An Introduction to All-Optical Quantum Controlled-NOT Gates

Abstract

Quantum computer promises to outperform classical computer fundamentally, due to its quantum superposition. Any operations to N qubits can be decomposed into several single-qubit operations and two-qubit controlled-NOT (CNOT) operations in theory. Linear optical quantum computing (LOQC) is one of the most prominent physical quantum systems, which has the advantage of long coherent time and convenience in implementing single qubit operations. However, the realization of two-qubit CNOT gate is the greatest challenge for LOQC, because two photons cannot directly interact with each other by nature. KLM protocol proves the feasibility of LOQC and spurs quantity of research on schematic design and experimental demonstration of CNOT gates by using linear quantum optics system. These researches are very important and nontrivial for LOQC, and this paper gives an overview of different schemes of the proposed CNOT gates and the experimental demonstration.