F. Mallet

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

Optimal design for the Josephson mixer

  1. J.-D. Pillet,
  2. E. Flurin,
  3. F. Mallet,
  4. and B. Huard
We present an optimal design in terms of gain, bandwidth and dynamical range for the Josephson mixer, the superconducting circuit performing three-wave mixing at microwave frequencies.
In a compact all lumped-element based circuit with galvanically coupled ports, we demonstrate non degenerate amplification for microwave signals over a bandwidth up to 50 MHz for a power gain of 20 dB. The quantum efficiency of the mixer is shown to be about 70% and its dynamical range reaches 5 quanta per inverse dynamical bandwidth.
arxiv pdf

Generating and verifying entangled itinerant microwave fields with efficient and independent measurements

  1. H. S. Ku,
  2. W. F. Kindel,
  3. F. Mallet,
  4. S. Glancy,
  5. K. D. Irwin,
  6. G. C. Hilton,
  7. L. R. Vale,
  8. and K. W. Lehnert
By combining a squeezed propagating microwave field and an unsqueezed vacuum field on a hybrid (microwave beam-splitter), we generate entanglement between the two output modes. We verify
that we have generated entangled states by making independent and efficient single-quadrature measurements of the two output modes. We observe the entanglement witness EW=−0.263+0.001−0.036 and the negativity N=0.0824+0.01−0.0004 with measurement efficiencies at least 26±0.1% and 41±0.2% for channel~1 and 2 respectively. These measurements show that the output two-mode state violates the separability criterion and therefore demonstrate entanglement. This shared entanglement between propagating microwaves provides an important resource for building quantum networks with superconducting microwave systems.
arxiv pdf

Quantum Heating of a nonlinear resonator probed by a superconducting qubit

  1. F. R. Ong,
  2. M. Boissonneault,
  3. F. Mallet,
  4. A. C. Doherty,
  5. A. Blais,
  6. D. Vion,
  7. D. Esteve,
  8. and P. Bertet
We measure the quantum fluctuations of a pumped nonlinear resonator, using a superconducting artificial atom as an in-situ probe. The qubit excitation spectrum gives access to the frequency
and temperature of the intracavity field fluctuations. These are found to be in agreement with theoretical predictions; in particular we experimentally observe the phenomenon of quantum heating.
arxiv pdf