  Several recent experiments in atomic, molecular and optical systems motivated
a huge interest in the study of quantum long-range %spin systems. Our goal in
this paper is to present a general description of their critical behavior and
phases, devising a treatment valid in $d$ dimensions, with an exponent
$d+\sigma$ for the power-law decay of the couplings in the presence of an
$O(N)$ symmetry. By introducing a convenient ansatz for the effective action,
we determine the phase diagram for the $N$-component quantum rotor model with
long-range interactions, with $N=1$ corresponding to the Ising model. The phase
diagram in the $\sigma-d$ plane shows a non trivial dependence on $\sigma$. As
a consequence of the fact that the model is quantum, the correlation functions
are anisotropic in the spatial and time coordinates for $\sigma$ smaller than a
critical value and in this region the isotropy is not restored even at
criticality. Results for the correlation length exponent $\nu$, the dynamical
critical exponent $z$ and a comparison with numerical findings for them are
presented.
