Experimental realisation of a deterministic secure quantum communication protocol based on entangled photons

Quantum cryptography protocols using entangled qubit pairs can increase the rate and security of information exchange through a quantum channel. Usually those protocols are non-deterministic, i.e. a classical bit is encoded in a quantum state, but the actual bit value cannot be determined. Therefore they request the transmission of additional information through a public channel. By contrast, the ping-pong coding scheme based on entangled photons allows for deterministic and direct key distribution. No qubit has to be discarded and information can be decoded without the need for a subsequent transmission of additional information. Communication through a public channel nevertheless is necessary to ensure asymptotical security against arbitrary eavesdropping attacks.