  We study the properties of spectra and eigenfunctions for a chain of $1/2-
$spins (qubits) in an external time-dependent magnetic field, and under the
conditions of non-selective excitation (when the amplitude of the magnetic
field is large). This model is known as a possible candidate for experimental
realization of quantum computation. We present the theory for finding
delocalization transition and show that for the interaction between nearest
qubits, the transition is very different from that to quantum chaos. We explain
this phenomena by showing that in the considered region of parameters our model
is close to an integrable one. According to a general opinion, the threshold
for the onset of quantum chaos due to the interqubit interaction decreases with
an increase of the number of qubits. Contrary to this expectation, for a
magnetic field with constant gradient we have found that chaos border does not
depend on the number of qubits. We give analytical estimates which explain this
effect, together with numerical data supporting our analysis. Random models
with long-range interactions are studied as well. In particular, we show that
in this case the delocalization and quantum chaos borders coincide.
