Hong-Ou-Mandel interferometry on a biphoton beat note

The ability to control the spectral and spatial properties of photons and harness non-classical phenomena such as interference, has led to remarkable progress in quantum theory and has enabled quantum information processing to reach for first practical applications. Among the many manifestations of non-classical interference, the Hong-Ou-Mandel (HOM) bunching effect is a particularly valuable tool in quantum information processing; it finds application in the characterization of single-photon sources or to tailor high-dimensional quantum states. In particular, the Hong-Ou-Mandel effect holds great promise for conceptually new schemes in sensing and metrology, where it can be used either to generate N00N states, or to perform very precise measurements of temporal delays.

With regards to temporal delay measurements, which play a central role e.g. for quantum coherence tomography, the broad consensus has been that the width of the HOM dip, i.e. the coherence time, imposes the ultimate limit on the precision of a HOM sensor. As a consequence, ultra-broad-band photon sources have long been hailed as a vital prerequisite for such applications, and great technological efforts are being dedicated to the development of suitable ultra-broadband biphoton sources.

In this article, we challenge this assumption and propose an arguably more practicalalternative. We beleve it is the route towards ultra-precise HOM interferometry. Our approach is based on superpositions of two well separated and entangled discrete frequency modes and coincidence detection on the bi-photon beat note. We explore the sensitivity limits as a function of the difference frequency of color-entangled states and find that the precision is mainly determined not by the coherence time of photons, but by the separation of the center frequencies of the state. Aside from promising improved precision, the approach allows to increase the dynamic range of a HOM-based sensor, provided the required frequency nondegenerate states can be generated in a tunable manner

The Fisher information in this scenario is analyzed to determine the optimum working points for a HOM sensor in presence of background noise and imperfect visibility. We also experimentally demonstrate an optimized HOM-based sensor based on discrete frequency entanglement that we use to detect delays introduced by temperature drifts in an optical fiber. The results obtained in this proof-of-concept experiment might indicate a new direction towards fully harnessing HOM interference in quantum sensing and quantum information processing. Quantum interference of unconventional frequency states on a beam splitter may provide a more practical way of increasing the timing resolution in HOM-based sensors.

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