The document summarizes research on using proximity-induced superconducting correlations in mesoscopic conductors to implement quantum detectors. It describes how superconductivity can modify the density of states in normal metals through the proximity effect. It then introduces the Superconducting QUantum Interference Proximity Transistor (SQUIPT), a novel quantum interferometer that uses this effect for high-sensitivity magnetic flux detection, and presents theoretical predictions and experimental results demonstrating its behavior and advantages over DC SQUIDs.

1. Universita’ di Perugia 15 Aprile 2010 Ruolo delle correlazioni superconduttive in conduttori mesoscopici: utilizzo per l’implementazione di rilevatori quantistici Francesco Giazotto NEST Istituto Nanoscienze-CNR & Scuola Normale Superiore Pisa, Italia

2. Collaboration J. T. Peltonen M. Meschke J. P. Pekola Low Temperature Laboratory, Helsinki University of Technology, 02015TKK, Finland

4. Andreev reflection in SN contacts BdG equations Andreev reflection BTK, PRB 25 , 4515 (1982)

5. Proximity effect and supercurrent S S N Metallic contact between a normal metal and a superconductor S S N Electron-hole correlations: proximity effect Supercurrent Andreev bound states (ABS) Reflected hole Incident electron Superconductor Normal metal (Semiconductor) Cooper pair Andreev reflection

6. Proximity effect in SNS systems: basic formalism LDOS properties: N (- E ) = N ( E ) E g for | E |  E g E g (  = 0)  3.2 E Th for  >> E Th E g (  =  ) = 0 Diffusive mesoscopic N wire: quasi-1D geometry L  >L >> l e D = diffusion coefficient  = superconducting order parameter  = macroscopic phase of the order parameter E Th =  D/L 2 Thouless energy Usadel equations LDOS

7. Modification of the LDOS in SNS systems due to proximity effect J. C. Hammer et al ., PRB 76 , 064514 (2007) Phase dependence J. C. Cuevas et al ., PRB 73 , 184505 (2006) Length and position dependence

8. Spatial spectroscopy of PE probed with tunnel junctions Al/Cu SN structure with tunnel probes

9. Phase-dependence of PE probed with STM spectroscopy Al/Ag SNS proximity SQUIDs

10. Phase-dependence of PE probed with STM spectroscopy Experiment to theory comparison H. le Sueur et al ., PRL 100 , 197002 (2008) Phase-evolution of PE Full phase-control of the minigap amplitude

11. I)  -tuning of specific heat: quantum control of a thermodynamic variable H. Rabani, F. Taddei, F. G. and R. Fazio, JAP 105 , 093904 (2009); H. Rabani, F. Taddei, R. Fazio, and F. G., PRB 78, 012503 (2008) Electron entropy Electron specific heat

12. II)  -tuning of e-ph interaction: quantum control of relaxation T. T. Heikkila and F. G., PRB 79 , 094514 (2009)

13. Sensitivity through proximity

14. SQUIPT: a novel quantum interferometer Active manipulation of the DOS of a proximity N metal Phase control (through magnetic flux) Detection (through tunnel junctions) High sensitivity for flux detection SQUIPT

15. SQUIPT: fabrication details and configurations Shadow-mask evaporation 27 nm Al @ 25  Oxidation 4.4 mbar 5’ (tunnel junctions) 27 nm Cu @ -25  60 nm Al @ 60  (clean SN interfaces) Fabrication details Geometry and materials details L  1.5  m Probe width  200 nm N wire width  240 nm SN overlapping  250 nm R t  50-70 k  L G  40 pH I J  3  A  = 200  eV

16. SQUIPT (theo): prediction of its behavior in the current-bias mode A-type configuration Usadel equations quasiparticle current

19. A-type SQUIPT (exp): current-voltage characteristic vs  R t = 50 k  T = 68 mK Coherent modulation of the N DOS R t = 50 k  T = 53 mK Theory

20. A-type SQUIPT (exp): Josephson coupling in the proximity metal R t = 50 k  T = 68 mK I J  17 pA R t = 50 k  T = 53 mK  0  0.17 Oe A  120  m 2

22. A-type SQUIPT (exp): transfer function R t = 50 k  T = 54 mK  V /   30  V/  0 @ 1 nA theory

23. B-type SQUIPT (exp): voltage modulation vs  and transfer function R t = 70 k  T = 53 mK  V  12  V @ 1 nA  V /   60  V/  0 @ 0.6 nA R t = 70 k  T = 53 mK doubled response in B-type SQUIPT

24. A-type SQUIPT (exp): temperature dependence R t = 50 k  I = 1 nA R t = 50 k  I = 1 nA change of concavity between 376 mK and 411 mK

25. SQUIPT: dissipation and flux sensitivity Power dissipation P diss = VI  100 fW increasing the probing junction resistance lowered DC SQUIDS 4-5 orders of magnitude smaller in the SQUIPT Ultralow dissipation cryogenic applications Flux sensitivity NEF = <V 2 N > 1/2 /|  V/  |  1/2 N Pre  1.2 nV/Hz 1/2 NEF  2  10 -5  0 /Hz 1/2 NEF  4  10 -7  0 /Hz 1/2 with Nb (  1.5 meV) and L = 150 nm

27. SQUIPT: future perspectives Short junction limit (  << E Th ) Al and L = 150 nm (i) (ii) V SNS junction SQUIPT C. Pascual Garcia and F. G., APL 94 , 132508 (2009) (iii) Noise? Both theory and experiment