Towards ultra strong-coupling quantum thermodynamics using a superconducting flux qubit.

In the rapidly advancing field of quantum technologies, managing heat presents a significant challenge for achieving peak performance in quantum devices. Thermodynamics in quantum circuits aims to find improved functionalities of thermal machines, highlight fundamental phenomena peculiar to quantum nature in thermodynamics, and point out limitations in quantum information processing due to coupling of the system to its environment. A key aspect of achieving these goals is exploring the strong coupling regime, which has so far been largely theoretical. Our objective is to experimentally demonstrate strong coupling effects in heat transport using a superconducting flux qubit, which has been shown to reach this regime. We present experimental evidence of strong coupling by observing a hybridized state between the qubit and the cavities it is coupled to, resulting in triplet-like thermal transport through the system near the qubit's minimum energy. This occurs at power levels in the range of femtowatts, which is an order of magnitude higher than previously demonstrated experiments experiments. Additionally, we achieve an on-off switching ratio of nearly 100% for heat current by applying a magnetic flux to the qubit. This experiment paves the way for investigating long-debated questions about the nature of heat in the strong coupling regime, bringing us closer to realizing true quantum heat engines and refrigerators with enhanced power and efficiency. Also, we present a simple theoretical model which is in agreement with the reported observations and explains the origin of the heat transport observed in our device.