Quantum refrigeration powered by dephasing in a superconducting circuit*

While dephasing noise frequently presents obstacles for quantum devices, it can become an asset in the context of quantum refrigerators. Quantum thermal machines have in recent years garnered more attention, but their implementation is hindered by the difficulties in measuring tiny heat flows. Here we demonstrate a novel quantum thermal machine that leverages dephasing noise to fuel a cooling engine in steady state. The device exploits symmetry selective couplings between a superconducting artificial molecule, comprised of two strongly flux tunable transmon qubits, and two microwave waveguides. These waveguides act as thermal reservoirs of different temperatures, which are regulated by employing synthesized thermal fields. The dephasing is controlled by injecting white noise through a third channel that is longitudinally coupled to the molecule. The interplay between individual thermalization to the baths and dephasing noise produces a heat current between the two baths, which we measure with a resolution below 1 aW using interleaved power spectral density measurements. By varying the temperature ratio of the thermal reservoirs, we demonstrated that the device can be operated as a quantum heat engine, thermal accelerator, and refrigerator. Our findings show the successful operation of a Brownian refrigerator and the simultaneous measurement of heat flows to multiple baths, opening new avenues for investigating quantum thermal machines in circuit quantum electrodynamics.