Second-order decoherence mechanisms of a transmon qubit probed with thermal microwave states

  1. J. Goetz,
  2. F. Deppe,
  3. P. Eder,
  4. M. Fischer,
  5. M. Müting,
  6. J. P. Martínez,
  7. S. Pogorzalek,
  8. F. Wulschner,
  9. E. Xie,
  10. K. G. Fedorov,
  11. A. Marx,
  12. and R. Gross
Thermal microwave states are omnipresent noise sources in superconducting quantum circuits covering all relevant frequency regimes. We use them as a probe to identify three second-order decoherence mechanisms of a superconducting transmon. First, we quantify the efficiency of a resonator filter in the dispersive Jaynes-Cummings regime and find evidence for parasitic loss channels. Second, we probe second-order noise in the low-frequency regime and demonstrate the expected T3 temperature dependence of the qubit dephasing rate. Finally, we show that qubit parameter fluctuations due to two-level states are enhanced under the influence of thermal microwave states. In particular, we experimentally confirm the T2-dependence of the fluctuation spectrum expected for noninteracting two-level states.
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