Kvanttiavainvaihto

Kvanttiavainvaihto, also known as Quantum Key Distribution (QKD), is a method for exchanging cryptographic keys between two parties using the principles of quantum mechanics. This technique allows for the secure distribution of a shared secret key that can then be used for encrypting and decrypting messages. The fundamental advantage of QKD lies in its ability to detect any eavesdropping attempt.

The most common protocols for QKD, such as BB84 and E91, rely on the properties of photons. Photons can be polarized in different bases, and according to quantum mechanics, attempting to measure a quantum system inevitably disturbs it. In QKD, one party (Alice) sends a sequence of polarized photons to the other party (Bob). Alice randomly chooses the polarization basis for each photon, and Bob also randomly chooses his measurement basis. After the transmission, Alice and Bob publicly compare the bases they used for each photon. For the photons where they used the same basis, they can establish a shared secret key.

Any attempt by an eavesdropper (Eve) to intercept and measure the photons will inevitably alter their quantum state, introducing detectable errors in the key generation process. Alice and Bob can then check for these errors by comparing a subset of their generated key bits over a public channel. If the error rate exceeds a certain threshold, they know that eavesdropping has occurred and discard the key. If the error rate is acceptably low, they can be confident that the key is secure and proceed with using it for classical encryption. QKD is a crucial technology for achieving information-theoretic security in communication.