Nanodevices for Quantum Computing Using Photons

Summary

The amount of research aiming to apply essential features of quantum theory such as the uncertainty principle, the quantum interference of wave functions, and quantum entanglement to information communication and information processing has been remarkable recently. For example, quantum cryptography, which completely excludes the possibility of eavesdropping, becomes possible when the uncertainty principle is applied. Also, the quantum computer has been proposed. By fully applying quantum superposition states and quantum entanglement to computation, a computation that would take billions of years using present supercomputers might be performed in several minutes. The research into quantum computers is still in its early stages, but for quantum cryptography, verification experiments over tens of kilometers have already been performed in various places and research aimed at practical use is proceeding. In this chapter, devices of all kinds which are needed in quantum cryptography and quantum computation are called quantum information devices.

Nanotechnology and quantum information devices are related very closely. For example, the discrete quantum states which are used to store the information in quantum computation can be observed only in nanostructures where the numbers of electrons and atoms are very small, and cannot be seen in macroscopic mate?rials. Also, during a quantum computing process, it is required that the quantum superposition state should never be broken. The time during which the superposition state is maintained is called the decoherence time. To prevent decoherence, the interactions with the outside which cannot be controlled must be blocked off. For this purpose, various nanotechnologies are required. In this chapter, the general concepts of quantum communication and quantum information processing are explained first, and then we describe the devices which are needed for quantum cryptography and quantum computers to be realized, while aiming specifically to deal with their relation to nanotechnology. In the last part of the chapter, we shall focus on nanodevices for quantum computing using photons.