Thermometry Based on a Superconducting Qubit

TL;DR

This paper demonstrates a method for measuring temperature using a superconducting transmon qubit by analyzing its energy level populations and coherence properties, providing insights into qubit thermalization and potential thermometry applications.

Contribution

It introduces a novel qubit-based thermometry technique that measures effective temperature through population analysis and coherence times, supported by experimental data and numerical modeling.

Findings

Effective qubit temperature closely follows cryostat temperature between 100-250 mK.

Signal-to-noise ratio depends on relaxation time, which decreases at higher temperatures.

Qubit thermalization involves multiple heat baths, affecting measurement accuracy.

Abstract

We report temperature measurements using a transmon qubit by detecting the population of its first three energy levels, after applying a sequence of $\pi$-pulses and performing projective dispersive readout. We measure the effective temperature of the qubit and characterize its relaxation and coherence times $\tau_{1,2}$ for three devices in the temperature range of $20-300$ mK. We analyze the process of qubit thermalization to its effective environment consisting of multiple heat baths and support it with experimental data. Signal-to-noise (SNR) ratio of the temperature measurement depends strongly on $\tau_1$, 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 $\pi$-pulses. The effective qubit temperature follows closely that of the cryostat in the range of $100 - 250$ mK. We present a numerical model of the qubit population distribution and compare it favorably with the experimental results. Finally, we compare our technique with previous works on qubit thermometry and discuss its application prospects.