Micius (Mozi) satellite quantum-communication mission

What it is

Micius carries three optical payloads. An SPDC source pumps a type-II BBO crystal to generate polarisation-entangled photon pairs at ~810 nm; two independent transmit telescopes downlink the two halves of each Bell pair to a pair of ground stations during a single overhead pass. A separate decoy-state BB84 transmitter sends weak coherent pulses for prepare-and-measure QKD. A receive telescope plus single-photon detectors close the loop for ground-to-satellite teleportation experiments. All three modes use the same fundamental channel — a free-space optical link through the atmosphere, with the satellite tracking the ground station for the duration of a usable pass. ^{Yin et al. 2017}

The 2020 entanglement-based QKD result is the architecturally distinctive one. The satellite plays the role of a midpoint Bell pair source between two ground stations 1,120 km apart, and the security argument rests on the entanglement itself rather than on trust in the satellite. The 4,600 km figure, by contrast, is assembled by chaining the satellite with the Beijing–Shanghai fibre trunk under a trusted-node model — the satellite is one more trusted hop, not an entanglement source for the whole span. ^{Yin et al. 2020}

Published experiments

The landmark results published from the mission, chronological. Secret key? marks whether the run produced shared key material rather than only moving or certifying quantum states. Trust breaks the security assumption down by the device it rests on — the relay node N that forwards a classical key, the source S that emits the quantum states, and the detector D that measures them. Entanglement-based QKD removes trust in the source; measurement-device-independent (MDI) QKD would remove trust in the detector, and device-independent (DI) QKD removes both. None of the Micius links are MDI or DI, so their receivers are always trusted. ^{Lu et al. 2022}

Every QKD link here also rests on assumptions no Micius experiment relaxes: the endpoints' own apparatus (Alice's and Bob's secure labs), the random-number source that picks bases and states, and an authenticated classical channel. Those stay trusted throughout; the node / source / detector split is only the part that varies across the experiments.

The Bell violation in the 2017 entanglement-distribution result is a one-time certification of the channel, not part of any key protocol. The 2020 BBM92 link runs no Bell test during operation — its security rests on the QBER staying below the secret-key threshold, with the entanglement removing the need to trust the source. The 2012 (100 km free-space teleportation) and 2013 (ground feasibility) demonstrations predate the 2016 launch and were not flown on Micius; they are out of scope here.

Verified claims

• 1,200 km satellite-to-two-ground-station Bell pair distribution, Delingha and Lijiang — polarisation entanglement preserved through the dual downlink with a Bell-inequality violation of 2.37σ to 4σ. ^{Yin et al. 2017}
• SPDC entangled-photon source on the satellite — type-II BBO crystal, ~810 nm, two independent transmit telescopes feeding the two ground stations. ^{Yin et al. 2017}
• 1,120 km entanglement-based QKD between Delingha and Nanshan — satellite as midpoint source; security argument independent of trust in the satellite. ^{Yin et al. 2020}
• Free-space optical channel, ~810 nm downlink — chosen for atmospheric transmission and detector quantum efficiency; not telecom-band.
• Operational mission since 2016, multiple follow-up experiments published — including ground-to-satellite teleportation (Ren et al., Nature 2017) ^{Ren et al. 2017} and the integrated 4,600 km space-ground demonstration. ^{Chen et al. 2021}

Things to note

• One satellite, one pass at a time. Useful link time per ground station is a few minutes per overhead pass; average key throughput over a day is bit/s to kbit/s class.
• Operating conditions for the free-space downlink. The standard operating window is clear-sky night passes over the participating ground stations; daylight QKD has been demonstrated at lower rates.
• The 4,600 km figure is trusted-node-assisted. It chains Micius with the Beijing–Shanghai fibre trunk, with the satellite acting as a trusted relay for the inter-segment hop. The trust model matches the terrestrial backbone rather than end-to-end entanglement.
• The 2020 entanglement-based QKD removes the trusted-satellite assumption for the link layer at bit/s-class secure-key rates. The result establishes the architecture at demonstration scale.
• Distinct system from the Beijing–Shanghai trunk. Micius is a separate physical platform with a separate vendor stack — see the china-backbone entry for the terrestrial side.

Related entries

• Entanglement distribution — free-space optical as a third channel alongside fibre and matter-qubit links.
• Applications — QKD as the primary use case for the Micius downlinks.
• Links — the free-space loss budget that LEO downlinks have to close.
• Beijing–Shanghai QKD backbone — the terrestrial side of the integrated 4,600 km space-ground network.
• Companies — the Chinese vendor ecosystem (QuantumCTek and others) that supplies the ground-segment hardware.