A quantum Szilard engine for two-level systems coupled to a qubit

The innate complexity of solid state physics exposes superconducting quantum circuits to interactions with uncontrolled degrees of freedom degrading their coherence. By using a simple stabilizer code we show that a superconducting fluxonium qubit is coupled to a two-level system (TLS) environment of unknown origin, with a relatively long energy relaxation time exceeding 50 ms. Implementing the quantum Szilard engine, the active feedback control loop allows us to decide whether the qubit heats or cools its mesoscopic TLS environment. The TLSs can either be heated up to a population inversion of ~80%, that in turn inverts the qubit population during free evolution, or they can be cooled down below the 20 mK base temperature of the dilution refrigerator. We show that the TLSs and the qubit are each other's dominant loss mechanism. Understanding and mitigating TLS environments is not only crucial to improving qubit lifetimes, but also to avoiding non-Markovian qubit dynamics.