We introduce an ultrafast, bright, entangled twin-beam source generated by type-0 parametric down-conversion in periodically-poled lithium niobate at MHz repetition rate, with continuously tunable Schmidt number K set by the pump pulse duration. Photon-number statistics characterization via g(2)(0) and singular-value decomposition of the signal spectral density matrix yield K≃1.05 and K≃1.03, respectively, maintained over multiple orders of magnitude in brightness. Group-delay dispersion of the pump drives a continuous transition from single-mode operation to a controlled multimode regime, consistent with the temporal gain window departing from the inverse phase-matching bandwidth. Strong non-degeneracy of the source (signal at 1.37 um, idler at 4.0 um, ∼100 fs duration) decouples a mid-infrared interaction wavelength, which overlaps with molecular vibrational resonances, from a near-infrared detection band, establishing a practical platform for quantum-enhanced metrology, nonlinear interferometry, and mid-infrared spectroscopic sensing. We show that in the bright few-mode limit, the total entanglement resource is clearly separated between modal and occupational degrees of freedom, and that our source allocates up to 95-97% of that resource to the occupational sector.
@article{arxiv.2605.15385,
title = {Single Spatio-Temporal Mode Bright Twin-Beam Source Across the Near- and Mid-Infrared},
author = {Gabriel Demontigny and Patrick Cusson and Amauri Perraton Elorza and Esteban Murillo Zapata and Eli Martel and Andrei Rasputnyi and Maria Chekhova and Stéphane Virally and Denis Seletskiy},
journal= {arXiv preprint arXiv:2605.15385},
year = {2026}
}