Quantum Optical Signatures of Band Topology in Solid-State High Harmonics

TL;DR

This paper presents a novel density-matrix-based theory of high-harmonic generation in solids, linking band topology to quantum light signatures and demonstrating topology-dependent quantum light phenomena like squeezing.

Contribution

It introduces a comprehensive density-matrix approach to HHG in solids, explicitly connecting band topology with quantum light properties and revealing topology-sensitive quantum light generation.

Findings

Topological phase exhibits stronger HHG response.

Cavity-matter interaction produces squeezed high-harmonic quantum light.

Quantum light properties are governed by current-current fluctuations.

Abstract

We develop a general theory of high-harmonic generation (HHG) in solid-state systems, based on a weak-correlation expansion of photonic and matter degrees of freedom. Unlike standard HHG theories, which treat light-matter dynamics through the Schrodinger equation, our approach employs density-matrix evolution, naturally capturing the mixed-state character of both the field and the matter - a critical aspect for describing complex solid-state band structures. We show explicitly that the properties of the emitted fields are governed by the quantum statistics and quantum geometry of the underlying solid. Taking the Su-Schrieffer-Heeger (SSH) model in a one-sided optical cavity as a paradigmatic example and considering the dual regime, we demonstrate that in the topological phase a system exhibits a stronger HHG response and stronger quantum-light signatures than in the trivial phase. Furthermore, we show that cavity-matter interaction gives rise to squeezed high-harmonic quantum light, whose properties are directly imprinted by the current-current fluctuations in the material system. Crucially, the observed squeezing does not rely on a separate quartic Kerr mechanism. In the mesoscopic regime, the genuine quantum Kerr term is higher order in light-matter coupling strength and negligible, while the relevant non-classical effect is governed by current-current fluctuations encoded in the complex susceptibility of the material. This work establishes a direct link between band topology and photon statistics, opening new avenues for topology-sensitive quantum light generation and photon statistics based spectroscopy of solid-state systems.