A Flexible Source of Non-Degenerate Entangled Photons Based on a Two-Crystal Sagnac Interferometer

TL;DR

This paper introduces a compact, flexible source of polarization-entangled photon pairs using a two-crystal Sagnac interferometer, allowing independent control of key parameters and demonstrating multiple quantum inequality violations.

Contribution

It presents a novel, highly adjustable entangled photon source based on a two-crystal Sagnac interferometer, enabling independent tuning of wavelength, phase, and entanglement quality.

Findings

High-quality entangled states produced with independent wavelength and phase control

Experimental violations of CHSH and Leggett inequalities demonstrated

First experimental test of the 'beautiful' Bell inequality

Abstract

Sources of entangled photon pairs are a key component in both fundamental tests of quantum theory and practical applications such as quantum key distribution and quantum computing. In this work, we describe and characterize a source of polarization entangled photon pairs based on two spontaneous parametric down-conversion (SPDC) crystals in a Sagnac interferometer. Our source is compact and produces high-quality entangled states in a very flexible manner. The wavelengths of the photon pairs, around 810 and 1550 nm, the phase between orthogonal components of the entangled state, and the tangle of the state are all independently adjustable. In addition to presenting basic characterization data, we present experimental violations of CHSH and Leggett inequalities, as well as an instance of the "beautiful" Bell inequality, which has not previously been tested experimentally.