Satellite Qkd Micius

Short answer: In 2017, papers in Nature, Science and Physical Review Letters reported experiments with the Micius low-Earth-orbit satellite that carried out decoy-state quantum key distribution over up to 1,200 kilometres, distributed entangled photons to two ground stations 1,203 kilometres apart, and ran entanglement-based QKD at a final key rate of 3.5 bits per second. These are research demonstrations with low key rates, not a general commercial service.

Why a satellite

The Nature paper says QKD has been limited to a few hundred kilometres because channel loss in optical fibres or terrestrial free space exponentially reduces the photon transmission rate. It says satellite-based QKD has the potential to help establish a global-scale quantum network, owing to the negligible photon loss and decoherence experienced in empty space. The Science paper says that before this, free-space entanglement demonstrations were limited to line-of-sight links across cities or between mountaintops, with link separations around 100 km because of scattering and coherence decay.

Decoy-state QKD from orbit

The Nature paper reports the development and launch of a low-Earth-orbit satellite for decoy-state QKD, a form that uses weak coherent pulses at high channel loss and is secure because photon-number-splitting eavesdropping can be detected. The authors report a key rate in the kilohertz range from the satellite to the ground over a distance of up to 1,200 kilometres, which they say is around 20 orders of magnitude greater than expected using an optical fibre of the same length. They describe this as paving the way to global-scale quantum networks. That comparison is the authors' own, against a hypothetical fibre of equal length.

Entanglement distribution

The Science paper reports satellite-based distribution of entangled photon pairs to two locations on Earth separated by 1,203 kilometres, through two satellite-to-ground downlinks with a summed length varying from 1,600 to 2,400 kilometres. The authors observed that two-photon entanglement survived and violated a Bell inequality by 2.37 plus or minus 0.09 under strict Einstein locality conditions. They say the effective link efficiency is orders of magnitude higher than direct transmission of the two photons through telecommunication fibres.

Entanglement-based QKD

The Physical Review Letters paper reports entanglement-based QKD between the satellite, which carries a space-borne entangled-photon source, and a ground observatory. One photon is measured at the satellite and the other is sent down to the receiver at the Delingha ground station. It measured link attenuation varying from 29 dB at 530 km to 36 dB at 1,000 km, and a two-photon entanglement state fidelity of at least 0.86. The authors obtained an average final key rate of 3.5 bits per second over 530 to 1,000 km.

What this does not tell you

These abstracts describe experiments from 2017. They do not describe later systems, and they do not say these links are available as a service. A key rate of a few bits per second is small, and the kilohertz figure applies to a different protocol and conditions. Each result depends on the particular satellite pass and ground stations.

FAQ

Why use a satellite for QKD?

Loss in optical fibre grows exponentially with length, while the Nature paper says photon loss in empty space is negligible.

How far did it work?

Decoy-state QKD over up to 1,200 km, and entanglement distribution to stations 1,203 km apart.

What key rates were reported?

A kilohertz-range key rate for decoy-state QKD, and 3.5 bits per second for entanglement-based QKD.

Is this a commercial network?

These papers are research demonstrations and do not describe a service.

Was entanglement preserved?

Yes. The Science paper reports a Bell inequality violation of 2.37 plus or minus 0.09, and the PRL paper a fidelity of at least 0.86.