Satellite Quantum Links
Satellite quantum links use free-space paths to distribute quantum signals over geographic distances difficult for direct fiber transmission. The research question is which orbit, optical design and protocol delivers a useful service under realistic visibility, loss and background. A successful pass is evidence of a link; it is not continuous worldwide availability.
Required background. Communication with quantum systems supplies the channel language; quantum key distribution sets the security task.
Geometry and availability enter the quantum budget
Section titled “Geometry and availability enter the quantum budget”Diffraction, pointing, atmospheric turbulence, absorption, receiver coupling and detection all affect transmission. The two downlinks of an entangled-pair source contribute jointly to coincidence probability. A crude rate model is
before background, dead time and protocol filtering. Integrate useful outcomes over a pass and include unavailable passes when quoting service per day. Weather correlations between sites can matter as much as average clear-sky probability.
Trusted satellite key relays and entanglement-based links impose different security assumptions. A ground separation, satellite slant range and optical path length should never be reported interchangeably.
Established demonstration and remaining questions
Section titled “Established demonstration and remaining questions”Yin and colleagues’ 2017 experiment distributed entangled photons to ground stations separated by more than 1,200 km and tested their correlations. This is established experimental evidence for long-distance satellite entanglement distribution. It does not by itself establish an on-demand quantum-memory network or a universal device-independent service. Primary report.
Active: daylight operation, robust acquisition and pointing, finite-key yields, interoperability with memories and service under intermittent coverage. Conjectural: a general economic or scientific advantage over terrestrial alternatives without a matched application and availability model. The repeater review provides the broader architectural comparison. Review.
Open project choices
Section titled “Open project choices”Can scheduling improve usable key or entanglement yield across competing ground stations? Which hardware improvement best increases accepted pairs after background rejection? Can temporal or spectral filtering improve fidelity without sacrificing all rate? What storage lifetime is needed to join non-simultaneous passes?
Graduate project: a pass-level link budget
Section titled “Graduate project: a pass-level link budget”Use a documented synthetic orbit or a public orbit record with its epoch to calculate slant range and visibility for two ground stations. Combine a simple diffraction model with declared atmospheric and detector parameters. Deliver pass-integrated coincidence yield, uncertainty bands and a sensitivity ranking. A laptop and standard orbital geometry are sufficient.
Compare with a constant-loss baseline. Failure tests perturb pointing jitter, add daytime background, correlate cloud outages and count all optical modes. Label synthetic weather as an assumption, not field evidence. Research begins with validated atmospheric data or a protocol improvement that increases a finite-size application yield under an independently tested link model.
Entry points and evidence scope
Section titled “Entry points and evidence scope”Read the primary experiment before comparing satellite and repeater architectures. Literature checked on 11 October 2026; selected evidence is the 2017 demonstration and 2023 review, not the latest launch or distance census.