfENhf

eVJE )(

2)( 12

0

2

0

2

0

2

0

2

0

2

P. K. Tien and J. P. Gordon, Phys Rev. 129, 647 (1963)

0

2

Frequency dependence

Linear dependence of sidebands observed.

Anti-crossing not observable since Ej=11eV (2.6 GHz)

22 )4/()(4 EcEjCgVgEchf

1: 186 eV

2: 193 eVc.f. 180 eV from transport

Power dependence30 GHz

0 1 2

EC=180 eV, =300 eV, EJ=11 eV

Multiple events occur

Possibly QP states excited too

J. M. Hergenrother et al., Physica B 203, 327 (1994)

Conclusions

• ~100 eV of QP barrier possible with thin film

• Reduced QP poisoning allows 2e-periodicity

• rf-measurement of 2e supercurrent shown • Observe individual QP poisoning events• Combination of PAT and CP resonant

tunneling observed

Future

• Experimental: investigate charge noise of thin film

• Experimental: further study individual QP poisoning events

• Theoretical: look at rf-supercurrent measurement as electrometer (ultimate sensitivity etc)