Unambiguous state discrimination of coherent states with linear optics: Application to quantum cryptography
Abstract
We discuss several methods for unambiguous state discrimination of N symmetric coherent states using linear optics and photodetectors. One type of measurement is shown to be optimal in the limit of small photon numbers for any N. For the special case of N=4 this measurement can be fruitfully used by the receiving end (Bob) in an implementation of the BennettBrassard 1984 (BB84) quantum key distribution protocol using faint laser pulses. In particular, if Bob detects only a single photon the procedure is equivalent to the standard measurement that he would have to perform in a singlephoton implementation of BB84, if he detects two photons Bob will unambiguously know the bit sent to him in 50% of the cases without having to exchange basis information, and if three photons are detected, Bob will know unambiguously which quantum state was sent.
 Publication:

Physical Review A
 Pub Date:
 October 2002
 DOI:
 10.1103/PhysRevA.66.042313
 arXiv:
 arXiv:quantph/0207138
 Bibcode:
 2002PhRvA..66d2313V
 Keywords:

 03.67.Dd;
 42.50.Ar;
 42.79.Sz;
 Quantum cryptography;
 Photon statistics and coherence theory;
 Optical communication systems multiplexers and demultiplexers;
 Quantum Physics
 EPrint:
 5 RevTeX pages, 2 eps figures