Fig. 1

An introductory overview of classical computers and quantum computers. The basic unit of information in classical computers is a digital bit whose value is either 0 or 1. Quantum computers consist of qubits, which have 0 and 1 states, as well as superposition states. The physical system of classical bits is a transistor, while there are many physical systems for qubits. The error rates of classical bits are extremely low, \(10^{-13}\)–\(10^{-18}\), and are caused by clock jitters, background radiation, etc. Temperature fluctuations, noises in microwaves and lasers for qubit manipulation, environmental electromagnetic fields can induce qubit errors at a rate of \(10^{-2}\)–\(10^{-4}\). The errors in classical bits can be corrected by copying the information to multiple bits. Because a quantum state cannot be copied, multiple physical qubits are entangled to form a logical qubit to reduce the error