Olli-Pentti Saira

Heat transistor: Demonstration of gate-controlled electronic refrigeration

We present experiments on a superconductor-normal-metal electron refrigerator in a regime where single-electron charging effects are significant. The system functions as a heat transistor; i.e., the heat flux out from the normal-metal island can be controlled with a gate voltage. A theoretical model developed within the framework of single-electron tunneling provides a full quantitative agreement with the experiment. This work serves as the first experimental observation of Coulombic control of heat transfer and, in particular, of refrigeration in a mesoscopic system.

Test of the Jarzynski and Crooks fluctuation relations in an electronic system.

Recent progress on micro- and nanometer-scale manipulation has opened the possibility to probe systems small enough that thermal fluctuations of energy and coordinate variables can be significant compared with their mean behavior. We present an experimental study of nonequilibrium thermodynamics in a classical two-state system, namely, a metallic single-electron box. We have measured with high statistical accuracy the distribution of dissipated energy as single electrons are transferred between the box electrodes. The obtained distributions obey Jarzynski and Crooks fluctuation relations. A comprehensive microscopic theory exists for the system, enabling the experimental distributions to be reproduced without fitting parameters.

Fast Electron Thermometry for Ultrasensitive Calorimetric Detection

We demonstrate radio-frequency thermometry on a micrometer-sized metallic island below 100 mK. Our device is based on a normal-metal-insulator-superconductor tunnel junction coupled to a resonator with transmission readout. In the first generation of the device, we achieve 90 mu K/root Hz noise-equivalent temperature with 10 MHz bandwidth. We measure the thermal relaxation time of the electron gas in the island, which we find to be of the order of 100 mu s. Such a calorimetric detector, upon optimization, can be seamlessly integrated into superconducting circuits, with immediate applications in quantum-thermodynamics experiments down to single quanta of energy.

Vanishing quasiparticle density in a hybrid Al/Cu/Al single-electron transistor

The achievable fidelity of many nanoelectronic devices based on superconducting aluminum is limited by either the density of residual nonequilibrium quasiparticles n_qp or the density of quasiparticle states in the gap, characterized by Dynes parameter \gamma. We infer upper bounds n_qp < 0.033 um^-3 and \gamma < 1.6*10^-7 from transport measurements performed on Al/AlOx/Cu single-electron transistors, improving previous results by an order of magnitude. Owing to efficient microwave shielding and quasiparticle relaxation, typical number of quasiparticles in the superconducting leads is zero.

Radio-frequency single-electron refrigerator.

We propose a cyclic refrigeration principle based on mesoscopic electron transport. Synchronous sequential tunneling of electrons in a Coulomb-blockaded device, a normal metal-superconductor single-electron box, results in a cooling power of approximately k(B)T x f at temperature T over a wide range of cycle frequencies f. Electrostatic work, done by the gate voltage source, removes heat from the Coulomb island with an efficiency of approximately k(B)T/Delta, where Delta is the superconducting gap parameter. The performance is not affected significantly by nonidealities, for instance by offset charges. We propose ways of characterizing the system and of its practical implementation.

Environmentally activated tunneling events in a hybrid single-electron box

We have measured individual tunneling events and Coulomb step shapes in single-electron boxes with opaque superconductor-normal metal tunnel junctions. We observe anomalous broadening of the Coulomb step with decreasing temperature in a manner that is consistent with activation of first-order tunneling events by an external dissipative electromagnetic environment. We demonstrate that the rates for energetically unfavourable tunneling events saturate to finite values at low temperatures, and that the saturation level can be suppressed by more than an order of magnitude by a capacitive shunt near the device. The findings are important in assessing the performance limits of any single-electronic device. In particular, master equation based simulations show that the electromagnetic environment realized in the capacitively shunted devices allows for a metrologically accurate charge pump based on hybrid tunnnel junctions.

Real-Time Observation of Discrete Andreev Tunneling Events

We provide a direct proof of two-electron Andreev transitions in a superconductor-normal-metal tunnel junction by detecting them in a real-time electron counting experiment. Our results are consistent with ballistic Andreev transport with an order of magnitude higher rate than expected for a uniform barrier, suggesting that only part of the interface is effectively contributing to the transport. These findings are quantitatively supported by our direct current measurements in single-electron transistors with similar tunnel barriers.

Tunnel junction thermometry down to millikelvin temperatures

We present a simple on-chip electronic thermometer with the potential to operate down to 1 mK. It is based on transport through a single normal-metal - superconductor tunnel junction with rapidly widening leads. The current through the junction is determined by the temperature of the normal electrode that is efficiently thermalized to the phonon bath, and it is virtually insensitive to the temperature of the superconductor, even when the latter is relatively far from equilibrium. We demonstrate here the operation of the device down to 7 mK and present a systematic thermal analysis.

Long hold times in a two-junction electron trap

The hold time τ of a single-electron trap is shown to increase significantly due to suppression of photon assisted tunneling events. Using two rf-tight radiation shields instead of a single one, we demonstrate increase of τ by a factor exceeding 103, up to about 10 h, for a trap with only two superconductor (S)—normal-metal (N) tunnel junctions and an on-chip resistor R ∼ 100 kΩ (R-SNS structure). In the normal state, the improved shielding made it possible to observe τ ∼ 100 s, which is in reasonable agreement with the quantum-leakage-limited level expected for the two-electron cotunneling process.

Incomplete measurement of work in a dissipative two level system

We discuss work performed on a quantum two-level system coupled to multiple thermal baths. To evaluate the work, a measurement of photon exchange between the system and the baths is envisioned. In a realistic scenario, some photons remain unrecorded as they are exchanged with baths that are not accessible to the measurement, and thus only partial information on work and heat is available. The incompleteness of the measurement leads to substantial deviations from standard fluctuation relations. We propose a recovery of these relations, based on including the mutual information given by the counting efficiency of the partial measurement. We further present the experimental status of a possible implementation of the proposed scheme, i.e. a calorimetric measurement of work, currently with nearly single-photon sensitivity.