  We propose an optical parametric down conversion (PDC) scheme that does not
suffer a trade-off between the state-purity of single-photon wave-packets and
the rate of packet production. This is accomplished by modifying the PDC
process by using a microcavity to engineer the density of states of the optical
field at the PDC frequencies. The high-finesse cavity mode occupies a spectral
interval much narrower than the bandwidth of the pulsed pump laser field,
suppressing the spectral correlation, or entanglement, between signal and idler
photons. Spectral filtering of the field occurs prior to photon creation rather
than afterward as in most other schemes. Operator-Maxwell equations are solved
to find the Schmidt-mode decomposition of the two-photon states produced.
Greater than 99% pure-state packet production is predicted to be achievable.
