We demonstrate simultaneous measurements of DC transport properties and flux noise of a hybrid superconducting magnetometer based on the proximity effect (superconducting quantum interference proximity transistor, SQUIPT). The noise is probed by a cr
yogenic amplifier operating in the frequency range of a few MHz. In our non-optimized device, we achieve minimum flux noise $sim 4;muPhi_0/Hz^{1/2}$, set by the shot noise of the probe tunnel junction. The flux noise performance can be improved by further optimization of the SQUIPT parameters, primarily minimization of the proximity junction length and cross section. Furthermore, the experiment demonstrates that the setup can be used to investigate shot noise in other nonlinear devices with high impedance. This technique opens the opportunity to measure sensitive magnetometers including SQUIPT devices with very low dissipation.
We present a quantum heat switch based on coupled superconducting qubits, connected to two $LC$ resonators that are terminated by resistors providing two heat baths. To describe the system we use a standard second order master equation with respect t
o coupling to the baths. We find that this system can act as an efficient heat switch controlled by the applied magnetic flux. The flux influences the energy level separations of the system, and under some conditions, the finite coupling of the qubits enhances the transmitted power between the two baths, by an order of magnitude under realistic conditions. At the same time, the bandwidth at maximum power of the switch formed of the coupled qubits is narrowed.
We analyse a quantum Otto refrigerator based on a superconducting qubit coupled to two LC-resonators each including a resistor acting as a reservoir. We find various operation regimes: nearly adiabatic (low driving frequency), ideal Otto cycle (inter
mediate frequency), and non-adiabatic coherent regime (high frequency). In the nearly adiabatic regime, the cooling power is quadratic in frequency, and we find substantially enhanced coefficient of performance $epsilon$, as compared to that of an ideal Otto cycle. Quantum coherent effects lead invariably to decrease in both cooling power and $epsilon$ as compared to purely classical dynamics. In the non-adiabatic regime we observe strong coherent oscillations of the cooling power as a function of frequency. We investigate various driving waveforms: compared to the standard sinusoidal drive, truncated trapezoidal drive with optimized rise and dwell times yields higher cooling power and efficiency.
We have measured the electronic heat capacity of thin film nanowires of copper and silver at temperatures 0.1 - 0.3 K; the films were deposited by standard electron-beam evaporation. The specific heat of the Ag films of sub-100 nm thickness agrees wi
th the bulk value and the free-electron estimate, whereas that of similar Cu films exceeds the corresponding reference values by one order of magnitude. The origin of the anomalously high heat capacity of copper films remains unknown for the moment. Based on the low heat capacity and the possibility to devise a tunnel probe thermometer on it, the Ag films form a promising absorber material, e.g., for micro-wave photon calorimetry.
We discuss a qubit weakly coupled to a finite-size heat bath (calorimeter) from the point of view of quantum thermodynamics. The energy deposited to this environment together with the state of the qubit provides a basis to analyze the heat and work s
tatistics of this closed combined system. We present results on two representative models, where the bath is composed of two-level systems or harmonic oscillators, respectively. Finally, we derive results for an open quantum system composed of the above qubit plus finite-size bath, but now the latter is coupled to a practically infinite bath of the same nature of oscillators or two-level systems.
We propose and analyze Maxwells demon based on a single qubit with avoided level crossing. Its operation cycle consists of adiabatic drive to the point of minimum energy separation, measurement of the qubit state, and conditional feedback. We show th
at the heat extracted from the bath at temperature $T$ can ideally approach the Landauer limit of $k_BTln 2$ per cycle even in the quantum regime. Practical demon efficiency is limited by the interplay of Landau-Zener transitions and coupling to the bath. We suggest that an experimental demonstration of the demon is fully feasible using one of the standard superconducting qubits.
We analyze the stochastic evolution and dephasing of a qubit within the quantum jump (QJ) approach. It allows one to treat individual realizations of inelastic processes, and in this way it provides solutions, for instance, to problems in quantum the
rmodynamics and distributions in statistical mechanics. As a solvable example, we study a qubit in the weak dissipation limit, and demonstrate that dephasing and relaxation render the Jarzynski and Crooks fluctuation relations (FRs) of non-equilibrium thermodynamics intact. On the contrary, the standard two-measurement protocol, taking into account only the fluctuations of the internal energy $U$, leads to deviations in FRs under the same conditions. We relate the average $langle e^{-beta U} rangle $ (where $beta$ is the inverse temperature) with the qubits relaxation and dephasing rates, and discuss this relationship for different mechanisms of decoherence.
We demonstrate experimentally that disorder enhanced Andreev current in a tunnel junction between a normal metal and a superconductor provides a method to measure electronic temperature, specifically at temperatures below 200 mK when aluminium is use
d. This Andreev thermometer has some advantages over conventional quasiparticle thermometers: for instance, it does not conduct heat and its reading does not saturate until at lower temperatures. Another merit is that the responsivity is constant over a wide temperature range.
We present an experimental realization of a Coulomb blockade refrigerator (CBR) based on a single - electron transistor (SET). In the present structure, the SET island is interrupted by a superconducting inclusion to permit charge transport while pre
venting heat flow. At certain values of the bias and gate voltages, the current through the SET cools one of the junctions. The measurements follow theoretical model down to about 80 mK, which was the base temperature of the current measurements. The observed cooling increases rapidly with decreasing temperature in agreement with the theory, reaching about 15 mK drop at the base temperature. CBR appears as a promising electronic cooler at temperatures well below 100 mK.
We consider a voltage-biased Normal metal-Insulator-Superconductor (NIS) tunnel junction, connected to a high-temperature external electromagnetic environment. This model system features the commonly observed subgap leakage current in NIS junctions t
hrough photon-assisted tunneling which is detrimental for applications. We first consider a NIS junction directly coupled to the environment and analyze the subgap leakage current both analytically and numerically; we discuss the link with the phenomenological Dynes parameter. Then we focus on a circuit where a low-temperature lossy transmission line is inserted between the NIS junction and the environment. We show that the subgap leakage current is exponentially suppressed as the length, $ell$, and the resistance per unit length, $R_0$, of the line are increased. We finally discuss our results in view of the performance of NIS junctions in applications.
