Free-space optical quantum key distribution (QKD) uses a beam of light to exchange quantum states between aligned transmitters and receivers, then uses an authenticated classical connection to turn their measurements into a shared secret key. QKD distributes key material; it does not carry or encrypt the user’s message by itself. In an air or satellite link, alignment, light loss, turbulence and background light all affect whether enough usable detections arrive.
What happens during a free-space QKD exchange?
A free-space QKD link needs optical equipment with a clear line of sight: a transmitter prepares light states, and a telescope-like receiver collects them and measures them. The quantum optical path carries those states. A separate, conventional classical connection lets the endpoints coordinate and process the results.
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From prepared states to a shared key
In a high-level example using the BB84 protocol, the sender chooses a random bit and a random encoding basis for each signal, then prepares the corresponding optical state. Practical systems can encode states in weak optical pulses. The receiver measures each arriving pulse using a randomly chosen basis.
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Once the optical exchange is over, the endpoints disclose their basis choices over an authenticated classical channel. They keep the events where their bases were compatible, estimate the error rate, and use error correction and privacy amplification to derive a shared secret key. The key can then be used by a separate encryption system; QKD itself is not the message-encryption step.
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Why disturbance matters—and what it does not guarantee
The security intuition is that an eavesdropper cannot generally learn information about unknown, non-orthogonal quantum states without causing detectable disturbance. That statement depends on the chosen protocol and its security model. It is not a blanket promise that every device, implementation or end-to-end system is invulnerable. Authentication is still necessary, and endpoint security remains important.
What makes an optical link through air or space different?
Unlike a fibre link, a free-space signal travels through the atmosphere or a space path. The beam spreads as it travels, so only a fraction may enter the receiver’s aperture. The optical terminals must also acquire and track each other: even small pointing errors can reduce the collected signal or make the channel fluctuate.
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Downlink and uplink trade-offs
In a satellite downlink, the signal travels through much of its path outside the atmosphere before reaching a ground receiver. Diffraction and the receiver telescope’s limited aperture are important sources of loss in the scenarios described in the 2017 review Progress in satellite quantum key distribution. In an uplink, the beam passes through the turbulent lower atmosphere while still narrow; the same review describes substantial additional uplink loss from turbulence. These are scenario-dependent engineering observations, not fixed loss values for every system.
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Common sources of impairment
- Diffraction and geometric spreading: The beam expands, reducing the fraction of light collected at the receiver.
- Pointing error and motion: Misalignment between moving terminals can cause loss and time-varying fading.
- Atmospheric turbulence: Refractive-index variations can distort or wander the beam, particularly near the ground.
- Atmospheric extinction: Absorption and scattering vary with the path and wavelength.
- Background light: Sky radiance can produce unwanted detector events and raise the error rate if collection, filtering, detector noise or timing are not managed.
Can free-space QKD work in daylight?
Yes, daylight operation has been demonstrated, though rejecting background light while retaining enough signal is a significant engineering challenge. In a 2017 Nature Photonics experiment, the authors demonstrated a 53 km free-space QKD link using 1550 nm light during the day, with approximately 48 dB total channel loss. They described it as a feasibility demonstration for daylight satellite-based quantum communication—not as an operational satellite service.
A 2021 field experiment, Adaptive-Optics-Enabled Quantum Communication: A Technique for Daytime Space-To-Earth Links, examined conditions representative of daytime space-to-Earth links. The team reported that higher-order adaptive optics improved quantum-channel efficiency beyond tip-tilt correction alone. Diffraction-limited spatial filtering rejected optical noise without requiring an ultranarrow spectral filter.
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These results show that daylight is not an absolute barrier; they do not establish that every free-space QKD link will work in daylight. Performance depends on the particular channel, optics, detectors and mitigation techniques.
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Satellite QKD is not one protocol. A 2026 review, The State of the Art in Satellite Quantum Key Distribution: Protocols, Technologies, and Physical Limits, surveys discrete-variable and continuous-variable approaches, as well as prepare-and-measure, entanglement-based, measurement-device-independent and twin-field protocols.
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The review assesses discrete-variable decoy-state BB84 for low Earth orbit downlinks as the most mature near-term option in its survey, including daylight operation when suitable filtering and pointing or turbulence mitigation are used. That is the review’s assessment, not a universal ranking or a claim that other approaches cannot work.
What determines whether a particular link is secure and practical?
Distance alone is not enough to judge a design. Two systems with the same nominal range can have different link geometry, background light, hardware and security assumptions. A meaningful evaluation considers:
- Protocol and security model: Which protocol is used, how much data is collected, and what implementation assumptions its security analysis makes.
- Geometry: Uplink or downlink, satellite altitude, viewing angle, pass duration and whether the required ground stations can see the relay at the same time.
- Optical channel: Wavelength, aperture, atmospheric attenuation, turbulence, pointing error, background radiance and channel fading.
- Hardware and mitigation: Source and detector performance, acquisition and tracking, spatial-mode coupling, optical filtering and adaptive optics.
- Trust and operations: How the classical channel is authenticated, whether a satellite or relay is trusted, and how finite data and device behavior are handled.
A security proof applies to its stated model. Real-world assurance also depends on whether devices behave as assumed, the link provides enough usable data, and the network’s trust and authentication requirements are met.
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