A theoretical and experimental study describes long-range quantum spectral entanglement in ultrafast, highly nondegenerate optical systems, where the frequency modes of a frequency comb become correlated beyond the usual neighbouring pairs. The work addresses how quantum correlations arise in regimes of intense pumping and large mode spacing, a scenario relevant to metrology and the generation of squeezed states.

The key is to combine multi-comb description frameworks with continuous-variable theory for the optical field. Working in a highly nondegenerate regime, the authors show that entanglement is not limited to adjacent modes but extends across the spectrum, giving rise to long-range correlations between frequencies separated by multiple comb intervals.

The central result is the identification of an extended spectral entanglement structure, with correlations persisting between modes very distant in frequency. This broadens the standard framework of quantum frequency combs, where correlations are usually described between neighbouring modes or around the carrier, and suggests that ultrafast systems can generate multimode states with a much higher degree of entanglement than expected.

The implications are relevant for high-precision optical metrology, the generation of squeezed light and the design of quantum information protocols based on continuous variables. Highly nondegenerate systems could offer richer quantum resources for sensors and for entanglement distribution in the spectral domain. The work also opens questions about the stability of these correlations against loss and dispersion, and about their possible exploitation in integrated photonic architectures.