Thermodynamics in quantum circuits aims to find improved functionalities of thermal machines, highlight fundamental phenomena peculiar to quantum nature in thermodynamics, and pointout limitations in quantum information processing due to coupling of the system to its environment. An important aspect to achieve some of these goals is the regime of strong coupling that has remained until now a domain of theoretical works only. Our aim is to demonstrate strong coupling features in heat transport using a superconducting flux qubit that has been shown to reach strong to deep-ultra strong coupling regimes. Here we show experimental evidence of strong coupling by observing a hybridized state of the qubit with the cavities coupled to it, leading to a triplet-like thermal transport via this combined system around the minimum energy of the qubit, at power levels of tens of femtowatts, exceeding by an order of magnitude from the earlier ones. We also demonstrate close to 100% on-off switching ratio of heat current by applying small magnetic flux to the qubit. Our experiment opens a way towards testing debated questions in strong coupling thermodynamics such as what heat in this regime is. We also present a theoretical model that aligns with our experimental findings and explains the mechanism behind heat transport in our device. Furthermore, we provide a new tool for quantum thermodynamics aimed at realizing true quantum heat engines and refrigerators with enhanced power and efficiency, leveraging ultra-strong coupling between the system and environment.
Superconducting circuits provide a versatile and controllable platform for studies of fundamental quantum phenomena as well as for quantum technology applications. A conventional techniqueto read out the state of a quantum circuit or to characterize its properties is based on rf measurement schemes involving costly and complex instrumentation. Here we demonstrate a simple dc measurement of a thermal spectrometer to investigate properties of a superconducting circuit, in this proof-of-concept experiment a coplanar waveguide resonator. A fraction of the microwave photons in the resonator is absorbed by an on-chip bolometer, resulting in a measurable temperature rise. By monitoring the dc signal of the thermometer due to this process, we are able to determine the resonance frequency and the lineshape (quality factor) of the resonator. The demonstrated scheme, which is a simple dc measurement, has a wide band up to 200 GHz, well exceeding that of the typical rf spectrometer. Moreover, the thermal measurement yields a highly frequency independent reference level of the Lorentzian absorption signal, unlike the conventional rf measurement. In the low power regime, the measurement is fully calibration-free. Our technique thus offers an alternative spectrometer for quantum circuits, which is in many ways superior with respect to conventional methods.
The study of quantum heat transport in superconducting circuits is significant for further understanding the connection between quantum mechanics and thermodynamics, and for possibleapplications for quantum information. The first experimental realisations of devices demonstrating photonic heat transport mediated by a qubit have already been designed and measured. Motivated by the analysis of such experimental results, and for future experimental designs, we numerically evaluate the photonic heat transport of qubit-resonator devices in the linear circuit regime through electromagnetic simulations using Sonnet software, and compare with microwave circuit theory. We show that the method is a powerful tool to calculate heat transport and predict unwanted parasitic resonances and background.
Here we present an architecture for the implementation of cyclic quantum thermal engines using a superconducting circuit. The quantum engine consists of a gated Cooper-pair box, capacitivelycoupled to two superconducting coplanar waveguide resonators with different frequencies, acting as thermal baths. We experimentally demonstrate the strong coupling of a charge qubit to two superconducting resonators, with the ability to perform voltage driving of the qubit at GHz frequencies. By terminating the resonators of the measured structure with normal-metal resistors whose temperature can be controlled and monitored, a quantum heat engine or refrigerator could be realized. Furthermore, we numerically evaluate the performance of our setup acting as a quantum Otto-refrigerator in the presence of realistic environmental decoherence.