Dispersive Hong-Ou-Mandel Interference with Finite Coincidence Windows

Hong-Ou-Mandel (HOM) interference is a fundamental tool for assessing photon indistinguishability in quantum information processing. While the effect of chromatic dispersion on HOM interference has been widely studied, the interplay between dispersion and the finite detection window of realistic measurement devices remains under-explored. In this work, we demonstrate that the rectangular coincidence window inherent to modern time-tagging modules, which effectively acts as a temporal filter, breaks the standard dispersion cancellation condition and restores sensitivity to symmetric group velocity dispersion. We derive an analytical model for type-II SPDC processes that predicts a modification of the HOM dip shape, specifically the emergence of characteristic oscillations and dip broadening. We experimentally validate this theoretical framework using a ppKTP source and transmission through optical fibers of lengths up to 29 km. The experimental data show excellent agreement with the model, confirming the presence of window-induced oscillations and allowing for the precise extraction of the fiber dispersion parameter. These findings underscore the importance of accounting for finite timing resolution in the design and characterization of dispersive quantum communication links.

💡 Research Summary

In this work the authors investigate how the finite, rectangular coincidence window employed by modern time‑tagging modules modifies Hong‑Ou‑Mandel (HOM) interference when the two photons have undergone chromatic dispersion. Starting from a general two‑photon state |ψ⟩ = ∫dt_i dt_s β(t_i,t_s)|t_i⟩⊗|t_s⟩, they introduce a relative delay τ in one arm, pass the photons through a balanced beam splitter with reflectivity η, and write the resulting two‑photon wavefunction ψ_τ(t_i,t_s) = η β(t_i,t_s‑τ) − (1‑η) β(t_s,t_i‑τ). The time‑resolved coincidence rate is obtained by imposing a δ‑function constraint σ = t_s‑t_i and integrating over a symmetric rectangular window