# Coherence and Entanglement Preservation of Frequency-Converted Heralded Single Photons

## TL;DR

This paper demonstrates quantum frequency conversion of entangled photons from near-infrared to telecom wavelengths while preserving their coherence and entanglement, enabling better integration into quantum communication networks.

## Contribution

It introduces a method for converting entangled photons to telecom wavelengths with high coherence preservation and verifies entanglement through Bell inequality violation.

## Key findings

- 8% overall conversion efficiency achieved
- Preservation of first and second order coherence confirmed
- Bell inequality violated with 88% visibility

## Abstract

We report on quantum frequency conversion of near-infrared photons from a wavelength of 854nm to the telecommunication O-band at 1310nm with 8% overall conversion efficiency. Entangled photon pairs at 854nm are generated via type-II spontaneous parametric down conversion. One photon is mixed with a strong pump field in a nonlinear ridge waveguide for its conversion to 1310nm. We demonstrate preservation of first and second order coherence of the photons in the conversion process. Based on this we infer the coherence function of the two-photon state and compare it with the actual measured one. This measurement demonstrates preservation of time-energy entanglement of the pair. With 88% visibility we violate a Bell inequality.