= 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.<The Physical Implementation of Quantum Computation by David P. DiVincenzo>[Section III, requirements 1–5]{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.
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