Post-quantum cryptography protects data with new maths. Quantum key distribution (QKD) takes a different route: it uses physics. Both have a place, but they solve different problems.
The basic idea
To encrypt a message, both sides need the same secret key. Quantum key distribution sends that key as a stream of single particles of light, called photons, over fibre-optic cable or through the air.
Measuring a quantum state disturbs it, and an unknown quantum state can’t be copied perfectly. So an eavesdropper who tries to read the photons leaves detectable traces. If the error rate is too high, the two parties throw the key away and try again.
From idea to real networks
The first QKD protocol, BB84, was proposed by Charles Bennett and Gilles Brassard in 1984. Since then it has moved from the lab to real networks:
- ◆China built a QKD backbone of about 2,000 km between Beijing and Shanghai, completed in 2017.
- ◆China’s Micius satellite, launched in 2016, demonstrated QKD between space and the ground, and in 2017 helped secure an intercontinental video call between Beijing and Vienna.
- ◆Test networks operate in Europe, South Korea, the UK and elsewhere.
Important limits
- ◆Distance: photons get lost in fibre, so direct links typically reach up to a few hundred kilometres. Longer networks use “trusted nodes” where the key is decrypted and re-sent, and those sites must be physically secured.
- ◆Identity: QKD doesn’t prove who you’re talking to. It still needs a separate authentication method, usually classical or post-quantum cryptography.
- ◆Special hardware: QKD needs dedicated optical equipment; it can’t be added to an app with a software update.
- ◆Real devices can leak: security proofs assume ideal equipment, and researchers have shown attacks on real hardware, such as blinding the light detectors with bright lasers.
What security agencies say
The US NSA and the UK NCSC do not currently recommend QKD for protecting government systems, and prefer post-quantum cryptography. They point to the limits above. Other countries, especially in Asia and Europe, are investing in QKD as an extra layer for specific high-value links.
When QKD makes sense
QKD can make sense for a small number of fixed, high-value links, such as between two data centres or government sites, where organisations want an extra layer on top of post-quantum cryptography. For most organisations, post-quantum cryptography is the practical first step.
task_altKey takeaways
- check_circleQKD sends keys on single photons so eavesdropping can be detected.
- check_circleIt is limited by distance, needs special hardware and still requires authentication.
- check_circleThe NSA and UK NCSC prefer post-quantum cryptography for government systems.