Deterministic preparation of optical qubits with coherent feedback control

TL;DR

This paper introduces a novel method for preparing optical qubits using coherent feedback control with linear optics, enabling efficient and arbitrary state transfer without spatial interferometers.

Contribution

It presents a new class of preparation schemes for optical qubits that use coherent feedback control, allowing arbitrary state transcription and unit efficiency independent of initial states.

Findings

Preparation schemes work with unit efficiency

Arbitrary polarisation states can be transcribed onto OAM systems

Measurement-based control channels are translated into coherent feedback schemes

Abstract

We propose a class of preparation schemes for orbital angular momentum and polarisation qubits carried by single photons or classical states of light based on coherent feedback control by an ancillary degree of freedom of light. The preparation methods use linear optics and include the transcription of an arbitrary polarisation state onto a two-level OAM system (swap) for arbitrary OAM values plus/minus "l" within a light beam, i.e. without spatial interferometer. The preparations can be carried out with unit efficiency independent from the potentially unknown initial state of the system. Moreover, we show how to translate measurement-based qubit control channels into coherent feedback schemes for optical implementation.