Satellite-to-Ground Entanglement-Based Quantum Key Distribution

We report on entanglement-based quantum key distribution between a low-Earth-orbit satellite equipped with a space borne entangled-photon source and a ground observatory. One of the entangled photons is measured locally at the satellite, and the other one is sent via a down link to the receiver in the Delingha ground station. The link attenuation is measured to vary from 29 dB at 530 km to 36 dB at 1000 km. We observe that the two-photon entanglement survives after being distributed between the satellite and the ground, with a measured state fidelity of $\ge 0.86$. We then perform the entanglement-based quantum key distribution protocol and obtain an average final key rate of $3.5\mathrm{bits}/\mathrm{s}$ at the distance range of 530–1000 km.

Figure 1

Illustration of the experimental setup. (a) Overview of the satellite-to-ground entanglement-based quantum key distribution. (b) The measurement module in the satellite. About 1% of the idler photons are reflected by the sampling mirror (SM) to be measured in the satellite. (c) Schematic of the space borne entangled photon source. (d) The measurement module in the ground station. Beam splitter (BS); polarization beam splitter (PBS); beam expander (BE); Wollaston prism (WP); interference filter (IF); half-wave plate (HWP); quarter-wave plate (QWP); dichroic mirror (DM); fast-steering mirror (FSM); piezo steering mirror (PI); periodically poled ${\mathrm{KTiOPO}}_4$ (PPKTP).

Figure 2

The characterization of the satellite-to-ground optical link. (a) The polarization visibilities using the calibration lasers in four specific states. (b) The attenuation of the down-link channel with different distances between the satellite and the ground. These diagrams show that the overall polarization visibilities and efficiency are sufficient to implement the satellite-to-ground QKD.

Figure 3

Measurement of the entangled photons after traveling the down-link channel. (a) Normalized two-photon coincidence counts in the $|H⟩/|V⟩$ basis. (b) Normalized two-photon coincidence counts in the $|+⟩/|-⟩$ basis.

Figure 4

Performance of the satellite-to-ground entanglement-based QKD during one orbit. The typical distance from the satellite to the ground station varies from 530 to 1000 km. (a) The coincident counts with different distances between the satellite and the ground. (b) The overall QBER with different distances between the satellite and the ground.