Thermometry based on a superconducting qubit

Abstract

We report temperature measurements using a transmon qubit by detecting the population of its first three energy levels after applying a sequence of π pulses and performing projective dispersive readout. We measure the effective temperature of the qubit and characterize its relaxation and coherence times τ1, τ2 for three devices in the temperature range from 20 to 300 mK. We analyze the process of qubit thermalization to its effective environment consisting of multiple heat baths and support this with experimental data. The signal-to-noise ratio of the temperature measurement depends strongly on τ1, which drops at higher temperatures due to quasiparticle excitations, adversely affecting the measurements and setting an upper bound of the dynamic temperature range of the thermometer. The measurement relies on coherent dynamics of the qubit during the π pulses. The effective qubit temperature follows closely that of the cryostat in the range from 100 to 200 mK. We present a numerical model of the qubit population distribution and find hat it compares favorably with the experimental results. Finally, we compare our technique with previous work on qubit thermometry and discuss its application prospects.