Exploring on-chip qusaiparticle thermodynamics through unconfined circuit quantum electrodynamics.

One of the current challenges in microwave-to-optical quantum transduction is the light-induced qusaiparticle (QP) generation at superconducting elements, particularly Aluminum-based Josephson junction (JJ). Comprehensive probes of chip-scale QP thermodynamics require integrating multiple thermometers over the chip, which has remained a challenging task. Although superconducting qubits (SCQs) already present in the devices are excellent candidates for such QP thermometry, thermally activated decoherence sources blur the spectrum of SCQs, confining the operation to low qusaiparticle density regimes. In this work, we reveal that the Aluminum JJ of a transmon still conserves the QP-induced temperature dependence which well follows Josephson physics even when the transmon is strongly driven to unconfined states, and thereby the circuit is fully linearized. By measuring dispersively coupled resonators, we can extract the temperature information from JJ almost without effects from other pure dephasing sources, and consequently, the probing range can be extended to the critical temperature of Aluminum. Our approach has a variety of distinct merits such as simplicity in measurement systems, multiplexibility in readouts, and well-established fabrication processes for transmons. Thereby, one can readily scale the number of QP thermometers on different positions of one device, establishing space-time-resolved temperature probes. Thanks to the aforementioned properties, this scheme can also find applications even outside of quantum transduction. Beyond the thermometry applications, the proposed approach will lead to the development of a new method to characterize the thermal properties of JJs.