 Quantum computer

A quantum computer is a physical device that performs controlled quantum computation and extracts an output through quantum measurement. A programmable quantum computer combines physical qubits, control hardware and classical processing that specifies operations and interprets repeated measurements.

David P. DiVincenzo organized the central physical requirements into five capabilities: scalable and characterized qubits; initialization in a known quantum state; sufficiently long relevant decoherence times compared with operation times; a universal set of quantum gates; and quantum measurement of selected qubits. These requirements connect an abstract quantum circuit with an experiment: knowing the intended gate sequence is useful only if the apparatus can initialize, control and read the encoded system accurately.^[ref]

The encoding can use different physical systems. Two selected internal levels of a trapped ion, for example, can represent the basis states of a qubit. The remaining levels and environmental couplings then become part of the physical control problem.