Optimized Proximity Thermometer for Ultrasensitive Detection

Abstract

We present a set of experiments to optimize the performance of a noninvasive thermometer based on proximity superconductivity. Current through a standard tunnel junction between an aluminum superconductor and a copper electrode is controlled by the strength of the proximity induced to this normal metal, which in turn is determined by the position of a direct superconducting contact from the tunnel junction. Several devices with different distances are tested. We develop a theoretical model based on Usadel equations and dynamic Coulomb blockade that reproduces the measured results and yields a tool to calibrate the thermometer and to optimize it further in future experiments. We also propose an analytic formula that reproduces the experimental data for a wide range of temperatures.

Quasi-probabilities of work and heat in an open quantum system

“…The second suggests to measure the energy quanta emitted by the system [15]. The main limitation in this case is that it works only for very specific systems [16][17][18][19][20][21] where only a channel is involved in the dissipative process. On the contrary, the present approach allows us to determine the dissipated heat by only manipulating the system-detector coupling leading to a protocol that does not depend on the physical platform used as a quantum system.…”

“…An alternative proposal suggests to measure the quanta emitted by the system [15]. Although closer to realistic implementation [15][16][17][18][19][20][21], these methods are indissolubly tied to a specific physical implementation. On the contrary, being based on the system degrees of freedom, the proposed approach can be implemented in any physical quantum system.…”

Calorimetry of a phase slip in a Josephson junction

“…As visible in Fig. 2b,c and already discussed in detail in [24][25][26], the zero-bias conductance of the tunnel junction, and therefore P out (V b = 0), is a sensitive probe of the electron temperature T in N. Accordingly, we set V b = 0 in the remainder of this work and use P out for both static and dynamic electron thermometry, after initial calibration under equilibrium conditions (Fig. 2c).…”

Note on possibility of proximity induced spontaneous currents in superconductor/normal metal heterostructures

“…In particular, the depth of Cooper pairs penetration into the N in the diffusive case is equal to ξ N ∼ = D N /2πT where D N = vl/3 is the diffusion coefficient, and becomes smaller if the condensate moves. The proximity effect is utilized in various S/N/S Josephson junctions [6][7][8] and other superconducting devices [9][10][11][12] as it leads to a number of interesting physical phenomena. The most famous of these is the advantageous Josephson coupling in S/N/S Josephson junctions with the N layer being significantly thicker (up to a few microns) than the insulating (I) barrier in tunnel Josephson junctions [6][7][8].…”