Filtering Noise and Photon Source Engineering for Noise-Resilient Nonlocal Multiphoton Quantum Applications

Filtering around photonic signals is the foundation of performing quantum optical applications in real-world noisy environments (deployed fiber, free-space, unideal devices, etc.). However, filtering noise photons across multiphoton/multidetector systems (entangled photons, quantum teleportation, and entanglement swapping) introduces more complex physics and has been less explored compared to single-detector filtering. Here, we discuss source-filter design considerations for noisy multiphoton applications including realistic imperfections such as reduced heralding efficiency when filtering a photon-pair source's joint spectrum, multiphoton emission during pair generation, and the single-mode purity of the filtered photons for enabling Hong-Ou-Mandel interference/Bell state measurements between independent sources. It is shown that filtering into a source's joint spectrum limits the ability to improve fidelity due to a lower single-photon number purity at a given coincidence rate. As a case study, we examine filtering for quantum networking over noisy optical fibers carrying classical communications, showing how optimization can enable multiphoton applications coexisting with mW-level classical power in C-band wavelengths and 100's of mW when using the O-band.