A Temperature‐Insensitive Optical Fiber Delay for Indistinguishability‐Preserving Multiplexing of Photons

Abstract Quantum state purity, namely indistinguishability, of photons having different propagation histories is a prerequisite of multiphoton quantum information processing such as quantum networking and quantum computing. Although photonic multiplexing techniques using optical fiber delays can enhance the scalability of quantum states, the sensitivity of optical fibers to ambient temperature fluctuations deteriorates the indistinguishability of photons. This work demonstrates a novel optical differential delay using polarization‐maintaining fibers, which suppresses temperature dependence by balancing thermal expansion with modal dispersion in a fully passive mechanism. Time‐multiplexed Hong–Ou–Mandel interferometry under temperature variations from 24 to 50 °C verifies the effectiveness of the devised structure in a practical quantum photonics environment.