Stabilizing two-qubit entanglement with dynamically decoupled active feedback

TL;DR

This paper presents a feedback protocol that stabilizes a maximally entangled state of two qubits using continuous measurement and dynamical decoupling, achieving high fidelity despite realistic noise and delays.

Contribution

It introduces a novel combined measurement and dynamical decoupling feedback scheme for entanglement stabilization that is robust against practical imperfections.

Findings

Achieves near-unit fidelity entanglement stabilization.

Effective mitigation of feedback-delay errors with forward-state estimation.

Steady state is globally attractive without ancillas, regardless of errors.

Abstract

We propose and analyze a protocol for stabilizing a maximally entangled state of two noninteracting qubits using active state-dependent feedback from a continuous two-qubit half-parity measurement in coordination with a concurrent, non-commuting dynamical decoupling drive. We demonstrate that such a drive can be simultaneous with the measurement and feedback, while also playing a key part in the feedback protocol itself. We show that robust stabilization with near-unit fidelity can be achieved even in the presence of realistic nonidealities, such as time delay in the feedback loop, imperfect state-tracking, inefficient measurements, dephasing from $1/f$-distributed qubit-frequency noise, and relaxation. We mitigate feedback-delay error by introducing a forward-state-estimation strategy in the feedback controller that tracks the effects of control signals already in transit. More generally, the steady state is globally attractive without the need for ancillas, regardless of the error state, in contrast to most known feedback and error correction schemes.