Deterministic generation of remote entanglement with active quantum feedback

TL;DR

This paper develops and compares feedback protocols for deterministic remote entanglement of qubits without direct interaction, demonstrating feasibility with current superconducting qubit technology.

Contribution

It introduces ASLO feedback protocols that do not require real-time state estimation and provides a hybrid protocol adaptable to various measurement efficiencies.

Findings

Deterministic entanglement achievable with high-efficiency feedback.

Hybrid protocol outperforms purely semiclassical or quantum strategies.

Simulations show feasibility with current superconducting qubit setups.

Abstract

We consider the task of deterministically entangling two remote qubits using joint measurement and feedback, but no directly entangling Hamiltonian. In order to formulate the most effective experimentally feasible protocol, we introduce the notion of average sense locally optimal (ASLO) feedback protocols, which do not require real-time quantum state estimation, a difficult component of real-time quantum feedback control. We use this notion of optimality to construct two protocols which can deterministically create maximal entanglement: a semiclassical feedback protocol for low efficiency measurements and a quantum feedback protocol for high efficiency measurements. The latter reduces to direct feedback in the continuous-time limit, whose dynamics can be modeled by a Wiseman-Milburn feedback master equation which yields an analytic solution in the limit of unit measurement efficiency. Our formalism can smoothly interpolate between continuous-time and discrete-time descriptions of feedback dynamics, and we exploit this feature to then derive a superior hybrid protocol for arbitrary non-unit measurement efficiency that switches between quantum and semiclassical protocols. Finally, we show using simulations incorporating experimental imperfections that deterministic entanglement of remote superconducting qubits may be achieved with current technology using the continuous-time feedback protocol alone.