Q. Ficheux

Dynamics of a qubit while simultaneously monitoring its relaxation and dephasing

  1. Q. Ficheux,
  2. S. Jezouin,
  3. Z. Leghtas,
  4. and B. Huard
Decoherence originates from the leakage of quantum information into unmonitored degrees of freedom. For a qubit the two main decoherence channels are relaxation and dephasing. Here,
we report an experiment on a superconducting qubit where we retrieve a significant part of the lost information in both of these channels. We demonstrate that raw averaging the corresponding measurement records provides a full quantum tomography of the qubit state where all three components of the effective spin-1/2 are simultaneously measured. From single realizations of the experiment, it is possible to infer the quantum trajectories followed by the qubit state conditioned on relaxation and/or dephasing channels. The incompatibility between these quantum measurements of the qubit leads to observable consequences in the statistics of quantum states. The high level of controllability of superconducting circuits enables us to explore many regimes from Zeno effect to underdamped Rabi oscillations depending on the relative strengths of driving, dephasing and relaxation.
arxiv pdf

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