Local Sensing with an AC Stark Spectrum Analyzer

  1. Andre Schneider,
  2. Jochen Braumüller,
  3. Lingzhen Guo,
  4. Patrizia Stehle,
  5. Hannes Rotzinger,
  6. Michael Marthaler,
  7. Alexey V. Ustinov,
  8. and Martin Weides
Analyzing weak microwave signals in the GHz regime is a challenging task if the signal level is very low and the photon energy widely undefined. Due to its discrete level structure, a superconducting qubit is only sensitive to photons of certain energies. With a multi-level quantum system (qudit) in contrast, the unknown photon frequency can be deduced from the higher level AC Stark shift. The measurement accuracy is given by the signal amplitude, its detuning from the discrete qudit energy level structure and the anharmonicity. We demonstrate an energy sensitivity in the order of 10−4 with a measurement range of 1 GHz. Here, using a transmon qubit, we experimentally observe shifts in the transition frequencies involving up to three excited levels. These shifts are in good agreement with an analytic circuit model and master equation simulations. For large detunings, we find the shifts to scale linearly with the power of the applied microwave drive.

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