KMB09 protocol is an alternative quantum key distribution protocol

Communications protocol

A communications protocol is a system of digital message formats and rules for exchanging those messages in or between computing systems and in telecommunications...

, where Alice and Bob

Alice and Bob

The names Alice and Bob are commonly used placeholder names for archetypal characters in fields such as cryptography and physics. The names are used for convenience; for example, "Alice sends a message to Bob encrypted with his public key" is easier to follow than "Party A sends a message to Party...

 use two mutually unbiased bases

Mutually unbiased bases

In quantum information theory, mutually unbiased bases in Hilbert space Cd are two orthonormal bases \In quantum information theory, mutually unbiased bases in Hilbert space Cd are two orthonormal bases \...

 with one of them encoding a ‘0’ and the other one encoding a ‘1’. The security of the scheme is due to a minimum index transmission error rate (ITER) and quantum bit error rate (QBER) introduced by an eavesdropper

Eavesdropping

Eavesdropping is the act of secretly listening to the private conversation of others without their consent, as defined by Black's Law Dictionary...

. The ITER increases significantly for higher dimensional photon states. This allows for more noise in the transmission line

Transmission line

In communications and electronic engineering, a transmission line is a specialized cable designed to carry alternating current of radio frequency, that is, currents with a frequency high enough that its wave nature must be taken into account...

, thereby increasing the possible distance between Alice and Bob without the need for intermediate nodes.