The idea: the uncertainty principle protects the key

Classical cryptography relies on mathematics that are hard to compute. Quantum key distribution (QKD) changes the terrain: it makes security rest on the laws of quantum physics, not on an adversary's computing power.

The exact goal of QKD

QKD does not encrypt messages. Its sole role is to let Alice and Bob share a secret key that they are certain no eavesdropper has learned. Once this key is established, they use it with a classical cipher (one-time pad, AES).

What makes it special: any interception attempt is detectable. It is not that an eavesdropper would struggle to break the key — it is that they cannot observe it without leaving a measurable trace.

Encoding a bit in a photon

Information travels as photons (grains of light) whose polarization (the orientation of the oscillation) is set. For example:

• vertical polarization = bit 0, horizontal = bit 1;
• or polarization at 45° = bit 0, at 135° = bit 1.

Two laws that protect the key

Two physical principles make eavesdropping detectable:

• The uncertainty principle: measuring a quantum state disturbs it. You cannot observe a photon "quietly": the measurement modifies the measured state.
• The no-cloning theorem: it is impossible to copy an unknown quantum state. An eavesdropper therefore cannot duplicate the photon to keep a copy and pass the original along intact.

Why an eavesdropper is trapped

   Alice                 canal quantique                 Bob
   [photon polarisé]  ───────────────────────────►  [mesure]

   Si Ève s'intercale :

   Alice ──► [ Ève mesure ] ──► [ Ève renvoie ] ──► Bob
                  │                    │
          perturbe l'état      ne peut pas cloner
          (incertitude)        (non-clonage)
                  │
          => introduit des erreurs détectables

Eve can neither copy the photon (no-cloning) nor measure it without disturbing it (uncertainty). Whatever her choice, she leaves traces that Alice and Bob will be able to spot.

A new kind of guarantee

With RSA, security is computational: it assumes a problem is too long to solve. With QKD, security is physical: it follows from fundamental properties of nature, independent of the adversary's power.

In summary

QKD shares a secret key by encoding bits in the polarization of photons. Thanks to the uncertainty principle (measuring disturbs) and the no-cloning theorem (copying is impossible), any interception leaves a detectable trace. Security rests on physics, not on computational hardness.