Circuit Quantum Thermodynamics - from photonic heat transport to ultra-sensitive nanocalorimetry

Quantum thermodynamics deals with open quantum systems. The word 'open' here means that the system is interacting with its environment, which in the thermodynamics context is a heat bath. Presently a lot of activity is devoted to questions in heat transport, heat engines, refrigerators and ultra-sensitive detectors facilitated by quantum systems as working medium. In this thesis, we investigate both experimentally and theoretically phenomena and devices in quantum thermodynamics realized by superconducting and metal circuits on a chip at low millikelvin temperatures. This is a novel area of research coined circuit quantum thermodynamics, cQTD. The building blocks in the experiments are formed of harmonic oscillators (superconducting cavities), non-linear oscillators (Josephson junctions), and heat baths formed of resistors and phonons on the chip substrate. These systems form well-characterized elements that can be described theoretically, quantitatively accurately, by means of theoretical tools applied earlier to structures in mesoscopic physics. What is new here is the full thermal description of these systems, including various thermal transport mechanisms, like radiative heat by thermal microwave photons, electronic heat transport in metals, superconductors and tunnel contacts, and electron-phonon heat transport. There are two central topics on which we present new results in the thesis. The first one is the utilization of photonic heat transport on a chip. We develop a theoretical model for a quantum Otto refrigerator, where a superconducting qubit is coupled alternately to two different heat baths, and by the cyclic variation of the qubit energy by external field one can pump heat from the cold bath to the hot one. We demonstrate explicitly the quantum contribution in this heat arising from coherences built into the qubit. We then propose ideas and develop theoretical models to describe superconducting transmon qubit-based quantum heat valves and rectifiers that were realized experimentally in our laboratory during the course of this thesis. The experimental achievement of the thesis is the demonstration of an ultra-sensitive thermal detector reaching the ultimate noise level dictated by the fundamental thermal fluctuations. This allows us to consider the scheme of detecting single microwave photons in a continuous manner, calorimetrically. The key ingredients of the calorimeter are an ultrasensitive proximity supercurrent thermometer (ZBA thermometer) and a tiny proximitized normal metal absorber. A scheme of coupling a superconducting qubit to this calorimeter is presented and we conclude positively about the possibility of having sufficient signal-to-noise ratio (SNR) in detecting a photon emitted by it. As a final boost to enhance the SNR, we propose splitting of the photon to two uncorrelated baths and performing a cross-correlation measurement of their temperatures.