  We propose, analyze, and evaluate a technique for the joint measurement of
time-frequency entanglement between two photons. In particular, we show that
the frequency sum and time difference of two photons could be simultaneously
measured through the sum-frequency generation process, without measuring the
time or frequency of each individual photon. We demonstrate the usefulness of
this technique by using it to design a time-frequency entanglement based
continuous variable superdense coding and a quantum illumination protocol.
Performance analysis of these two protocols suggests that the joint measurement
of strong time-frequency entanglement of non-classical photon pairs can
significantly enhance the performance of joint-measurement based quantum
communication and metrology protocols.
