Experimental demonstration of conjugate-Franson interferometry

TL;DR

This paper introduces conjugate-Franson interferometry, a new quantum measurement technique that measures photon-pair arrival-time correlations, validated through high-visibility interference and demonstrating sensitivity to spectral phase variations.

Contribution

The paper presents the experimental demonstration of conjugate-Franson interferometry as a novel method for certifying time-energy entanglement and analyzing spectral phase effects.

Findings

Achieved 96% interference visibility surpassing classical threshold

Validated conjugate-Franson interferometry as an alternative entanglement certification method

Demonstrated sensitivity to spectral phase variations in biphoton states

Abstract

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.