N. Cottet

Observing a quantum Maxwell demon at work

  1. N. Cottet,
  2. S. Jezouin,
  3. L. Bretheau,
  4. P. Campagne-Ibarcq,
  5. Q. Ficheux,
  6. J. Anders,
  7. A. Auffèves,
  8. R. Azouit,
  9. P. Rouchon,
  10. and B. Huard
In apparent contradiction to the laws of thermodynamics, Maxwell’s demon is able to cyclically extract work from a system in contact with a thermal bath exploiting the information
about its microstate. The resolution of this paradox required the insight that an intimate relationship exists between information and thermodynamics. Here, we realize a Maxwell demon experiment that tracks the state of each constituent both in the classical and quantum regimes. The demon is a microwave cavity that encodes quantum information about a superconducting qubit and converts information into work by powering up a propagating microwave pulse by stimulated emission. Thanks to the high level of control of superconducting circuits, we directly measure the extracted work and quantify the entropy remaining in the demon’s memory. This experiment provides an enlightening illustration of the interplay of thermodynamics with quantum information.
arxiv pdf

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