Pieces of Quantum Information and Quantum Logical Gates

Abstract

The basic idea of quantum computer theory is that computations can be performed by some quantum systems that evolve in time. Accordingly, by applying Schrodinger’s equation, it is natural to assume that quantum information is processed by special examples of unitary operators (called quantum logical gates), which transform in a reversible way the pure states of the quantum systems that store the information in question. This chapter gives a synthetic presentation of the main “mathematical characters” of the quantum computational game: qubits, quregisters, mixtures of quregisters, quantum logical gates. The last Section of the Chapter illustrates possible physical implementations of some quantum logical circuits by means of special variants of the Mach-Zehnder interferometer, an apparatus that has played an important role in the philosophical debates about quantum theory.