Microwave Spectroscopy of the radiofrequency Cooper Pair Transistor

1
Microwave Spectroscopy of the radio-frequency
Cooper Pair Transistor

• A. J. Ferguson, N. A. Court R. G. Clark

Centre for Quantum Computer Technology,
University of New South Wales, Sydney
2
Summary

• Engineering the properties of superconducting
  aluminium
• The single cooper pair transistor (SCPT)
• Radio frequency operation of the SCPT
• The superconducting transport processes
• Microwave spectroscopy

3
Aluminium Devices
Superconducting Qubits
I. Chiorescu et al Science 299 1869 (2002)
Y. Nakamura et al Nature 398 786 (1999)
Single electron (Cooper-pair) transistors
4
Aluminium Materials Science
Thin films dramatic change in superconducting
properties
Bc
Tc, D
R. Meservey and P. M. Tedrow J. Appl. Phys. 42,
51 (1971)
d (nm)
Pauli-limited Bc spin effects in superconducting
SETs. A. J. Ferguson et al. on cond-mat soon
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The thin-film SCPT
7 nm islands used for these devices
1K of quasiparticle barrier
7 nm
D 300 mV
D 200 mV
D 200 mV
30 nm
30 nm
30nm
30 nm
7 nm
Films evaporated onto LN2 cooled stage at 0.1
nms-1 Electrically continuous films to 5 nm
possible
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Single Cooper pair transistor
EJ,C1
EJ,C2
In a 2-band model
Cg
ECe2/(C1C2Cg)
EJ/EC0.5
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Why do it? QP poisoning
Careful filtering required to avoid
non-equilibrium qps These qps tunnel on and
poison supercurrent
G1
2D
A QP barrier reduces poisoning rate
G1/G2exp(D2-D1/kT)
The device itself becomes a qp filter
J. Aumentado et al., Phys. Rev. Lett, 92, 066802
(2004)
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rf-SET
Main idea LC circuit matches high resistance of
SET towards 50 Ohms.
rf (321MHz)
Amplitude of reflected signal (S11), related to
resistance (R) of SET.
Reflected signal either diode or mixer detected.
R. J. Schoelkopf et al., Science 280 1238 (1998)
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rf-SCPT
IrfltIsw R0 W IrfgtIsw Rgt0 W
Resistance is now Reff(Irf, Isw), use to find
reflection coefficient in the usual way.
Single shot QP poisoning events
J. Aumentado et al., cond-mat\0511026
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B0T Diamonds
2e supercurrent enabled by thin-island
Device II Parameters
0
Ec180 meV RS71 kW EJ11 meV EJ/EC0.06
Mixer out (a.u.)
1
2e supercurrent
DJQP
JQP
2D1 2D2 1.05 meV
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Resonant CP tunnelling
E(n2)-E(n)0
E(n2)-(E(n)-2eV)0
DJQP resonance QPs involved
Supercurrent occurs when resonance occurs for a
CP on both junctions.
D. B. Haviland et al., PRL 73, 1541 (1994)
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Microwave Spectroscopy
40GHz
No m-waves
Suppression of supercurrent Frequency dependent
sidebands on supercurrent Frequency dependent
sidebands on resonant CPT
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)
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PAT resonant CPT
0g
1g
2g
P. K. Tien and J. P. Gordon, Phys Rev. 129, 647
(1963)
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Frequency dependence
Linear dependence of sidebands observed. Anti-cro
ssing not observable since Ej11meV (2.6 GHz)
1g 186 meV
2g 193 meV c.f. 180 meV from transport
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Power dependence
EC180 meV, D300 meV, EJ11 meV
Multiple g events occur Possibly QP states
excited too
J. M. Hergenrother et al., Physica B 203, 327
(1994)
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Conclusions

• 100 meV 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

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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)

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Switching current measurement
Device I Parameters R 18 kW EJ 43 meV Ec
77 meV EJ/EC 0.56
2e-periodic Isw