We describe a free space Quantum cryptography system which is designed to allow continuous unattended key exchanges for periods of several days, and over ranges of a few kilometres. The system uses a four laser faint pulse transmission system running at a pulse rate of 10MHz to generate the required four alternative polarization states. The receiver module similarly automatically selects a measurement basis and performs polarization measurements with four avalanche photodiodes. The controlling software can implement the full key exchange including sifting, error correction, and privacy amplification required to
generate a secure key.
We describe the principles and performance of a novel form of hybrid electronic/photonic correlator which exploits recent developments in both electronics and photonics to provide a fast, compact and rugged processor. In the earlier Vander Lugt correlator the input image is Fourier Transformed optically and multiplied optically with the conjugate Fourier Transform of a reference pattern; the required correlation function is completed by taking the inverse Fourier Transform of the product optically. The correlator described here is similar, but performs the initial Fourier Transforms and multiplication electronically. In this scheme the Fourier Transforms of both the input scene and reference pattern are reduced to a binary phase-only format, when the multiplication process simplifies to a simple Boolean logic XOR function. The output of this XOR gate is displayed on a state-of-the-art Fast Bit Plane Spatial Light Modulator (FBPSLM). This device is electrically addressed, and employs a ferroelectric liquid crystal so its operation is inherently binary and fast. A consequence of the use of binary phase-only data is the appearance of a second (unwanted) peak in the output plane, but we demonstrate its elimination through the introduction of a chirp function. The complete correlator is demonstrated operating at 10,500 correlations/sec.
A GEC-Marconi optically addressed spatial light modulator was used in a nonlinear joint transform correlator to demonstrate an application in speckle metrology. A particular input architecture used in this correlator potentially allows the design of a more compact correlator architecture.
Conference Committee Involvement (1)
Electro-Optical and Infrared Systems: Technology and Applications