Neither free-space nor fiber quantum key distribution (QKD) is universally better. Fiber suits fixed terrestrial routes with suitable optical infrastructure; terrestrial free-space links can connect sites with a clear line of sight, and satellite links can bridge distant regions without carrying photons through fiber for the whole route. The trade-off is between fixed-route infrastructure and an optical path that must be aligned and can be affected by the atmosphere, background light, or limited satellite contact windows. Range figures only make sense when you compare the same kind of path.
What does “range” mean for QKD?
A fiber distance is measured along an installed cable route. A terrestrial free-space distance is a horizontal line-of-sight span between optical terminals. A satellite figure is typically the distance along a moving satellite-to-ground path during a pass. These are different geometries, not interchangeable measures of how far one uninterrupted QKD link can reach.
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Network reach can mean something else again. Networks may connect multiple links through trusted relays; their end-to-end extent is not the range of a single direct quantum channel. For example, the 2021 integrated network combined fiber and satellite links to connect nodes up to 4,600 km apart using trusted relays. That figure describes the reach of that network architecture, not a direct 4,600-km link. Nature’s 2021 report also describes more than 700 fiber QKD links across a ground network spanning more than 2,000 km.
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How do the channel choices compare?
| Consideration | Fiber QKD | Terrestrial free-space QKD | Satellite free-space QKD |
|---|---|---|---|
| Path | Fixed route along installed fiber | Line of sight between optical terminals | Moving satellite-to-ground optical path |
| Continuity | Can provide ongoing connectivity while the route and equipment are available | Can operate while line of sight and acceptable link conditions persist | Limited to satellite contact periods; conditions change during a pass |
| Key-rate considerations | Channel loss, system design, protocol, and security analysis | Daylight and background light, turbulence, alignment, and system configuration | Pass geometry, pointing and tracking, atmospheric path, and finite contact time |
| Deployment needs | Suitable fiber and channel planning with the network operator | Optical terminals, clear line of sight, alignment, and management of atmospheric effects and background light | Ground optical station, satellite access, acquisition and tracking, and scheduling around passes |
| Good fit | Fixed terrestrial routes with appropriate infrastructure | Connecting nearby sites where laying or accessing fiber is impractical | Bridging remote regions beyond practical terrestrial routes |
What ranges and key rates have been demonstrated?
Satellite-to-ground links
A 2017 satellite-to-ground experiment reported a kilohertz key rate over distances of up to 1,200 km. This is a satellite-path result, not a demonstration of 1,200 km of ground-level fiber or continuous service at that distance. The Nature paper reports the experiment.
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In the 2021 integrated network, the satellite link averaged 47.8 kb/s for a typical pass. A pass-limited average rate and a fiber link’s rate over an ongoing operating interval are not directly comparable without accounting for how long each link is available and how its rate is measured. The network report describes the integrated architecture and its results.
Terrestrial free-space links
A 2023 metropolitan study demonstrated QKD over a 1.7-km line-of-sight link in Jena. Its system averaged 5.4 kb/s secure key rate at night and achieved more than 2.5 kb/s in direct daylight. The authors estimated kilobit-per-second performance at 10 km, but that was an extrapolation from measurements, not a 10-km demonstration. These rates belong to that experimental setup; they are not universal free-space QKD rates. The study details the system and conditions.
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Fiber distance
There is no single fiber-distance figure that answers how far QKD will work in a particular deployment. Attenuation accumulates along the route, while the result also depends on the system, protocol, channel conditions, and security analysis. Network operators should assess the actual path and equipment rather than treat an experimental distance or a network-wide reach as a universal limit.
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Reliability depends on what can interrupt a given route. A fixed fiber path may support continuous service when its infrastructure and equipment are available, but its suitability depends on the actual channel and route conditions. Free-space links must additionally maintain line of sight and optical alignment; atmospheric turbulence, weather, pollution, and background light can reduce key generation or make a link unavailable.
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ETSI’s 2010 QKD use-case document notes that atmospheric conditions and pollution can attenuate free-space links, and that adverse conditions can make QKD impossible or significantly reduce the achievable key-generation rate. In the 2023 Jena experiment, performance was stable at night and more variable in daylight amid sunlight and changing clouds. Those observations describe that implementation, not every free-space system. ETSI GS QKD 002 V1.1.1 sets out the atmospheric caveat.
Satellite availability adds a scheduling constraint: the optical link exists only during a suitable pass, and its geometry and conditions vary as the satellite moves. A 2017 review gives roughly 150 useful night passes per year for an illustrative low-Earth-orbit satellite and a ground station at 40 degrees latitude. That example is specific to the discussed orbit and site, not a general annual availability figure for all satellites or stations. The review explains the satellite-link constraints.
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What should be checked before deploying fiber QKD?
Distance alone is not a sufficient fiber-planning test. ETSI’s deployment-parameter guidance suggests discussing the real optical channel and operating conditions with the network operator. The details matter because a route’s loss, noise, stability, and available channels can shape whether the planned QKD system is viable. ETSI GS QKD 012 V1.1.1 lists parameters to consider.
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- Loss and equipment: What attenuation is measured, at what wavelength, and are there active components or bypass options on the route?
- Channel use: Is the fiber dark or multiplexed? What other optical powers and cross-talk could affect it?
- Stability and noise: How much instability or vibration is present, how stable is polarization, and what noise conditions should the QKD system expect?
These are questions for a specific deployment, not a requirement that every QKD link use one fixed channel configuration.
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When does each approach make sense?
Choose fiber for a fixed terrestrial route
Fiber is the natural option when suitable infrastructure connects the sites and the channel’s availability, loss, noise, and stability have been assessed. It avoids the need for an atmospheric line of sight, but it does not remove the need to characterize the route or agree on operating conditions.
Choose terrestrial free space to bridge a line-of-sight gap
A free-space link can be useful when two sites can see one another optically but a fiber route is unavailable or impractical. Its feasibility depends on terminal placement and alignment, atmospheric conditions, and ambient light; off-the-shelf, plug-and-play terrestrial free-space solutions were less mature than fiber in the 2023 study’s account.
Consider satellite links for geographically separated sites
Satellite QKD can bridge regions that are difficult to connect with terrestrial fiber, while most of the optical path is through vacuum rather than long lengths of fiber. It still requires suitable optical ground stations, pointing and tracking, an atmospheric path near the ground, and coordination around finite passes. Its broad geographic reach comes with intermittent contact rather than an always-available connection.
What the range numbers do—and do not—say about security
A distance or key-rate result does not by itself establish the security of an entire deployed network. The integrated network’s end-to-end connections used trusted relays, so its reported reach should not be read as an end-to-end architecture without relay trust assumptions. Channel-level QKD security properties and the practical security of devices and network components are separate considerations; deployment decisions should account for both.
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