Quantum interference probed by thermal spectrometer

Superconducting circuits present a platform for investigations of fundamental quantum phenomena as well as for quantum technology applications. A conventional method to read out the state of a quantum device or to characterize its properties is based on RF measurement schemes. Here we present a simple DC measurement of a thermal spectrometer to investigate properties of a superconducting coplanar waveguide resonator [1]. A fraction of the microwave photons in the resonator is absorbed by an on-chip bolometer, which is in the form of a normal-metal mesoscopic resistor combined with NIS junction thermometry [2-3], resulting in a measurable temperature rise. By monitoring the DC signal of the thermometer, we are able to determine the resonance frequency and the lineshape (quality factor) of the resonator. The demonstrated scheme offers a wide frequency band potentially reaching up to 200 GHz, far exceeding that of the typical RF spectrometer. In the low power regime, the measurement is calibration-free. The technique offers an alternative spectrometer for quantum circuits. Furthermore, we use the spectrometer as an on-chip engineered thermal bath to detect quantum interference [5]. We measure quantum features in the heat transfer between a qubit and a thermal bath in a system formed of a driven flux qubit galvanically coupled to a λ/4 coplanar-waveguide resonator that is coupled to the spectrometer. We detect interference patterns in the heat current due to driving-induced coherence. In particular, resonance peaks in the heat transferred to the bath are found at driving frequencies which are integer fractions of the resonator frequency. A selection rule on the even/odd parity of the peaks holds at the qubit symmetry point. The studied system provides a platform for studying the role of coherence in quantum thermodynamics. Our work opens the possibility to demonstrate a true quantum thermal machine where heat is measured directly.