We consider the focusing 3D quantum many-body dynamic which
models a dilute Bose gas strongly confined in two spatial directions. We
assume that the microscopic pair interaction is attractive and given by
, where
and
matches the
Gross–Pitaevskii scaling condition. We carefully examine the effects of the fine interplay
between the strength of the confining potential and the number of particles on the 3D
-body
dynamic. We overcome the difficulties generated by the attractive interaction in 3D and
establish new focusing energy estimates. We study the corresponding BBGKY hierarchy,
which contains a diverging coefficient as the strength of the confining potential tends
to
.
We prove that the limiting structure of the density matrices counterbalances this
diverging coefficient. We establish the convergence of the BBGKY sequence and
hence the propagation of chaos for the focusing quantum many-body system. We
derive rigorously the 1D focusing cubic NLS as the mean-field limit of this
3D focusing quantum many-body dynamic and obtain the exact 3D-to-1D coupling
constant.