Technology
Quantum Key Distribution over Metropolitan Optical Networks
Quick fact
In QKD, any attempt to eavesdrop on the key transmission necessarily disturbs the quantum states, alerting the legitimate users — this is guaranteed by quantum mechanics, not by computational assumptions.
Why this is interesting
Imagine encrypting a message with a key that is mathematically impossible to copy — because the laws of physics prevent it. How can we send such unbreakable keys across a city's fiber-optic network?
Read the full explanation
Understanding Quantum Key Distribution over Metropolitan Optical Networks
Think of QKD as a way to send a secret code (the encryption key) using light particles (photons). A photon can be prepared in different states, like polarization (up/down, diagonal). The sender (Alice) randomly uses one of two bases (rectilinear or diagonal) to encode bits (0 or 1). The receiver (Bob) also randomly chooses a measurement basis for each photon. After transmission, they compare bases over a public channel and keep only the bits where they used the same basis. This becomes the secret key. In a metropolitan optical network, this happens over standard fiber-optic cables, but the challenge is that photons get lost over distance due to attenuation, so the range is limited to tens of kilometers without repeaters.
A deeper explanation
The security of QKD relies on two quantum principles: the no-cloning theorem (you cannot make an exact copy of an unknown quantum state) and the observer effect (measuring a quantum state disturbs it). If an eavesdropper (Eve) tries to intercept the photons, she must measure them, which introduces errors. By comparing a small sample of the key, Alice and Bob can estimate the quantum bit error rate (QBER). If it's below a threshold, they can use error correction and privacy amplification to distill a secure key. In metropolitan networks, QKD systems typically use either weak coherent pulses (with decoy states) or entangled photons. The main challenge is signal loss in fibers (~0.2 dB/km), which limits the reach to about 100 km without trusted relays. Metropolitan networks are ideal because they have shorter distances and existing fiber infrastructure. Real-world deployments include the Beijing-Shanghai backbone and city-scale networks in several countries, often using trusted relay nodes to extend coverage.