Sub-Kelvin Thermometer for On-Chip Measurements of Microwave Devices Utilizing Two-Level Systems in Superconducting Microresonators

Two-level systems (TLS) are well-known to be a source of decoherence in superconducting qubits. Extensive research has been carried out to understand its physics and to mitigate its negative effects. In this talk, however, we describe how to make use of TLS physics to create a useful device. We present a superconducting microresonator thermometer based on the TLS-induced frequency shift that is drop-in compatible with cryogenic microwave systems such as qubits. The operational temperature range is 50-1000 mK (which may be extended to 5 mK). The miniature footprint can be conveniently attached to the feedline of the microwave devices. The key advantage is that it enables easy and accurate measurement of the true chip temperature without additional wiring or readout electronics. We demonstrate the practical use of these TLS thermometers to investigate static and transient chip heating in a kinetic inductance traveling-wave parametric amplifier (KIT) operated with a strong pump tone. We describe how the insights from these temperature measurements have guided us to the successful mitigation of excess thermal loads in our cryogenic testbed. TLS thermometry may find broad applications for use with superconducting qubit and detector systems.