Quantum Key Distribution

Using quantum mechanics to establish secure cryptographic keys between two parties, with security guaranteed by physics.

Quantum Key Distribution (QKD) enables two parties (traditionally Alice and Bob) to generate a shared secret key with security guaranteed by the laws of quantum physics, not computational assumptions.

The Goal

Create a shared random key that:

1. Only Alice and Bob know
2. Any eavesdropping attempt is detectable
3. Security doesn’t depend on computational hardness

Why Quantum?

Classical key exchange (like Diffie-Hellman) relies on mathematical problems being hard to solve. A powerful enough computer (or algorithm) could break it.

QKD security comes from physics:

• No-cloning theorem: Can’t copy unknown quantum states
• Measurement disturbance: Observing qubits changes them
• Bell inequality violations: Certify genuine quantum correlations

Major Protocols

BB84

The original QKD protocol (1984):

• Uses single photons in two bases
• Detect eavesdropping via error rate

E91

Entanglement-based (1991):

• Uses entangled photon pairs
• Security from Bell inequality violations

Continuous-Variable QKD

Uses coherent states and homodyne detection:

• Compatible with telecom infrastructure
• Different noise analysis

Basic QKD Steps

1. QUANTUM TRANSMISSION
   Alice sends quantum states to Bob
   (photons encoding bits)

2. BASIS RECONCILIATION
   Alice & Bob compare measurement bases (publicly)
   Keep only matching-basis results

3. ERROR ESTIMATION
   Compare subset of key bits (publicly)
   High error rate → eavesdropper detected

4. ERROR CORRECTION
   Fix remaining errors in the key

5. PRIVACY AMPLIFICATION
   Shrink key to remove eavesdropper's information

Result: Shorter but perfectly secure key

Security

Information-Theoretic Security

QKD can provide unconditional security: secure against any attack, even with unlimited computing power.

Assumptions Still Required

• Quantum mechanics is correct
• Devices work as specified
• No side channels

Device-Independent QKD

Device-independent QKD removes device assumptions using Bell inequality violations. Security is certified by observed quantum correlations.

Practical Considerations

Quantum Networks

For multiple users, QKD networks are being developed:

• Trusted node networks (relay through secure stations)
• Quantum repeaters (future: extend range without trust)
• Satellite QKD (demonstrated by China’s Micius)

Relation to Post-Quantum Cryptography

Both are responses to quantum computing threats but work differently.

See also: BB84 Protocol, E91 Protocol, No-Cloning Theorem, Post-Quantum Cryptography, Quantum Random Number Generator, Device-Independent Security