Experimental Free-Space Distribution of Entangled Photon Pairs Over 13 km: Towards Satellite-Based Global Quantum Communication

We report free-space distribution of entangled photon pairs over a noisy ground atmosphere of 13 km. It is shown that the desired entanglement can still survive after both entangled photons have passed through the noisy ground atmosphere with a distance beyond the effective thickness of the aerosphere. This is confirmed by observing a spacelike separated violation of Bell inequality of $2.45\pm 0.09$. On this basis, we exploit the distributed entangled photon source to demonstrate the Bennett-Brassard 1984 quantum cryptography scheme. The distribution distance of entangled photon pairs achieved in the experiment is for the first time well beyond the effective thickness of the aerosphere, hence presenting a significant step towards satellite-based global quantum communication.

Figure 1

Schematic diagram of locations in our experiment. The source of entangled photons is located at the foot of a high television tower, on the top of Dashu Mountain. Alice is located on the west campus of USTC, and Bob is located at Feixi, a county of Hefei city. Photons from the sender to the receivers experience a noisy city environment. Therefore, strongly influenced by the air pollution and noisy background lights, even after rain with less air pollution, the background count rate can reach about 30 000 per second at night without using interference filters.

Figure 2

Block diagram of the experiment and optical setup at the receivers. As shown in the left part, we combine the entangled photons with the synchronous pulsed laser beam utilizing a dichroic mirror (DM) at the sender and separate them at each receiver with a DM. Then it follows with single-photon polarization analysis and time synchronization. In the lower part, a beam splitter (BS) is used to achieve random basis selection, and a half-wave plate (HWP) together with a polarization beam splitter (PBS) makes up an apparatus for polarization measurement. Interference filters (IF) are used to get rid of noisy background light.

Figure 3

Verification of the distributed quantum entanglement. In order to test the quality of the entangled state between the two receivers, we measured the coincident count as a function of Alice’s polarization angle. Four curves correspond to four polarization angles $(H,V,+45,-45)$ set at Bob. The data are best fitted with sin functions, showing that the visibility is 94% in the $H/V$ basis, and it is 89% in the $+45/-45$ basis. The average visibility has reached 91%, which is far beyond the threshold required for a violation of the Bell inequality.