What is quantum key distribution, and how is it different from post-quantum cryptography?
Quantum key distribution, or QKD, uses the physics of single particles of light to let two parties share a secret key and detect eavesdropping, under specific assumptions about the protocol and equipment. Post-quantum cryptography, or PQC, is ordinary software built on maths problems that quantum computers aren't known to solve. Both aim to keep data safe from quantum attacks, but they work in completely different ways, and security agencies recommend PQC as the main defence.
How QKD works
QKD sends a stream of photons between two points over dedicated optical fibre or a line-of-sight link. Measuring a quantum state disturbs it. In a well-designed protocol running on sound equipment, interception raises the error rate the two ends see, which lets them detect and limit how much an eavesdropper could learn. If the error rate stays low enough, they keep the bits they agree on as a shared key. QKD also needs an authenticated ordinary channel to work at all, so it still relies on conventional authentication, and real devices can have weaknesses the theory doesn't cover. Some schemes use entangled pairs; in 2017 the Chinese satellite Micius sent entangled photons to ground stations about 1,200 km apart.
QKD versus PQC
| QKD | Post-quantum cryptography | |
|---|---|---|
| What it is | Physics: special hardware sending single photons | Maths: software algorithms such as ML-KEM and ML-DSA |
| Runs on | Dedicated fibre or free-space links, limited distance | Existing computers and networks |
| Proves who you're talking to | No: needs separate authentication | Yes: includes signature standards |
| Long distances | Often needs trusted relay stations | Works over the internet |
| Agency view | Not recommended as a sole or main solution | Recommended primary defence |
What security agencies say
The UK's National Cyber Security Centre says post-quantum cryptography is the best mitigation for the quantum threat. In a 2025 paper it said it will not support QKD for government or military use, and that other sectors should not rely on QKD alone for generating and distributing keys. The US National Security Agency does not recommend QKD for protecting national security systems, citing the need for separate authentication, special equipment, the cost and insider risk of trusted relays, the difficulty of validating real implementations, and vulnerability to denial of service.
QKD has specialised applications, but its infrastructure and operational requirements make it unsuitable as a general replacement for software-based post-quantum migration.
Why the confusion matters
Vendors sometimes market QKD as "unhackable" or "the quantum-safe solution". Real QKD systems depend on engineering, not physics alone, and published attacks have targeted the hardware. A bank that confuses the two risks buying the wrong thing, or delaying the software migration that regulators and standards bodies expect. The post-quantum cryptography lesson explains the standards banks are adopting.
Common confusions
- "Quantum-safe means quantum hardware." No. PQC is quantum-safe and runs on ordinary computers.
- "QKD sends messages faster than light." No. It shares keys over a physical link at ordinary speeds.
- "QKD is provably unbreakable." The theory is strong under its assumptions; real devices have been attacked through their engineering.
- "QKD replaces signatures." No. It shares keys; proving who sent something still needs other cryptography.
Sources and further reading
Tags show what kind of source each one is (how sources are used). Checked against these sources on 12 October 2026. Spotted an error? Email hello@plainquantum.com and it will be corrected, with a note.
- Government guidanceQuantum networking technologiesUK National Cyber Security Centre, 2025
- Government guidanceQuantum Key Distribution (QKD) and Quantum Cryptography (QC)US National Security Agency
- StandardPost-Quantum Cryptography project (status of FN-DSA and other work)NIST Computer Security Resource Center