Microwave nanobolometer based on proximity Josephson junctions

Abstract

Govenius, J.; Lake, Russell; Tan, Kuan; Pietilä, V.; Julin, J.K.; Maasilta, I.J.; Virtanen, P.; Möttönen, Mikko Microwave nanobolometer based on proximity Josephson junctions We introduce a microwave bolometer aimed at high-quantum-efficiency detection of wave packet energy within the framework of circuit quantum electrodynamics, the ultimate goal being single microwave photon detection. We measure the differential thermal conductance between the detector and its heat bath, obtaining values as low as 5 fW/K at 50 mK. This is one tenth of the thermal conductance quantum and corresponds to a theoretical lower bound on noise-equivalent power of order 10 −20 W/ √ Hz at 50 mK. By measuring the differential thermal conductance of the same bolometer design in substantially different environments and materials, we determine that electron-photon coupling dominates the thermalization of our nanobolometer.

Towards quantum thermodynamics in electronic circuits

“…A sensitive thermometer, currently being developed in several laboratories [45][46][47], could then detect the temperature variations of the absorber that has minimal heat capacity. First estimates show that the present realizations are only about one order of magnitude away from single photon resolution [47].…”

Energy fluctuations of a finite free-electron Fermi gas

“…It is the purpose of this work to address the problem of energy fluctuations for a particular system of a metallic calorimeter representing a textbook model of free electron gas at low temperatures T far below the Fermi temperature T F . In a typical experimental setting, T /T F ∼ 10 −6 [14][15][16]. In the present work we will derive the distribution function for a finite Fermi gas and show that its shape is determined by only one dimensionless parameter-the heat capacity divided by the Boltzmann constant, k B .…”

“…We will analytically derive the distribution of the energies of a finite sample of the Fermi gas kept at a given temperature and analyze its properties including moments and skewness absent in the thermodynamic limit. Since the heat capacity of a metallic conductor of sub-micron dimensions at standard sub-kelvin (∼10-100 mK) experimental temperatures is of the order of (10 2 -10 3 )k B [4,13], our results have potential impact on sensitive bolometers [4,10,[14][15][16][17]. In future, we plan to use the obtained distribution for analysis of heat exchange between a quantum device, e.g., a qubit, and a mesoscopic metallic calorimeter.…”

“…Energy and temperature fluctuations in a single electron box [8] were considered in [9]; several works experimental and theoretical, e.g., [10][11][12], were devoted to temperature fluctuations. Such fluctuations are particularly relevant for ultra-small calorimeters aiming at extreme energy resolution, ultimately down to single microwave photon level [10,[13][14][15]. The effective temperature of the calorimeter, which has heat capacity C, changes by E/C upon absorbing a photon of energy E. The resolution, ∝ √ C, where C is the heat capacity of the absorber of the calorimeter, needs thus to be minimized.…”

Radial phononic thermal conductance in thin membranes in the Casimir limit: Design guidelines for devices

“…This level of G is usually achieved with the help of long and narrow beams 35,36 or with nanoscale hot-electron systems. 37,38 We therefore propose that a roughened but thin full membrane can possibly do the same job, but it would be mechanically more robust (no narrow beams) and would not require nanolithography to create a small electron volume.…”