  We have generated a new type of biphoton state by cavity-enhanced
down-conversion in a type-II phase-matched, periodically-poled KTiOPO_4 (PPKTP)
crystal. By introducing a weak intracavity birefringence, the
polarization-entangled output was modulated between the singlet and triplet
states according to the arrival-time difference of the signal and idler
photons. This cavity-enhanced biphoton source is spectrally bright, yielding a
single-mode fiber-coupled coincidence rate of 0.7 pairs/s per mW of pump power
per MHz of down-conversion bandwidth. Its novel biphoton behavior may be
utilized in sensitive measurements of weak intracavity birefringence.

Quantum Physics