# Fully guided-wave photon pair source for quantum applications

## TL;DR

This paper introduces a compact, stable, fully guided-wave source of polarization-entangled photons using a lithium niobate waveguide in a Sagnac interferometer, demonstrating high-quality entanglement and versatility for quantum applications.

## Contribution

The work presents a novel guided-wave photon pair source with high entanglement quality, stability, and tunable spectrum, suitable for quantum communication and metrology.

## Key findings

- Achieved a Bell inequality violation with S=2.75 ± 0.02
- Demonstrated conversion to energy-time entanglement without postselection
- Showcased application in quantum metrology for fiber dispersion measurement

## Abstract

We report a fully guided-wave source of polarisation entangled photons based on a periodically poled lithium niobate waveguide mounted in a Sagnac interferometer. We demonstrate the source's quality by converting polarisation entanglement to postselection-free energy-time entanglement for which we obtain a near-optimal $S$-parameter of $2.75 \pm 0.02$, i.e. a violation of the Bell inequality by more than 35 standard deviations. The exclusive use of guided-wave components makes our source compact and stable which is a prerequisite for increasingly complex quantum applications. Additionally, our source offers a great versatility in terms of photon pair emission spectrum and generated quantum state, making it suitable for a broad range of quantum applications such as cryptography and metrology. In this sense, we show how to use our source for chromatic dispersion measurements in optical fibres which opens new avenues in the field of quantum metrology.