Curriculum / Quantum Networking / Satellite Quantum Key Distribution
Satellite Quantum Key Distribution
Explore how satellite QKD overcomes fiber distance limits and model the Micius satellite experiment.
Satellite Quantum Key Distribution
Fiber-based QKD is limited to ~300 km without quantum repeaters. Satellites offer a path to global QKD coverage: at high altitudes, the atmosphere is thin, and free-space optical channels can reach thousands of kilometers. China's Micius satellite demonstrated intercontinental QKD in 2017, validating satellite-based quantum networking as a practical approach.
Why Satellites Extend QKD Range
Fiber loss is 0.2 dB/km: exponential over long distances. Free-space loss follows a different model: it scales with distance squared (beam divergence) rather than exponentially. More importantly, most of the atmosphere (and thus most atmospheric absorption and turbulence) is below 20 km. A satellite link traverses:
- •~10-20 km of dense atmosphere (high loss, turbulence)
- •~500+ km of near-vacuum (minimal loss)
The effective path loss for a satellite at 500 km altitude is comparable to 10-50 km of fiber: much more manageable than 500 km of fiber (100 dB of loss).
The Micius Satellite
Launched in 2016 by the Chinese Academy of Sciences, Micius is a 635 kg low-earth-orbit (LEO) satellite at 500 km altitude. Its quantum payload includes:
- •A polarization-entangled photon pair source (using SPDC)
- •A decoy-state laser source for BB84 QKD
- •A clock synchronization payload
- •Telescopes for optical uplink and downlink
Key demonstrations:
- •QKD at 1200 km: Between Xinglong and Urumqi ground stations (2017)
- •Entanglement distribution at 1200 km: Bell violation at intercontinental distances (2017)
- •Intercontinental video call: Vienna-Beijing encrypted with quantum key (2017)
Link Budget Analysis
The secret key rate of a satellite QKD link depends on:
Channel transmissivity: Determined by telescope aperture, pointing accuracy, beam divergence, atmospheric turbulence, and slant path through the atmosphere.
Secret key rate:
This is the opening of the lesson. The full walkthrough, the interactive circuit, and the graded challenge continue inside myqubit.
How this lesson works
A hands-on coding challenge. You write Qiskit-compatible Python in the browser editor, run it instantly via WebAssembly, watch the circuit and Bloch sphere react, and pass automatic output checks. The AI tutor Qubitus gives Socratic hints if you get stuck.
Part of: Quantum Networking
Build quantum networks from the ground up, entanglement distribution, quantum repeaters, the quantum internet, and satellite-based QKD.
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