Revisiting Quantum Feedback Control: Disentangling the Feedback-induced Phase from the Corresponding Amplitude

Coherent time-delayed feedback allows the control of a quantum system and its partial stabilization against noise and decoherence. The crucial and externally accessible parameters in such control setups are the round-trip-induced delay time $τ$ and the frequencies $ω$ of the involved optical transitions which are typically controllable via global parameters like temperature, bias or strain. They influence the dynamics via the amplitude and the phase $ϕ= ωτ$ of the feedback signal. These quantities are, however, not independent. Here, we propose to control the feedback phase via a microwave pump field. Using the example of a $Λ$-type three-level system, we show that the Rabi frequency of the pump field induces phase shifts on demand and therefore increases the applicability of coherent quantum feedback control protocols.

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