Locally optimal measurement-based quantum feedback with application to multi-qubit entanglement generation

TL;DR

This paper introduces a measurement-based quantum feedback method using proportional and quantum state-based components to efficiently generate high-fidelity multipartite entanglement, including remote entanglement, in multi-qubit systems.

Contribution

It develops a general PaQS feedback framework that reproduces known protocols and applies it to create various multi-qubit entangled states with high fidelity, even for large N.

Findings

Perfect measurement efficiency yields near-unity fidelity entangled states.

Markovian protocols achieve fidelities above 94%.

High-fidelity entanglement generation demonstrated for up to 100 qubits.

Abstract

We present a general approach to measurement-based quantum feedback that employs proportional and quantum state-based (PaQS) feedback components to obtain locally optimal protocols. To demonstrate the power of the method, we first show that it reproduces many known feedback protocols, and then apply it to generation of multipartite entanglement with an emphasis on remote entanglement, which requires spatially local feedback Hamiltonians. The symmetry of both measurement and feedback operators is found to be essential for construction of effective protocols. We show that under perfect measurement efficiency, entangled states can be reached with fidelity approaching unity under non-Markovian feedback control protocols, while Markovian protocols resulting from optimizing the feedback unitaries on ensemble averaged states still yield fidelities above 94%. Application of the PaQS approach to generation of N-qubit W, general Dicke and GHZ states shows that such entangled states can be efficiently generated with high fidelity, for up to N = 100 in some cases.