Kavli Affiliate: Birgitta Whaley
| First 5 Authors: Tathagata Karmakar, Tathagata Karmakar, , ,
| Summary:
Time-continuous quantum measurement allows for the tracking of a quantum
system in real time via sequences of short, and individually weak, measurement
intervals. Such measurements are necessarily invasive, imparting backaction to
the system, and allowing the observer to update their state estimate based on
stochastic measurement outcomes. Feedback control then involves real-time
interventions by an observer, conditioned on the time-continuous measurement
signal that they receive. We here consider diffusive quantum trajectories, and
focus on the "noise-canceling" subset of feedback protocols that aim to
minimize the degree of stochasticity in the dynamics. We derive such a class of
feedback operations, showing that under the idealized assumptions of pure
states, unit measurement efficiency, and zero time-delay in implementing
feedback operations, perfectly noise-canceling feedback always exists. We
consider the resulting noise-canceled dynamics generated by an effective
non-Hermitian Hamiltonian; while non-Hermitian Hamiltonians from continuous
monitoring generally describe rare dynamics (accessible by costly
post-selection), the use of noise-canceling feedback here leads to
non-Hermitian dynamics that occur deterministically. We demonstrate this via
examples of entangled state preparation and stabilization. We then illustrate
the potential for the application of noise-cancellation to boost success rates
in magic state distillation protocols. We show that adding feedback based on
noise-cancellation into a time-continuous 5-to-1 distillation protocol leads to
higher probabilities of successful distillation across a range of input errors,
and extends the threshold on input errors for which the protocol is effective.
Our results highlight the efficacy of noise-canceling feedback-aided protocols
for quantum state preparation and stabilization tasks.
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