P. Six

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.
arxiv pdf

Observing quantum state diffusion by heterodyne detection of fluorescence

  1. P. Campagne-Ibarcq,
  2. P. Six,
  3. L. Bretheau,
  4. A. Sarlette,
  5. M. Mirrahimi,
  6. P. Rouchon,
  7. and B. Huard
A qubit can relax by fluorescence, which prompts the release of a photon into its electromagnetic environment. By counting the emitted photons, discrete quantum jumps of the qubit state
can be observed. The succession of states occupied by the qubit in a single experiment, its quantum trajectory, depends in fact on the kind of detector. How are the quantum trajectories modified if one measures continuously the amplitude of the fluorescence field instead? Using a superconducting parametric amplifier, we have performed heterodyne detection of the fluorescence of a superconducting qubit. For each realization of the measurement record, we can reconstruct a different quantum trajectory for the qubit. The observed evolution obeys quantum state diffusion, which is characteristic of quantum measurements subject to zero point fluctuations. Independent projective measurements of the qubit at various times provide a quantitative validation of the reconstructed trajectories. By exploring the statistics of quantum trajectories, we demonstrate that the qubit states span a deterministic surface in the Bloch sphere at each time in the evolution. Additionally, we show that when monitoring fluorescence, coherent superpositions are generated during the decay from excited to ground state. Counterintuitively, measuring light emitted during relaxation can give rise to trajectories with increased excitation probability.
arxiv pdf