Abstract

If a single quantum system is prepared in an completely unknown state, it is impossible to devise a completely reliable procedure for either revealing or reproducing this state. However, for a quantum system whose state is chosen secretly from a known, non-orthogonal pair |ψ_±〉, there exist strategies for correctly revealing or perfectly replicating the state with non-zero probability. The possibility of unambiguous state discrimination was investigated by Ivanovic[1] who showed that |ψ_±〉 could be mapped onto orthogonal states, which can be discriminated by a von Neumann measurement. More recently, Guan and Duo [2] have demonstrated the feasability of an analogous exact copying machine. A tripartite interaction between the copier, the original particle in one of the states |ψ_±〉, and a particle in a ’blank’ state |χ〉 can leave the pair of particles in the state |ψ_±〉|ψ_±〉. However, the probability of either operation being successful cannot be unity for non-orthogonal states, and will sometimes fail, although we always know whether or not it has succeeded.

© 1998 IEEE