Technology
Quantum Key Distribution over Metropolitan Fiber Networks
Quick fact
In metropolitan fiber networks, QKD systems can deliver secure keys over distances of 50–100 km, but at 100 km, the signal becomes so weak that fewer than one photon in a billion sent by Alice reaches Bob, and the key rate drops to a few thousand bits per second.
Why this is interesting
You've probably shared a secret over the internet, but what if a hacker could intercept your message without you ever knowing? Quantum key distribution makes such spying impossible by using the weird rules of quantum physics.
Read the full explanation
Understanding Quantum Key Distribution over Metropolitan Fiber Networks
Imagine you want to send a secret code to a friend across a busy city. In classical communication, a hacker could copy your code without leaving a trace. But quantum physics says you can't copy an unknown quantum state—like the polarization of a single photon—exactly. QKD uses this to let two parties, Alice and Bob, build a secret key. Alice sends Bob a series of single photons over an optical fiber, encoding each bit in one of two bases (like rectilinear or diagonal polarization). Bob randomly chooses a basis to measure each photon. After the transmission, they compare (over a public channel) which bases they used, discarding mismatches. If an eavesdropper (Eve) tries to intercept, her measurements inevitably disturb the photons, introducing errors that Alice and Bob detect. This is the essence of QKD over metropolitan fibers: the fiber is the physical channel, and the laws of physics ensure security.
A deeper explanation
The mechanism behind QKD rests on two quantum principles: the no-cloning theorem and the measurement disturbance trade-off. For polarization-encoded BB84, Alice sends a random sequence of bits, each encoded in one of two conjugate bases (e.g., horizontal/vertical vs. diagonal/anti-diagonal). Bob measures each photon in a randomly chosen basis. The no-cloning theorem prevents Eve from making a perfect copy of the photon; any attempt to measure it forces her to choose a basis. If she picks the wrong basis, she introduces a 25% error rate in the bits that Alice and Bob later use (because Bob's measurement outcomes will be altered probabilistically). After the transmission, Alice and Bob publicly compare a subset of their key bits to estimate the quantum bit error rate (QBER). If QBER exceeds a threshold (typically around 11% for BB84), they abort the key, because Eve is too active. In metropolitan fiber networks, the practical challenge is signal loss: photons are absorbed and scattered in the fiber, and the probability of a photon arriving decreases exponentially with distance. At ~100 km, the key rate becomes very low, and above ~150 km, the QKD becomes impractical without quantum repeaters. This means QKD over metropolitan fibers works best in dense urban areas, where distances are manageable and secure key rates (kbps to Mbps) are achievable. Quantum key distribution thus offers a way to secure communications with information-theoretic security, but its real-world deployment requires careful engineering of single-photon sources, low-loss fibers, and high-efficiency detectors.