Quantum communication technologies: Quantum detection circuit for quantum channel coding

Abstract

Light, main carrier in the present information technology, is electromagnetic wave, and also an ensemble of energy quanta, photon as well. At present only the fact is used that light propagates as an energy flux, and the wave nature of light is never used any more so far. Conventional information theory is readily capable of designing communication system based on the wave nature of light, and providing its performance limit. However the ultimate performance limit of optical communication is eventually given by the law of quantum mechanics that governs photon dynamics. Quantum channel capacity is then determined by the distinguishability of optical quantum states. In this article one of the most basic quantum signal system, symmetric states of a single photon polarization, is considered which is often used for quantum key distribution. It is clarified what is the maximum amount of information that can be extracted from that source and how the optimal detector for attaining it can be implemented. Conventionally detection of polarization modulation signals is made by using a polarizer. For a single photon state, the binary output detection based on a polarizer is the standard measurement, which is called the von Neumann measurement. On the other hand the optimal solution in quantum information theory is given by the detector with three outputs at most regardless of the number of the signal components. This type of detector can be implemented by the polarization interferometer. Such a detector was developed and the 96 percent of the predicted limit which is superior to the conventional von Neumann limit could be demonstrated.