Chang Liu

Hong-Ou-Mandel interference is essentially a quantum mechanical phenomenon, which is a key part in quantum communication protocols and linear optical quantum computation, especially for measurement-device-independent quantum key distribution. The main challenge of the two-photon interference is to keep the indistinguishability of the two photons. This requirement becomes much more crucial on a large scale and even in free space, where both fluctuations and photon loss overwhelm the fragile interference phenomenon. Here we report a field test of the Hong-Ou-Mandel interference with one arm being a 220 m free-space path. Our experiment is a step towards long distance free-space measurement-device-independent quantum key distribution.