Reversing quantum trajectories with analog feedback

Understand

We demonstrate the active suppression of transmon qubit dephasing induced by dispersive measurement, using parametric amplification and analog feedback.

• By real-time processing of the homodyne record, the feedback controller reverts the stochastic quantum phase kick imparted by the measurement on the qubit.
• The feedback operation matches a model of quantum trajectories with measurement efficiency $\tilde{\eta} \approx 0.5$, consistent with the result obtained by postselection.
• We overcome the bandwidth limitations of the amplification chain by numerically optimizing the signal processing in the feedback loop and provide a theoretical model explaining the optimization result.