# Inelastic scattering of photon pairs in qubit arrays with subradiant states

## TL;DR

This paper presents a theoretical analysis of inelastic photon pair scattering in qubit arrays, highlighting the role of subradiant states in enhancing scattering and enabling quantum information storage.

## Contribution

It introduces a rigorous approach to analyze photon scattering in qubit arrays, revealing how subradiant states can be engineered and utilized for quantum information applications.

## Key findings

- Strong scattering enhancement at double-excited subradiant resonances
- Subradiant and twilight states produce long-lived photon correlations
- N-excitation subradiant states require more than 2N qubits to engineer

## Abstract

We develop a rigorous theoretical approach for analyzing inelastic scattering of photon pairs in arrays of two-level qubits embedded in a waveguide. Our analysis reveals strong enhancement of the scattering when the energy of incoming photons resonates with the double-excited subradiant states. We identify the role of different double-excited states in the scattering such as superradiant, subradiant, and twilight states, being a product of single-excitation bright and subradiant states. Importantly, the N-excitation subradiant states can be engineered only if the number of qubits exceeds 2N. Both the subradiant and twilight states can generate long-lived photon-photon correlations, paving the way to a storage and processing of quantum information.