Spin relaxation and decoherence of two-level systems

摘要

We revisit the concepts of spin relaxation and spin decoherence of two-level (spin-1/2) systems. From two toy models, we clarify two issues related to the spin relaxation and decoherence: (1) For an ensemble of two-level particles each subjected to a different environmental field, there exists an ensemble relaxation time T_1~*, which is fundamentally different from T_1. When the off-diagonal coupling of each particle is in a single mode with the same frequency but a random coupling strength, we show that T_1~* is finite while the spin-relaxation time of a single-spin T_1 and the usual ensemble decoherence time T_2~* are infinite. (2) For a two-level particle under only a random diagonal coupling, its relaxation time T_1 shall be infinite, but its decoherence time T_2 is finite.