  Phonon spectra of the paraelectric phase, structure and distribution of
polarization in the polar ground state, and energies of ferroelectrically and
antiferroelectrically ordered phases were calculated for stress-free
(KNbO$_3$)$_1$(KTaO$_3$)$_n$ superlattices ($n = {}$1--7) from first principles
within the density-functional theory. Quasi-two-dimensional ferroelectric state
with the polarization localized in the KNbO$_3$ layer was revealed to be the
ground state for the superlattices with $n \ge 2$. The interaction between
these polarized layers decreases exponentially with increasing the interlayer
distance, and the ground-state structure transforms to an array of nearly
independent ferroelectrically ordered planes at large $n$.
