- Letter
- Open Access
Sensing quantum fluctuations via forbidden harmonics in quantum materials
Phys. Rev. B 114, L140301 – Published 1 September, 2026
DOI: https://doi.org/10.1103/v886-ypf7
Abstract
High-harmonic generation in solids provides a powerful window into light-driven electron dynamics on attosecond timescales, yet the role of quantum fluctuations of the driving field remains largely unexplored. Here, we show that the quantum state of light offers a nonclassical degree of freedom for controlling strong-field dynamics in quantum materials. Using graphene driven by bicircular quantum light composed of counterrotating and components, we theoretically predict that squeezing-induced fluctuations break the dynamical symmetry of the classical bicircular field and activate otherwise forbidden harmonics. By analyzing the helicity-resolved harmonic spectra, we uncover robust polarization signatures uniquely dictated by the squeezed mode's profile, including a relative-phase-controlled helicity switching in the lowest orders. Crucially, second-order intensity correlations reveal that these forbidden harmonics exhibit strongly enhanced, super-Poissonian photon-number fluctuations, confirming that they inherit and directly transduce the nonclassical statistics of the squeezed driving vacuum. Our findings establish forbidden harmonics as high-fidelity optical probes of the quantum state of light and provide additional opportunities at the intersection of quantum optics, strong-field physics, and lightwave engineering in quantum materials.
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