Thermal and Residual Excited-State Population in a 3D Transmon Qubit

TL;DR

This study investigates the excited-state population of a 3D transmon qubit across a range of temperatures, revealing thermal equilibrium behavior and a saturation at very low temperatures, with implications for qubit lifetime and quasiparticle effects.

Contribution

The paper provides a systematic measurement of the excited-state population in a 3D transmon, demonstrating thermal equilibrium behavior and identifying an effective temperature near 35 mK.

Findings

Excited-state population follows Maxwell-Boltzmann distribution from 35-150 mK.

Below 35 mK, the population saturates at 0.1%.

Estimated qubit lifetime of 108 μs based on effective temperature.

Abstract

We present a systematic study of the first excited-state population in a 3D transmon qubit mounted in a dilution refrigerator with a variable temperature. Using a modified version of the protocol developed by Geerlings et al. [1], we observe the excited-state population to be consistent with a Maxwell-Boltzmann distribution, i.e., a qubit in thermal equilibrium with the refrigerator, over the temperature range 35-150 mK. Below 35 mK, the excited-state population saturates to 0.1%, near the resolution of our measurement. We verified this result using a flux qubit with ten-times stronger coupling to its readout resonator. We conclude that these qubits have effective temperature T_{eff} = 35 mK. Assuming T_{eff} is due solely to hot quasiparticles, the inferred qubit lifetime is 108 us and in plausible agreement with the measured 80 us.