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Free-space quantum key distribution (QKD) uses an optical beam through air or space to help two endpoints create a shared secret key. They send and measure quantum states, then use an authenticated classical connection to compare results, check for errors, and distill the key. QKD distributes key material; it does not carry or encrypt the user’s message by itself. The optical path, the classical channel, and the equipment at both ends all matter to security and performance.
What happens during a free-space QKD exchange?
At a high level, a free-space QKD system has an optical transmitter and a receiver with a clear line of sight, plus a conventional communications channel for coordination. In a prepare-and-measure protocol such as BB84, the transmitter chooses a random bit and encoding basis, then prepares an optical state representing that choice. Practical systems can encode states in weak optical pulses. The receiver measures each arriving pulse using a randomly chosen basis.
After the optical exchange, the endpoints communicate over an authenticated classical channel. They disclose which bases they used, keep compatible detection events, estimate the error rate, and apply error correction and privacy amplification to produce a shared key. This is a high-level description, not an implementation specification; the exact steps and security assumptions depend on the protocol and system.
The security intuition is that measuring unknown, non-orthogonal quantum states to learn information generally disturbs them, and that disturbance can show up in the endpoints’ error checks. A security proof applies to a defined protocol and model. It is not a blanket guarantee that every device, installation, or endpoint is secure.
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Why use a free-space optical link?
Instead of travelling through a glass fibre, the quantum signal travels through an atmospheric or space path. Telescope-like transmit and receive optics form and collect a narrow beam. The receiver must gather enough arriving light and couple it into its detector; acquisition and tracking systems must also keep the endpoints aligned. Those needs become especially demanding when one endpoint is moving.
Free-space transmission can connect ground stations to satellites, and photons on a satellite path can spend much of the journey above the atmosphere. But the link still faces loss, atmospheric effects, pointing constraints, and pass geometry. Free-space QKD is therefore an optical-link engineering problem as well as a protocol problem.
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Satellite uplinks and downlinks are not equivalent
| Link direction | What makes it distinct | Key considerations |
|---|---|---|
| Downlink: satellite to ground | Most of the path is above the atmosphere before the signal reaches the denser air near the receiver. | Diffraction and the limited ground receiver aperture can be major sources of loss in modeled scenarios; pointing, turbulence, atmospheric extinction, and background light still matter. |
| Uplink: ground to satellite | The beam crosses the turbulent lower atmosphere while it is still relatively narrow, before continuing toward space. | Turbulence can add substantial loss in modeled scenarios, alongside diffraction, pointing errors, extinction, and fading. |
These are scenario-dependent engineering observations, not universal numerical rules. Satellite altitude, viewing angle, wavelength, aperture, weather, platform motion, and the chosen optical system all affect the result.
What degrades the optical signal?
- Diffraction and geometric spreading: the beam expands as it travels, so the receiver collects only a fraction of the transmitted light.
- Pointing error and platform motion: a narrow beam is sensitive to misalignment. Acquisition and tracking must keep the beam on target; residual errors can cause loss and time-varying fading.
- Atmospheric turbulence: variations in air’s refractive index can distort or wander the beam and make transmission fluctuate, with the lower atmosphere particularly relevant.
- Atmospheric extinction: absorption and scattering reduce transmission, with effects that depend on the path and wavelength.
- Background light: daylight and sky radiance can produce unwanted detector events, potentially increasing the error rate unless collection optics, filtering, detector noise, or timing are managed.
Can QKD work in daylight?
Yes, daylight operation has been demonstrated, though it is an engineering challenge rather than a guarantee that any free-space system will work in bright conditions. In a 2017 experiment reported in Nature Photonics, researchers demonstrated a 1550 nm free-space QKD link over 53 km during the day, with approximately 48 dB total channel loss. The authors presented it as a feasibility demonstration for daylight satellite-based quantum communication, not as an operational satellite service.
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A 2021 field experiment reported in Physical Review Applied examined conditions representative of daytime space-to-Earth links. Higher-order adaptive optics improved quantum-channel efficiency beyond tip-tilt correction alone, while diffraction-limited spatial filtering rejected optical noise without requiring an ultranarrow spectral filter. These techniques address important impairments, but their value depends on the link and hardware.
Which QKD protocol does a satellite link use?
Satellite QKD is not one protocol. A 2026 review in Advanced Quantum Technologies surveys discrete-variable and continuous-variable approaches, as well as prepare-and-measure, entanglement-based, measurement-device-independent, and twin-field designs. It assesses discrete-variable decoy-state BB84 on low Earth orbit downlinks as the most mature near-term option in that review. Treat that as the review’s assessment, not a universal ranking or a claim that other approaches are unusable.
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Protocol choice changes what the system must implement and what security assumptions it relies on. Comparing designs by headline distance alone misses important differences in channel geometry, hardware, data volume, and trust model.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What determines whether a link is practical and secure?
A sound comparison needs to account for both achievable key generation and the system’s security and operating assumptions. For a specific proposal, check:
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- Protocol and security model: which protocol family is used, what its proof assumes, and how the implementation is assessed.
- Link geometry: uplink or downlink, satellite altitude, viewing angle, pass duration, and whether the required ground stations can see a relay at the same time.
- Optical channel: wavelength, transmitter and receiver apertures, atmospheric attenuation, turbulence, pointing error, background radiance, and whether fading is included in the analysis.
- Hardware and mitigation: source and detector performance, acquisition and tracking, optical and spatial filtering, mode coupling, and any adaptive optics.
- Operational security: finite data size, authenticated classical communication, endpoint and device implementation, and whether a satellite or relay is trusted.
Satellite-link analyses distinguish modeled channel performance from practical security. As discussed in the 2021 Physical Review Research study of fundamental bounds and practical security, channel effects and finite-size data can affect achievable rates. A protocol-level security proof alone does not establish that every deployed component behaves as modeled or that the overall network meets a particular trust requirement.
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