# Modulation Instability-Induced Multimode Squeezing in Quadratic Frequency Combs

## TL;DR

This paper demonstrates how modulation instability in lithium niobate microring resonators can induce multimode squeezing in quadratic frequency combs, advancing integrated quantum photonics for quantum information and measurement.

## Contribution

It introduces a universal framework for analyzing multimode squeezing and reveals modulation instability as a key mechanism for multimode entanglement in quadratic frequency combs.

## Key findings

- Modulation instability enables formation of frequency combs and multimode squeezing.
- A bipartite entanglement criterion for quantum frequency combs is proposed.
- On-chip multimode squeezing facilitates collective noise suppression.

## Abstract

Lithium niobate (LN) microring resonators, characterized by an exceptionally high second-order nonlinear coefficient and superior electro-optic tunability, serve as an outstanding platform for the precise control of integrated quantum frequency combs (QFCs). In this study, we introduce a bipartite entanglement criterion to investigate the pairwise entanglement characteristics of QFCs generated via the spontaneous parametric down-conversion (SPDC) process in lithium niobate microring resonators operating below threshold. Furthermore, we propose a universal framework for analyzing multimode squeezing in quadratic frequency combs, enabling the realization of ultrabroadband and high-degree multimode squeezing. We further reveal the underlying physical mechanism: modulation instability (MI), regulated by temporal walk-off control, not only enables the formation of frequency combs but also induces multimode squeezing in the corresponding resonant modes. This study uncovers the previously unexplored role of on-chip multimode squeezing in quadratic frequency combs while facilitating collective noise suppression across multiple modes, thus holding substantial potential for advancing quantum precision measurement and quantum information processing.