Decoherence time

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.