Quantum key distribution

In plain language

Quantum key distribution, or QKD, is a method for two people to agree a secret key that they can later use to protect messages. The key is carried by light: the sender encodes random bits in very weak pulses, often single photons, and the receiver measures them. The security comes from the rules of quantum physics rather than from how hard a calculation is to run, so an eavesdropper who tries to read the light leaves a disturbance that the two parties can check for.

QKD only creates the key. The messages themselves are still encrypted with ordinary methods that use the key. Experiments have sent keys down long fibre spools and from a satellite to ground stations, but the rates fall quickly with distance and the equipment is specialised. For that reason, the UK's National Cyber Security Centre does not endorse QKD for government or military use, and it advises against relying on QKD alone for business-critical networks.

Technical explanation

QKD protocols encode information in a quantum property of single photons or weak light pulses, such as polarisation or phase. The two parties compare part of their measurements to estimate how much an eavesdropper could have learned, then remove that information from the raw key. The security of practical systems, which differ from idealised models, is analysed in a review by Scarani and colleagues (2009). That review covers discrete-variable, continuous-variable and distributed-phase-reference protocols, and it presents the theory used to assess the security of each.

In 2017, Liao and colleagues reported satellite-to-ground QKD using the decoy-state method, which varies pulse intensities so that certain attacks can be detected. They reported a kilohertz key rate from the satellite to the ground over distances up to 1,200 km. They also state that the satellite link is up to 20 orders of magnitude more efficient at transmitting photons than an optical fibre of the same length, a comparison of photon transmission rather than key rate. In 2023, Liu and colleagues reported twin-field QKD over 1,002 km of fibre, with a secure key rate of 9.53 × 10^-12 per pulse in the asymptotic regime and 8.75 × 10^-12 per pulse at 952 km when finite-size effects are included.

The practical limits are key rate, hardware and authentication. A QKD link still needs an authenticated classical channel, so the two parties must already have a reliable way to confirm each other's identity. The UK NCSC advises that robust quantum-safe authentication mechanisms should be implemented alongside QKD.

What the evidence says

Each statement is typed by the strength of its evidence and linked to the source that supports it.

Limitations and open problems

• Secure key rates fall steeply with distance. The twin-field result over 1,002 km gives 9.53 × 10^-12 per pulse in the asymptotic regime (Liu and colleagues, 2023), which is far below what most applications would need from a single link.
• QKD still needs an authenticated classical channel, so it cannot establish identity on its own (NCSC, 2020).
• The UK NCSC does not endorse QKD for government or military applications and cautions against sole reliance on QKD for business-critical networks (NCSC, 2020).
• Security proofs must account for imperfect real hardware, which makes practical QKD harder to certify than the idealised theory (Scarani and colleagues, 2009).
• The sources reviewed here do not give verified cost figures for commercial QKD equipment.

Possible future uses

Timeline

1. 29 Sept 2009Scarani and colleagues publish a review of the security of practical QKD in Reviews of Modern Physics. Sources: [1]
2. Sept 2017Liao and colleagues report satellite-to-ground decoy-state QKD with kilohertz key rates over distances up to 1,200 km in Nature. Sources: [2]
3. 24 Mar 2020The UK National Cyber Security Centre publishes its QKD guidance, stating that it does not endorse QKD for government or military applications. Sources: [4]
4. 2023Liu and colleagues report twin-field QKD over 1,002 km of fibre in Physical Review Letters. Sources: [3]

Futures this could help bring about

How far away?

A quantum internet

Engineering prototypes · 4 of 7 steps done

How far away?

An internet protected against quantum attacks

Scaling up · 3 of 8 steps done

Related breakthroughs

• Twin-field quantum key distribution runs over a 254 km commercial telecom network at 110 bits per second (23 Apr 2025)
• A quantum microsatellite distributes real-time keys to portable ground stations, linking Beijing and Stellenbosch (19 Mar 2025)

Glossary

Quantum key distribution

A method for two parties to create a shared secret key using quantum states of light, where interception normally leaves a detectable disturbance.

Decoy-state method

A QKD technique in which the sender randomly varies pulse intensities so that certain attacks on multi-photon pulses can be detected.

Twin-field QKD

A QKD protocol in which both users send light to a central station that combines it, allowing longer fibre distances than direct exchange.

Authenticated classical channel

An ordinary communication link in which each message is checked to come from the claimed sender.

Sources

1. peer reviewed paper
   The security of practical quantum key distribution
   Valerio Scarani, Helle Bechmann-Pasquinucci, Nicolas J. Cerf et al.. Reviews of Modern Physics, 29 Sept 2009. DOI: 10.1103/RevModPhys.81.1301. Accessed 21 Oct 2026.
2. peer reviewed paper
   Satellite-to-ground quantum key distribution
   Sheng-Kai Liao, Jian-Wei Pan. Nature, Sept 2017. DOI: 10.1038/nature23655. Accessed 21 Oct 2026.
3. peer reviewed paper
   Experimental Twin-Field Quantum Key Distribution Over 1000 km Fiber Distance
   Yang Liu. Physical Review Letters, 25 May 2023. DOI: 10.1103/PhysRevLett.130.210801. Accessed 21 Oct 2026.
4. government report
   Quantum security technologies
   UK National Cyber Security Centre, 24 Mar 2020. Accessed 21 Oct 2026.

Revision history

3 changes since this profile was created.