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arXiv:1704.04762 · Physical Review Applied

Nanosecond Thermometry with Josephson Junctions

M. Zgirski, M. Foltyn, A. Savin, K. Norowski, M. Meschke, J. Pekola

Published Oct 29, 2018• 45 citations
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Abstract

A feverish rate of change: Thermometry is key to thermodynamics, but measuring temperature changes at the nanoscale is challenging. In this investigation, a superconducting weak link (Josephson junction) is used to measure rapidly changing electron temperature in a long superconducting wire, with nanosecond resolution. The approach used here extends the range of existing experiments to faster time scales, paving the way for cutting-edge work on the thermodynamics of low-temperature quantum circuits and the development of ultralow-energy calorimetry and bolometry.

DOI
10.1103/physrevapplied.10.044068

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A feverish rate of change: Thermometry is key to thermodynamics, but measuring temperature changes at the nanoscale is challenging. In this investigation, a superconducting weak link (Josephson junction) is used to measure rapidly changing electron temperature in a long superconducting wire, with nanosecond resolution. The approach used here extends the range of existing experiments to faster time scales, paving the way for cutting-edge work on the thermodynamics of low-temperature quantum circuits and the development of ultralow-energy calorimetry and bolometry.
Abstract In a Josephson junction, which is the central element in superconducting quantum technology, irreversibility arises from abrupt slips of the gauge-invariant quantum phase difference across the contact. A quantum phase slip (QPS) is often visualized as the tunneling of a flux quantum in the transverse direction to the superconducting weak link, which produces dissipation. Here, we detect the instantaneous heat release caused by a QPS in a Josephson junction using time-resolved electron thermometry on a nanocalorimeter, signaled by an abrupt increase of the local electronic temperature in the weak link and subsequent relaxation back to equilibrium. Beyond providing a cornerstone in experimental quantum thermodynamics in form of observation of heat in an elementary quantum process, this result sets the ground for experimentally addressing the ubiquity of dissipation, including that in superconducting quantum sensors and qubits.
For very small, very cold objects, merely knowing how cold they actually are is a challenge. The authors present a ``Josephson bolometer'' that detects weak electrical fluctuations and indicates their strength directly via frequency shift in a microwave resonator, without relying on power-spectrum analysis by external electronics. This combined experimental and theoretical study advances nanoscale thermometry and the detection of weak electrical signals, offering prospects not just for high-resolution calorimetry, but also for manipulating nonclassical microwave fields, and microwave amplification.
Quantum calorimetry, the thermal measurement of quanta, is a method of choice for ultrasensitive radiation detection ranging from microwaves to gamma rays. The fundamental temperature fluctuations of the calorimeter, dictated by the coupling of it to the heat bath, set the ultimate lower bound of its energy resolution. Here we reach this limit of fundamental equilibrium fluctuations of temperature in a nanoscale electron calorimeter, exchanging energy with the phonon bath at very low temperatures. The approach allows noninvasive measurement of energy transport in superconducting quantum circuits in the microwave regime with high efficiency, opening the way, for instance, to observe quantum jumps, detecting their energy to tackle central questions in quantum thermodynamics.
High-resolution (sub-microsecond to nanosecond) time-resolved measurements of the dynamics of superconducting electronic devices, such as Josephson tunnel junctions and SQUIDs, are indispensable for fundamental physics research, such as quantum mechanics of macroscopic variables. We describe the development of a time-resolved measurement technique that enables direct measurements, in the time domain, of the temporal evolution of Josephson junctions with a nanosecond resolution. The technique was applied to study escape dynamics of Josephson junctions, as macroscopic quantum systems, in the quantum tunnelling regime. The measured probability of junctions remaining in the initial metastable state—the survival probability—as a function of time decayed exponentially, agrees very well with the theoretical prediction of the incoherent tunnelling process.
In this work, normal metal and superconducting tunnel junctions have been studied. In particular applications in low temperature electronic thermometry are investigated. The double oxidation method for achieving high resistance junctions is described and shown to produce high quality devices. A new invention, the single junction thermometer (SJT), is described, and proof of the concept experiments demonstrating the functionality of the device are described. A key feature of the SJT is a well controlled electromagnetic environment surrounding a single junction. It is shown that embedding a single junction within arrays of other junctions is a superior way for controlling the environment as compared to previously proposed methods. Improved thermalisation of electrons is shown to be beneficial for the performance of superconducting RSFQ (Rapid Single Flux Quantum) devices. The so called grey zone of the most basic of these devices, the balanced comparator, is investigated. Improved performance based on metallic cooling fins has been achieved. In the same spirit improved thermalisation of the Coulomb blockade thermometers extends their operation towards lower temperatures. A method employing thick metallic islands was developed and tested to this end.