High-harmonic generation driven by temporal-mode quantum states of light

We develop a theoretical framework for high-harmonic generation (HHG) driven by quantum states of light based on a temporal-mode expansion of the electromagnetic field. This approach extends previous single plane-wave mode treatments to realistic pulse configurations, resolving conceptual inconsistencies arising from non-normalizable infinite plane waves and establishing consistency between analytical and numerical methods. We derive a correction factor that quantifies deviations from the single-mode approximation and show that it remains below 10^-4 for intensities typical of HHG $sim 10¹⁴ $W/cm$²$. This result confirms that free-space HHG driven by any quantum state of light is accurately described by averaging semi-classical calculations over the Husimi distribution, with no observable genuine quantum effects. The absence of such effects is attributed to the large photon numbers $sim 10¹¹$ required to reach HHG intensities in free space, which render quantum fluctuations negligible. We discuss nanophotonic environments with ultrasmall mode volumes as potential platforms where few-photon strong-field processes could exhibit genuine quantum signatures.