Abstract:

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

Authors:  X. Y. Jin, A. Kamal, A. P. Sears, T. Gudmundsen, D. Hover, J. Miloshi, R. Slattery, F. Yan, J. Yoder, T. P. Orlando, S. Gustavsson, and W. D. Oliver

Reference: Phys. Rev. Lett. 114, 240501 (2015)

Abstract:

Remarkable advancements in coherence and control fidelity have been achieved in recent years with
cryogenic solid-state qubits. Nonetheless, thermalizing such devices to their milliKelvin environments has
remained a long-standing fundamental and technical challenge. In this context, we present a systematic
study of the first-excited-state population in a 3D transmon superconducting qubit mounted in a dilution
refrigerator with a variable temperature. Using a modified version of the protocol developed by Geerlings
et al., 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 at approximately 0.1%. 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 Teff ¼ 35 mK. Assuming Teff is due solely to hot quasiparticles, the inferred qubit lifetime is
108 μs and in plausible agreement with the measured 80 μs.