Construction and characterization of a Sagnac-based entanged-photon source

TL;DR

This paper reports the construction and detailed characterization of a high-fidelity, Sagnac-based polarization-entangled photon source, demonstrating state-of-the-art performance with significant implications for quantum technology development in Romania.

Contribution

It presents the first entangled-photon source built in Romania, utilizing a Sagnac interferometer with a PPKTP crystal, achieving high visibility and fidelity, and violating Bell inequality significantly.

Findings

Bell-CHSH violation by 22 standard deviations

Visibility up to 98.9%

Fidelity of 97%

Abstract

Entangled photons are crucial for the development of quantum technologies and especially important in quantum communications. Hence it is paramount to have a reliable, high-fidelity source of entangled photons. Here we describe the construction and characterization of a polarization-entangled photon source. We generate maximally-entangled Bell states using a type-II PPKTP nonlinear crystal inside a Sagnac interferometer. We characterize the source in terms of brightness, visibility, Bell-CHSH test and we perform quantum-state tomography of the density matrix. Our source violates Bell-CHSH inequality $|S|\le 2$ by $n=22$ standard deviations. With visibilities up to ${\cal V}= 98.9\%$ and fidelity ${\cal F}= 97\%$, our source is highly competitive, with state-of-the-art performance. To the best of our knowledge, this is the first source of entangled-photons designed and build in Romania and as such, represents an important step for the development of quantum technologies in our country. We envisage that our results will stimulate the progress of quantum technologies in Romania and will educate the future generation of quantum engineers.