Theory of symmetry-protected two-photon coherence
Phys. Rev. A 111, 063710 – Published 24 June, 2025
DOI: https://doi.org/10.1103/gqpg-1nv3
Abstract
In a recent article [X. Lai et al., Phys. Rev. Lett. 133, 033601 (2024)], the coherence time of degenerate entangled photon pairs (biphotons) generated via backward spontaneous four-wave mixing in a cold atomic ensemble was shown to be immune to optical loss and dephasing. This finding is crucial for practical applications in quantum information processing, quantum communication, and networking, where loss is inevitable. However, in studying the underlying mechanism for this loss- and dephasing-insensitive biphoton coherence time, the previous article did not take quantum noise into account. In this work, we employ the Heisenberg-Langevin approach to study this effect and provide a rigorous theoretical proof of the symmetry-protected biphoton coherence by taking quantum noise into consideration, as compared to the perturbation theory in the interaction picture.
