Hong-Ou-Mandel Photon Bunching (2D)

Two-Photon Interference (top-down)

Photon delay Δt

Arrival mismatch 0 fs

Indistinguishability

Visibility V (polarization / spectral overlap) 100%

Wavepacket width (coherence time) 80 fs

Fire rate 6 /s

Live Statistics

Pairs fired

Coincidences

Measured P(coinc)

—

Theory P(coinc)

0.0%

Last event

—

Two identical photons enter a 50:50 beamsplitter from opposite sides (top-down view). If they are perfectly indistinguishable and arrive at exactly the same instant, quantum interference forces them to always exit together (bunch) — the two detectors never click at once. Slide the delay or reduce visibility to make the photons distinguishable again and watch coincidences reappear: this is the Hong–Ou–Mandel dip, the core test of photon-source quality for photonic qubits.

How this maps to photonic qubits

A dual-rail photonic qubit gate needs its two photons to be quantum-mechanically identical in every degree of freedom except the one carrying information. The coincidence probability follows:

P(Δt) = ½·[1 − V·exp(−(Δt/τ)²)]

Δt — timing mismatch between the two arms (path-length / time-bin error)
τ — photon coherence time (set by source bandwidth)
V — visibility: 1 means perfectly indistinguishable, 0 means fully distinguishable (e.g. crossed polarizations)

At Δt=0 with V=1 the dip bottoms out at P=0 — perfect bunching, zero coincidences. This is exactly the interference measurement used to certify single-photon sources (SPDC, quantum dots) before wiring them into a linear-optical quantum circuit.

Photon A Photon B Detector click