dephasing

Dephasing is the process by which a quantum system loses a definite relative phase between components of a superposition due to coupling to its environment. It suppresses quantum interference without necessarily changing the populations of energy levels, distinguishing it from energy relaxation.

In the density-matrix description, the coherence is encoded in the off-diagonal elements. These elements decay over time with a characteristic dephasing time T2. The population decay is governed by T1, the energy-relaxation time. The observed relation 1/T2 = 1/(2T1) + 1/T_phi separates true dephasing from energy loss, where T_phi is the pure dephasing time associated with fluctuations that shift energy levels without transitions.

Dephasing arises from various environmental interactions, including fluctuations of electromagnetic fields, phonons, charge noise, and spin baths. In solid-state and quantum-information platforms, dephasing is a major limitation to coherence. In ensembles, inhomogeneous broadening leads to an even shorter apparent time T2*, while in single systems the intrinsic dephasing time T2 can be longer and recovered with appropriate dynamical decoupling.

Experimental signatures include the decay of Ramsey fringes, where coherence decays as a function of free evolution time, and the broadening of spectral lines. Echo techniques, such as Hahn echo or more elaborate dynamical decoupling sequences, can reveal the true T2 by refocusing static inhomogeneities.

Mitigation strategies encompass improved isolation and shielding, quieter materials, crystallographic or isotopic purification, lower temperatures, and the use of dynamical decoupling protocols. Understanding and suppressing dephasing is central to advancing coherent control in quantum information, precision measurements, and spectroscopy.