Practical QKD: Real-World Deployments

Practical QKD: Real-World Deployments

QKD has moved from laboratory demonstrations to real commercial deployments. Toshiba, ID Quantique, Huawei, and others offer commercial QKD hardware. China has deployed the world's largest quantum network. Understanding the engineering constraints reveals where QKD works and where it faces fundamental limitations.

Fiber-Based QKD

The primary deployment medium is optical fiber, using single photons (or weak coherent pulses approximating single photons) at telecom wavelengths (1310 nm or 1550 nm). Key characteristics:

Fiber loss: ~0.2 dB/km at 1550 nm. Over 100 km, this is 20 dB of loss, meaning only 1% of photons arrive. Over 200 km, loss is 40 dB (0.01% arrival rate). This limits QKD range to roughly 300-400 km without repeaters.

A commercial QKD system at 100 km generates approximately 1 kbps of secure key. At 10 km, this can reach 100 kbps or more.

Quantum Repeaters

Classical optical repeaters amplify signals. Quantum signals cannot be amplified (the no-cloning theorem again). Instead, quantum repeaters work by entanglement swapping: establish entanglement between adjacent repeater nodes, then "stitch" them together to create long-range entanglement.

This requires quantum memory (storing entangled states while waiting for heralding signals) and Bell state measurements at each repeater node. Current quantum memories have coherence times of milliseconds to seconds: sufficient for moderate distances but challenging for continental links.

Satellite-based QKD offers an alternative: the free-space channel has low loss at high altitudes, and a low-earth-orbit satellite can establish links with ground stations hundreds of kilometers apart.

China's Quantum Network