ABSTRACT
Almut Beige, Berthold-Georg Englert, Christian Kurtsiefer, and Harald Weinfurter
Abstract All schemes for quantum cryptography that have been implemented in real-life experiments have in common that single qubits (carried by photons) are used for the exchange of quantum information between the parties. As a consequence, these schemes are probabilistic: they involve random processes that decide whether the next qubit will contribute to the shared key sequence or not. This is also an essential element in (almost) all generalizations of the simplest schemes, be it by introducing more choices in the same state space or by enlarging the state space. The situation is completely different in a scheme proposed recently. It uses qubit pairs for the transmission of quantum information (each pair carried by a single photon) and, as a benefit of this doubling, the scheme is deterministic: each and every qubit pair sent supplies one key bit. It is, therefore, even possible to communicate confidential messages securely and directly, that is: without the need for establishing a shared key first. We present a detailed analysis of the security of the qubit-pair schemes for cryptography and communication under eavesdropping attacks in which the quantum information in transit is intercepted and a substitute message is forwarded to conceal the interception.
