  In quantum computation, it is of paramount importance to locate the parameter
space where maximal coherence can be preserved in the qubit system. In recent
years environment-induced decoherence based the quantum Brownian motion (QBM)
models have been applied to two level systems (2LS) interacting with an
electromagnetic field, leading to the general belief that 2LS are easily
decohered. In a recent paper C. Anastopoulos and B. L. Hu [Phys. Rev. A62,
(2000) 033821] derived a new exact non-Markovian master equation at zero
temperature, from which they showed that this belief is actually misplaced. For
a two-level atom (2LA)- electromagnetic field (EMF) system the decoherence time
is rather long, comparable to the relaxation time. Theoretically this is
because the dominant interaction is the $\hat \sigma_{\pm}$ type of coupling
between the two levels (what constitutes the qubit) and the field, not the
$\hat \sigma_z$ type, which shows the QBM behavior. Depending on the coupling
the field can act as a resonator (in an atom cavity) or as a bath (in QBM) and
produce very different decoherent behavior in the system. This is not new to
Cavity QED experimentalists: the 2LA-EMF system maintaining its coherence in
sufficiently long duration is the reason why they can manipulate them so well
to show interesting quantum coherence effects.
