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Bulletin of the American Physical Society

Temperature Dependence of Relaxation-Time Fluctuations in Transmon Qubits

Sudeep Dutta, Kungang Li, Rui Zhang, Dylan Poppert, Shahriar Keshvari, Christopher J. Lobb, F. C. Wellstood

Published Mar 4, 2020• 0 citations
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We have measured temporal fluctuations in the energy relaxation timeT1of a series of 3D transmon qubits. At 20 mK, the relaxation times of the devices have average values between 2 and 190 μs, with standard deviations as large as 20% of the averages. Over the different devices, this fluctuation magnitude roughly scales as a power law in the averageT1, with an exponent near 1.5. With increasing temperature,T1decreases due to a higher density of thermally-generated quasiparticles. For an individual device measured up to 250 mK, the fluctuation magnitude appears to be proportional toT1. We present a model of quasiparticle dissipation channels that reproduces both of the observed scaling relationships.
Materials improvements in transmon qubits have yielded remarkable advances in qubit lifetimes (T1)and coherence times (T2). However, state-of-the-art superconducting qubits experience temporal fluctuations in their performance, which limit gate fidelities and quantum processor performance. We observe correlated increases in qubit relaxation and dephasing rates that last for minutes to hours, which we call T1 and T2 excursions. Using interleaved lifetime and coherence time measurements, noise spectroscopy, and two-level-system (TLS) spectroscopy, we find that these excursions are caused by single or few strongly-coupled TLS defects that spectrally diffuse into resonance with the qubit frequency. Our measurement protocol provides a powerful tool for understanding the noise sources that cause temporal fluctuations in superconducting qubit performance.