Using Spontaneous Emission of a Qubit as a Resource for Feedback Control

  1. P. Campagne-Ibarcq,
  2. S. Jezouin,
  3. N. Cottet,
  4. P. Six,
  5. L. Bretheau,
  6. F. Mallet,
  7. A. Sarlette,
  8. P. Rouchon,
  9. and B. Huard
Persistent control of a transmon qubit is performed by a feedback protocol based on continuous weak measurement of its fluorescence. By driving the qubit and cavity with microwave signals whose amplitudes depend linearly on the instantaneous values of the quadratures of the measured fluorescence field, we demonstrate the permanent stabilization of the qubit in any direction of the Bloch sphere. Using a Josephson mixer as a phase-preserving amplifier, it was possible to reach a total measurement efficiency η=35%, leading to a maximum of 59% of excitation and 44% of coherence for the stabilized states. The experiment demonstrates multiple-input multiple-output (MIMO) analog markovian feedback in the quantum regime.
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