Recoherence

Recoherence refers to the partial or full restoration of quantum coherence in a system that has previously decohered due to interaction with an environment. Coherence denotes a definite relative phase between components of a quantum superposition, while decoherence describes the loss of this phase as the system becomes entangled with its surroundings, yielding statistics that appear classical. Recoherence, therefore, describes a revival of coherence, typically seen as a revival of the off-diagonal terms in the system's density matrix.

In practice, recoherence arises through time-reversal or control techniques that reverse dephasing, such as spin echo or dynamical decoupling. In nuclear magnetic resonance and quantum information experiments, sequences like spin echo, CPMG, or Uhrig decoupling can refocus phase errors caused by inhomogeneous fields, leading to temporary revival of coherence. Recoherence can also occur due to non-Markovian environments where information flows back from the environment to the system, producing partial revivals of visibility. It can be affected by temperature, coupling strength, and reservoir structure.

The concept highlights that decoherence is not necessarily permanent and that coherence can be preserved or recovered with appropriate control or design. It has implications for quantum memory, computation, and interference experiments. Observations typically measure coherence through interference visibility or by tracking the off-diagonal density-matrix elements. The term is used across quantum optics, condensed matter, and quantum information contexts.