Observation of Low Thermal Excited State Population of Transmon Qubit

We have measured the thermal excited-state population of a 3D Transmon qubit from 12 mK up to 150 mK. The Al/AlOx/Al qubit had a g-to-e transition frequency of 3.6 GHz, an e-to-f transition frequency of 3.4 GHz, and T1 of 17 microseconds. The device was mounted in a rectangular Al cavity with a transition frequency of 7.9 GHz and the cavity input line had custom-made attenuators ensuring that noise on the input line was well-filtered and thermalized[1]. The residual excited state population of the qubit was measured by measuring the response of the system when driving Rabi oscillations between the e and f states, with no initial preparation pulse applied to the qubit[2] for one of the pulse sequences. With the refrigerator at 12 mK, we found that the residual excited state population of the qubit in the e state was 0.17% with uncertainty of 0.02%, corresponding to a remarkably small effective qubit temperature of just 28 mK. For refrigerator temperatures T above 40 mK, the effective temperature T_n of the qubit was very close to T.
[1]J.-H. Yeh, J. LeFebvre, S. Premaratne, F. C. Wellstood, and B. S. Palmer, J. Appl. Phys. 121, 224501 (2017).
[2]K. Geerlings, et al. Phys. Rev. Lett. 110, 120501 (2013).