It is predicted that (i) spin echoes have two kinds of generic long-time decays: either simple exponential, or a superposition of a monotonic and an oscillatory exponential decays; and (ii) the long-time behavior of spin echo and the long-time behavior of the corresponding homogeneous free induction decay are characterized by the same time constants. This prediction extends to various echo problems both within and beyond nuclear magnetic resonance. Experimental confirmation of this prediction would also support the notion of the eigenvalues of time evolution operators in large quantum systems.Calculations of the spin echo (SE) envelope observed by nuclear magnetic resonance (NMR) in solids are, in general, very difficult and, in practice, mostly limited to simple sudden-jump models (e.g., [1]), which cannot capture complex non-Markovian dynamics induced by spin-spin interactions. The latter difficulty is closely related to presently insufficient understanding of the role of chaos in many-body quantum systems. In this Letter, I show that a promising approach, which can both simplify the quantitative analysis of SEs and elucidate the role of chaos, is to exploit the notion of eigenvalues of the time evolution operator. In large isolated quantum systems, these eigenvalues are expected to be analogous to Pollicott-Ruelle (PR) resonances [2,3] in classical chaotic systems. PR resonances and their quantum analogs manifest themselves in the behavior of temporal correlation functions, and as such have been investigated, e.g., in the studies of deterministic diffusion [3], microwave billiards [4], spin dynamics [5-10], and classical Loschmidt echoes [11]. If an exponential decay appearing in one correlation function originates from a true PR resonance, then the same decay should appear in all correlation functions having the same symmetry properties.The specific goal of this Letter is to make the following two propositions:(1) The generic long-time behavior of SE envelope S2 obtained from the sequence [=2-pulse-time --pulse-time -detection] has the form eitheror S2 e ÿ2 C 1 C 2 cos2! S ;where C, C 1 , C 2 , , !, and S are some constants.(2) The constants and ! describing the long-time SE decay are the same as the long-time constants of the corresponding ''homogeneous'' free induction decay (FID) defined by the pulse sequence [=2-pulse-time tdetection] and exhibiting generic long-time behavior [10] of the form either Ft Ae ÿt ;(3)
counterpart of Eq. (1), orFt Ae ÿt cos!t F ;(4) counterpart of Eq. (2). Here A and F are some additional constants. (The meaning of the homogeneous FID will be specified below.) An important consequence of the above propositions is that the long-time constants ( and !) of the SE envelope can be obtained from a much simpler FID calculation. At present, a number of approximations exist (see, e.g., [5,12] and references therein), which can be used to calculate the FID long-time constants with reasonable accuracy.In the following, I shall first consider the case of spinspin interaction only, and then...