How to use the dispersion in the χ ( 3 ) tensor for broadband generation of polarization-entangled photons
Valeria Vento, Francesco Ciccarello, Sakthi Priya Amirtharaj, Christophe Galland · Physical Review Research · 2025
Polarization-entangled photon pairs are a widely used resource in quantum optics and technologies and are often produced using a nonlinear process. Most sources based on spontaneous parametric down-conversion have relatively narrow optical bandwidths because the pump, signal, and idler frequencies must satisfy a phase-matching condition. Extending the bandwidth—for example, to achieve spectral multiplexing—requires changing some experimental parameters such as temperature, crystal angle, poling period, etc. Here we demonstrate broadband (tens of THz for each photon) generation of polarization-entangled photon pairs by spontaneous four-wave mixing in a diamond crystal, with a simple colinear geometry requiring no further optical engineering. Our approach leverages the quantum interference between electronic and vibrational contributions to the χ ( 3 ) tensor. Entanglement is characterized by a single realization of a Bell test over the entire bandwidth using fiber dispersion spectroscopy and fast single-photon detectors. The results agree with the biphoton wave function predicted from the knowledge of the χ ( 3 ) and Raman tensors and demonstrate the general applicability of our approach to other crystalline materials.