Abstract

Practical quantum cryptography over long fiber is of current research interest. Quantum cryptography based on four single-photon polarizations (vertical, horizontal, right-circular. left-circular)¹ is difficult in fiber, although experimental efforts have been reported.² Two orthogonal components of the single-photon Field can be transmitted through two arms of Mach-Zebnder interferometer.³ The four polarizations correspond to relative phase 0, π, π/2, or 3π/2, which is even more difficult to maintain between two fibers. Time-division^3,4 and frequency-division⁵ multiplexing schemes have been proposed to circumvent the difficulty by putting two arms into a fiber. They both require precisely matched multiplexer and demultiplexer.

© 1997 Optical Society of America