  Nuclear spins are promising candidates for quantum information processing
because their good isolation from the environment precludes the rapid loss of
quantum coherence. Many strategies have been developed to further extend their
decoherence times. Some of them make use of decoupling techniques based on the
Carr-Purcell and Carr-Purcell-Meiboom-Gill pulse sequences. In many cases, when
applied to inhomogeneous samples, they yield a magnetization decay much slower
than the Hahn echo. However, we have proved that these decays cannot be
associated with longer decoherence times as coherences remain frozen. They
result from coherences recovered after their storage as local polarization and
thus they can be used as memories. We show here how this freezing of the
coherent state, which can subsequently be recovered after times longer than the
natural decoherence time of the system, can be generated in a controlled way
with the use of field gradients. A similar behaviour of homogeneous samples in
inhomogeneous fields are demonstrated. It is emphasized that the effects of
inhomogeneities in solid state NMR, independently of their origin, should not
be disregarded as they play a crucial role in multipulse sequences.
