  Photonic time-frequency entanglement is a promising resource for quantum
information processing technologies. We investigate swapping of
continuous-variable entanglement in the time-frequency degree of freedom using
three-wave mixing in the low-gain regime with the aim of producing heralded
biphoton states with high purity and low multi-pair probability. Heralding is
achieved by combining one photon from each of two biphoton sources via
sum-frequency generation to create a herald photon. We present a realistic
model with pulsed pumps, investigate the effects of resolving the frequency of
the herald photon, and find that frequency-resolving measurement of the herald
photon is necessary to produce high-purity biphotons. We also find a trade-off
between the rate of successful entanglement swapping and both the purity and
quantified entanglement resource (negativity) of the heralded biphoton state.
