Reversing Quantum Trajectories with Analog Feedback
G. de Lange, Diego Ristè, M. J. Tiggelman, Christopher Eichler, Lars Tornberg, Göran Johansson, Andreas J. Wallraff, Raymond N. Schouten, L. DiCarlo · Physical Review Letters · 2014
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 a measurement efficiency η ≈ 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.