Certification of linear optical quantum state preparation

TL;DR

This paper introduces a fidelity measure and witnessing methods for certifying the quality of multi-photon states in linear optical quantum computing, addressing the challenge of photon indistinguishability.

Contribution

It proposes a new fidelity measure and experimental witnessing techniques tailored for photonic quantum states, improving certification in linear optical quantum platforms.

Findings

Developed a fidelity measure suitable for indistinguishable photons

Implemented an optimal witness based on the discrete Fourier transform

Successfully certified multi-photon states experimentally

Abstract

Certification is important to guarantee the correct functioning of quantum devices. A key certification task is verifying that a device has produced a desired output state. In this work, we study this task in the context of photonic platforms, where single photons are propagated through linear optical interferometers to create large, entangled resource states for metrology, communication, quantum advantage demonstrations and for so-called linear optical quantum computing (LOQC). This setting derives its computational power from the indistinguishability of the photons, i.e., their relative overlap. Therefore, standard fidelity witnesses developed for distinguishable particles (including qubits) do not apply directly, because they merely certify the closeness to some fixed target state. We introduce a measure of fidelity suitable for this setting and show several different ways to witness it, based on earlier proposals for measuring genuine multi-photon indistinguishability. We argue that a witness based upon the discrete Fourier transform is an optimal choice. We experimentally implement this witness and certify the fidelity of several multi-photon states.