High-order correlations and ultrafast Wigner negativities in bright-squeezed-vacuum-driven high-harmonic generation

High-harmonic generation (HHG) is traditionally understood through semiclassical models where matter interacts with a classical laser field. This paper investigates whether using nonclassical light—specifically bright squeezed vacuum (BSV)—can induce quantum-optical effects in the emitted harmonics and the driving field itself that cannot be explained by classical ensemble sampling.

The researchers solve the fully quantized light-matter dynamics of a two-level system (TLS) coupled to a driving mode and a continuum of emission modes. To overcome the computational difficulty of simulating broadband, strongly squeezed fields, they employ a squeezed-frame transformation. This allows them to reach the infinite-photon limit while maintaining a converged multimode Hilbert space, enabling the direct calculation of higher-order photon correlations and the Wigner distribution of the driving field.

The study reveals that BSV driving produces harmonic emission spectra with qualitatively distinct second- and third-order photon correlations compared to coherent excitation. Notably, the back-action of the TLS on the driving field generates pronounced Wigner-function negativities. These nonclassical features evolve on attosecond timescales and reach maximum negativity at half-cycle intervals, coinciding with the vector potential maxima. The authors show that these quantum signatures are highly nonlinear and accumulate over the interaction time, providing a clear distinction from semiclassical models that preserve positive Wigner distributions by construction.

This work establishes a rigorous quantum-electrodynamical framework for broadband strong-field dynamics. By demonstrating that BSV driving induces measurable quantum back-action and unique correlation patterns, the paper provides a theoretical foundation for future quantum-HHG experiments. It suggests that higher-order photon correlations could serve as sensitive probes for light-matter dynamics in complex systems like diamond NV centers or excitonic materials.