Hong–Ou–Mandel effect simulator

Send one photon into each input of a 50:50 beam splitter. If the photons are identical, they never leave through different outputs. They bunch and exit together.

What happens

Classically you would expect the two photons to exit through opposite ports half of the time. Quantum mechanically, the two ways of ending up in different ports cancel by destructive interference, leaving the states with both photons in the same port.

The HOM dip

Delay one photon relative to the other and count coincidences between the two detectors. Coincidences drop to a minimum when the photons overlap perfectly, forming a dip. Its width is set by the photon coherence length, and its depth shows how indistinguishable the photons are.

Why it matters

The effect, first measured by Hong, Ou and Mandel in 1987, is a standard test of photon indistinguishability and a building block of linear-optics quantum computing.

Try it

Open the simulator and change the relative delay and photon distinguishability to see the dip open and close.