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A decoherence time is a characteristic timescale for loss of specified off-diagonal coherence in a density matrix. In a simple qubit dephasing model, ${\rho }_{01}(t)={\rho }_{01}(0)e^{-t/T_2}$, so $T_2$ is the time for that coherence magnitude to fall by a factor $e$.

In Ramsey interferometry, two separated pulses convert accumulated phase into an interference signal; scanning the pulse separation measures the decay of its contrast. The inferred timescale depends on the basis, environment and pulse sequence. It differs from the energy-relaxation time, and nonexponential decay may require a stated threshold or fit convention rather than a unique decay constant.

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.