# Ultra-broadband SPDC for spectrally far separated photon pairs

## TL;DR

This paper demonstrates the existence of phase-matching conditions in certain nonlinear crystals that produce ultra-broadband, widely non-degenerate photon pairs, enabling new applications in quantum optics and spectroscopy.

## Contribution

It introduces a novel broadband phase-matching method in quasi-phase matched crystals for generating widely non-degenerate photon pairs with large bandwidths.

## Key findings

- Achieved SPDC bandwidths between 15 and 25 THz in specific crystals.
- Experimentally demonstrated broadband phase-matching in ppKTP at 800 nm and 3800 nm.
- Produced photon pairs with a measured bandwidth of 15 THz in experiments.

## Abstract

Generating photon pairs via spontaneous parametric down-conversion (SPDC) in nonlinear crystals is important for a wide range of quantum optics experiments with spectral properties such as their bandwidths often being a crucial concern. Here, we show the generic existence of particular phase-matching conditions in quasi-phase matched KTP, MgO:LN and SLT crystals that lead to ultra-broadband, widely non-degenerate photon pairs. It is based on the existence of group-velocity matched, far apart wavelength pairs and for 2 mm long crystals results in SPDC bandwidths between 15 and 25 THz (FWHM) for photon pairs with the idler photon in the technologically relevant mid-IR range 3-5 {\mu}m and the signal photon in the NIR below 1100 nm. We experimentally demonstrate this type of broadband phase-matching in ppKTP crystals for photon pairs centered at 800 nm and 3800 nm and measure a bandwidth of 15 THz. This novel method of generating broadband photon-pairs will be highly beneficial for SPDC-based imaging, spectroscopy, refractometry and OCT with undetected mid-IR photons.