Thermometry Based on a Superconducting Qubit

We report temperature measurements using a transmon qubit by detecting the population of the first three levels of it, after employing 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 τ1,2 for three devices in the temperature range 20-300 mK. Signal-to-noise (SNR) 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 100-250 mK. We present a numerical model of the qubit population distribution and compare it favorably with the experimental results.