Polarization-entanglement-conserving frequency conversion of photons

Entangled photons play a pivotal role in the distribution of quantum information in quantum networks. However, the frequency bands for optimal transmission and storage of photons are often not the same. Here, we experimentally demonstrate the coherent frequency conversion of photons entangled in their polarization, a widely used degree of freedom in photonic quantum information processing. We verify the successful entanglement conversion by violating a Clauser-Horne-Shimony-Holt (CHSH) Bell inequality and fully characterize our near-perfect entanglement transfer using both state and process tomography. Our implementation is robust and flexible, making it a practical building block for future quantum networks.