  The quantum state of the photon pair generated from type-II spontaneous
parametric down-conversion pumped by a ultrafast laser pulse exhibits strong
decoherence in its polarization entanglement, an effect which can be attributed
to the clock effect of the pump pulse or, equivalently, to distinguishing
spectral information in the two-photon state. Here, we propose novel temporal
and spectral engineering techniques to eliminate these detrimental decoherence
effects. The temporal engineering of the two-photon wavefunction results in a
universal Bell-state synthesizer that is independent of the choice of pump
source, crystal parameters, wavelengths of the interacting photons, and the
bandwidth of the spectral filter. In the spectral engineering technique, the
distinguishing spectral features of the two-photon state are eliminated through
modifications to the two-photon source. In addition, spectral engineering also
provides a means for the generation of polarization-entangled states with novel
spectral characteristics: the frequency-correlated state and the
frequency-uncorrelated state.
