Nonclassical Cutoff Fluctuations in Squeezed-Light-Driven High-Harmonic Generation

TL;DR

This paper demonstrates that quantum squeezing of the driving laser field can suppress the fluctuations in high-harmonic generation cutoff, providing a new way to observe quantum effects in nanoscale strong-field physics.

Contribution

It introduces a Wigner phase-space model showing how squeezing reduces cutoff variance below the standard quantum limit in HHG, revealing quantum uncertainty redistribution.

Findings

Amplitude squeezing suppresses HHG cutoff variance below SQL.

Variance decays exponentially with squeezing parameter, independent of vacuum amplitude.

Optimal phase noise reduces variance near r_opt ~ 1.6.

Abstract

Strong-field high-harmonic generation (HHG) is conventionally described semiclassically, with the driving laser treated as a classical field. This approximation becomes insufficient in nanoscale interaction geometries, where extreme spatial confinement raises the vacuum-field amplitude to the ~10^-2 level relative to the driving-field amplitude. When the quantum fluctuations of the driving field are redistributed between conjugate quadratures by squeezing, they can be directly imprinted onto macroscopic HHG observables. To model this interaction, we employ a Wigner phase-space approach that maps the quantum-optical driver onto a stochastic ensemble of time-dependent Schrodinger equation realizations. Although each realization remains classically simulable, the sub-vacuum quadrature covariance structure of squeezed states cannot be reproduced by any field admitting a non-negative Glauber-Sudarshan P-representation. Within this single-mode Gaussian framework, we show that amplitude squeezing suppresses the shot-to-shot variance of the HHG cutoff below the standard quantum limit (SQL). To leading order in the vacuum-to-driver ratio, the normalized cutoff variance decays exponentially with the squeezing parameter, independent of the absolute vacuum-field amplitude and therefore robust against uncertainties in the effective nanoscale mode volume. A subleading phase-noise contribution from the anti-squeezed quadrature produces a variance minimum near r_opt ~ 1.6, providing a concrete experimental target. These results establish the HHG cutoff variance ratio as a nonlinear, self-calibrating Heisenberg witness in which sub-SQL scaling directly reflects the redistribution of quantum uncertainty in the driving field.