ua nt - p h / 01 06 08 3 v 1 1 5 Ju n 20 01 ‘ Circular type ’ quantum key distribution

Circular type’ interferometric system for quantum key distribution is proposed. The system has naturally self-alignment and compensation of birefringence and also has enough efficiency against polarisation dependence. Moreover it is easily applicable to multi-party. Key creation with 0.1 photon per pulse at a rate of 1.2KHz with a 5.4% QBER over a 200m fiber was realized. Quantum key distribution is expected as one of the most important technology on information security in the near future and provides two remote parties, Alice and Bob, with a common key in private manner. Many groups have reported its realization on optical fibers or others[1][5]. Almost all reports based on interferometric system have a problem on alignment and compensation of birefringence. Geneva group has solved this problem using Faraday mirror excellently[6, 7]. Their solution is self-alignment and compensation, that is, the two optical paths constituting its interferometric system are the same physical path and therefore the difference between the two paths is compensated automatically. But the solution has a flaw that it is sensitive to the polarisation dependent loss because the first and second paths are discriminated each other by polarisation states with the polarisation beam splitter[7]. Our proposal depicted in Fig.1 adopts circular-type optics instead of a Faraday mirror and a polarisation beam splitter. ∗Phone:+81-467-41-2181, Fax:+81-467-41-2185, E-mail: [email protected]