  We study the conditional preparation of single photons based on parametric
downconversion, where the detection of one photon from a given pair heralds the
existence of a single photon in the conjugate mode. We derive conditions on the
modal characteristics of the photon pairs, which ensure that the conditionally
prepared single photons are quantum-mechanically pure. We propose specific
experimental techniques that yield photon pairs ideally suited for
single-photon conditional preparation.
