title: Thermal and Residual Excited-State Population in a 3D Transmon Qubit.
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 T(eff)=35  mK. Assuming T(eff) is due solely to hot quasiparticles, the inferred qubit lifetime is 108  μs and in plausible agreement with the measured 80  μs.
authors.affiliation: Research Laboratory of Electronics, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA.
authors.forename: X Y
authors.initials: XY
authors.lastname: Jin
authors.affiliation: Research Laboratory of Electronics, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA.
authors.forename: A
authors.initials: A
authors.lastname: Kamal
authors.affiliation: MIT Lincoln Laboratory, 244 Wood Street, Lexington, Massachusetts 02420, USA.
authors.forename: A P
authors.initials: AP
authors.lastname: Sears
authors.affiliation: MIT Lincoln Laboratory, 244 Wood Street, Lexington, Massachusetts 02420, USA.
authors.forename: T
authors.initials: T
authors.lastname: Gudmundsen
authors.affiliation: MIT Lincoln Laboratory, 244 Wood Street, Lexington, Massachusetts 02420, USA.
authors.forename: D
authors.initials: D
authors.lastname: Hover
authors.affiliation: MIT Lincoln Laboratory, 244 Wood Street, Lexington, Massachusetts 02420, USA.
authors.forename: J
authors.initials: J
authors.lastname: Miloshi
authors.affiliation: MIT Lincoln Laboratory, 244 Wood Street, Lexington, Massachusetts 02420, USA.
authors.forename: R
authors.initials: R
authors.lastname: Slattery
authors.affiliation: Research Laboratory of Electronics, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA.
authors.forename: F
authors.initials: F
authors.lastname: Yan
authors.affiliation: MIT Lincoln Laboratory, 244 Wood Street, Lexington, Massachusetts 02420, USA.
authors.forename: J
authors.initials: J
authors.lastname: Yoder
authors.affiliation: Research Laboratory of Electronics, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA.
authors.forename: T P
authors.initials: TP
authors.lastname: Orlando
authors.affiliation: Research Laboratory of Electronics, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA.
authors.forename: S
authors.initials: S
authors.lastname: Gustavsson
authors.affiliation: Research Laboratory of Electronics, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA.|MIT Lincoln Laboratory, 244 Wood Street, Lexington, Massachusetts 02420, USA.
authors.forename: W D
authors.initials: WD
authors.lastname: Oliver