  The correlation properties of the pump field in spontaneous parametric
down-conversion are crucial in determining the degree of entanglement of
generated signal and idler photons. We find theoretically that
continuous-variable entanglement of the transverse positions and momenta of
these photons can be achieved only if the coherence of the pump beam is
sufficiently high. The positions of signal and idler photons are found to be
correlated, even for an incoherent pump. However, the momenta of the signal and
idler photons are not anti-correlated, even though transverse momentum is
conserved.
