Using the method of photon addition into both beams of a multi-mode twin beam, a new kind of highly nonclassical states is obtained. As these states exhibit tight photon-number cross-correlations and are endowed with marginal sub-Poissonian photon-number distributions, we suggest to call them sub-Poissonian twin beams. We reveal their properties in the model as well as experimentally using mesoscopic twin beams. An intensified CCD camera as a photon-number resolving detector is applied to prepare as well as characterize the states with up to three photons added into each beam and containing from 2.8 to 7.5 photons in average in each beam. Their nonclassicality as well as non-Gaussianity are determined to demonstrate their prominent features. Spatial correlations of photons in the beams are exploited to improve the properties of the generated states. The beams are promising as the quantum light source for virtual-state entangled-photon spectroscopy. Their properties are prospective for monitoring and manipulating two-photon light–matter interactions.
We demonstrate the generation of sub-Poissonian twin fields via near-degenerate spontaneous four-wave mixing (SFWM) in warm
85Rb vapor at 795~nm. When seeded with a weak coherent field, the generated twin beams exhibit approximately
5.5 dB of intensity-difference squeezing in free space and retain about
3 dB after coupling into polarization-maintaining (PM) fibers. Under vacuum seeding, time-resolved photon-counting measurements yield Mandel parameters of
Q≈−0.7 for each individual field, demonstrating strong photon-number squeezing. To explain these observations, we develop a finite-resource saturation model in which occupation-dependent SFWM gain, arising from competition for a finite nonlinear gain resource, suppresses large photon-number fluctuations within an effective collective mode selected by the PM-fiber spatial projection, thereby producing the observed negative Mandel-
Q parameters. The temporal cross-correlation between the twin photons exhibits a distinctive flat-topped profile resulting from the interplay of multiple
χ(3) processes in the atomic medium and is in excellent agreement with the theoretical model. Combining high photon flux, near-resonant operation, robust sub-Poissonian photon statistics, and fiber compatibility, this source provides a promising platform for scalable quantum-enhanced sensing and quantum information processing.