525-nm two-photon NOON-state is generated by quantum frequency up-conversion of 1547-nm photon pairs. The up-converted two photons are highly indistinguishable with nearly perfect Hong-Ou-Mandel-interference and two-photon visibility high enough to beat the standard quantum limit.
Precise measurement is at the heart of physical science, and taking advantage of quantum effects, where practicable, affords sensitivity beyond the reach of classical methods. Rather than increasing photon number to reduce the de Broglie wavelength and achieve superresolution via quantum optics, the authors use quantum frequency up-conversion to realize 525-nm maximally entangled photons. This two-photon NOON state offers a visibility better than the threshold for beating the standard quantum limit, and could be used for high-precision metrology in many applications.
Ultra-broadband frequency entangled biphotons have exceptionally short temporal duration and can achieve narrow Hong–Ou–Mandel (HOM) interference patterns, making them vital for quantum metrology applications. However, the bandwidth of previously demonstrated biphotons used for HOM interference has been limited to less than 400 nm, which is not wide enough to achieve ultra-high precision in quantum metrology. In order to push the precision to its limit, we have theoretically designed a 20-mm-long chirped periodically poled lithium niobate (CPPLN) crystal with type-II phase matching. Pumped by a 405 nm laser, the down-converted signal and idler photons can cover the wavelength range from 420 to 5200 nm, approaching the transparent upper bound of the LN crystal. In the simulated Hong–Ou–Mandel (HOM) interference, the full-width at half-maximum of the interference pattern is 965 attoseconds, with a visibility of 99.44%. Following a similar approach, we also designed a type-0 CPPLN crystal to achieve higher brightness. These ultra-broadband biphoton sources can generate ultra-narrow quantum interference patterns, which have the potential for attosecond-level high-precision quantum metrology.
We experimentally demonstrated the Hong-Ou-Mandel (HOM) interference between two photons after visible-to-telecommunication wavelength conversion. In the experiment, we prepared a heralded single photon by using spontaneous parametric down conversion (SPDC) and the other photon from a weak laser source at 780 nm. We converted the wavelength of both photons to the telecommunication wavelength of 1522 nm by using difference-frequency generation in a periodically poled lithium niobate, and then observed the HOM interference between the photons. The observed visibility is $0.76\ifmmode\pm\else\textpm\fi{}0.12$ which clearly shows the nonclassical interference of the two photons. The high-visibility interference is an important step for fiber-based quantum communications of photons generated from visible-photon emitters.