Microwave Spectroscopy of the radio-frequency 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. 10 3 1 Tc (K) B (T) 2 0.1 1 0.3 0.1 0.01 d-1 (nm-1) 1 10 100 1000 An alternative approach to O2 doping J. Aumentado et al., PRL 92, 066802 (2004) 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

5. 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

6. h Single Cooper pair transistor EJ,C2 EJ,C1 In a 2-band model Cg EC=e2/(C1+C2+Cg) EJ/EC=0.5

7. G2 2D2 2D2 2D1 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/G2~exp(D2-D1/kT) The device itself becomes a qp filter J. Aumentado et al., Phys. Rev. Lett, 92, 066802 (2004)

8. 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)

9. Device I: Parameters R = 18 kW EJ = 43 meV Ec = 77 meV EJ/EC = 0.56 Imin Imax rf-SCPT Irf<Isw: R~0 W Irf>Isw: R>0 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

10. Imin Imax B=0T Diamonds 2e supercurrent enabled by thin-island Device II: Parameters 0 Ec=180 meV RS=71 kW EJ=11 meV EJ/EC=0.06 Mixer out (a.u.) 1 2e ‘supercurrent’ DJQP JQP 2D1 + 2D2 = 1.05 meV

11. V 2 3 4 1 2 3 A B @ A @ B Resonant Dissipative Dissipative Resonant Resonant Resonant 2 V 0 2 2 V 0 0 Resonant CP tunnelling E(n+2)-E(n)=0 E(n+2)-(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)

12. -19 dBm -25 dBm 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)

13. 2 2 0 0 2 2 0 0 2 2 2 0 0 0 PAT + resonant CPT 2g 1g 0g P. K. Tien and J. P. Gordon, Phys Rev. 129, 647 (1963)

14. Frequency dependence Linear dependence of sidebands observed. Anti-crossing not observable since Ej=11meV (2.6 GHz) 1g: 186 meV 2g: 193 meV c.f. 180 meV from transport

15. 30 GHz 1 0 2 Power dependence EC=180 meV, D=300 meV, EJ=11 meV Multiple g events occur Possibly QP states excited too J. M. Hergenrother et al., Physica B 203, 327 (1994)

16. 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

17. 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)