  Franson interferometry is a well-known quantum measurement technique for
probing photon-pair frequency correlations that is often used to certify
time-energy entanglement. We demonstrate the complementary technique in the
time basis, called conjugate-Franson interferometry, that measures photon-pair
arrival-time correlations, thus providing a valuable addition to the quantum
toolbox. We obtain a conjugate-Franson interference visibility of $96\pm 1$%
without background subtraction for entangled photon pairs generated by
spontaneous parametric down-conversion. Our measured result surpasses the
quantum-classical threshold by 25 standard deviations and validates the
conjugate-Franson interferometer (CFI) as an alternative method for certifying
time-energy entanglement. Moreover, the CFI visibility is a function of the
biphoton's joint temporal intensity and is therefore sensitive to that state's
spectral phase variation, something which is not the case for Franson
interferometry or Hong-Ou-Mandel interferometry. We highlight the CFI's utility
by measuring its visibilities for two different biphoton states, one without
and the other with spectral phase variation, and observing a 21% reduction in
the CFI visibility for the latter. The CFI is potentially useful for
applications in areas of photonic entanglement, quantum communications, and
quantum networking.
