Quantum Skyrmions in Mixed States of Light and their Nested Topology

TL;DR

This paper demonstrates that topological skyrmions can exist within mixed quantum states of light, extending their realization beyond pure states and exploring their robustness and potential for quantum information encoding.

Contribution

It introduces a framework for topological skyrmions in mixed quantum states using a coherence-Stokes vector, enabling their realization without full coherence or entanglement.

Findings

Skyrmions can be encoded in the density matrix of mixed states of light.

Topological skyrmions persist in bipartite entangled photon pairs and their subspaces.

Skyrmions show robustness to environmental noise and mixed quantum states.

Abstract

Topological quasiparticles of light, such as classical and quantum optical skyrmions, have so far relied on fully coherent or pure quantum states whose topology is encoded in the entanglement between polarization and two-dimensional spatial modes. Here we show that skyrmionic topology can emerge directly within the density matrix of a mixed quantum state. We introduce a framework in which a coherence-Stokes vector defines a topological texture over the density matrix, enabling the realization of quantum skyrmions using only a pseudospin and a real or synthetic one-dimensional space of modes. For a single photon, the density matrix is analogous to the coherence matrix of classical light, and can be encoded using partially coherent electromagnetic fields. We further analyze the topological texture encoded in a bipartite entangled photon pair, showing how skyrmions arise not only in the full bi-photon system, but also in its reduced subspaces with any pseudospin-mode combination simultaneously. We then explore the robustness of such skyrmions to environmental noise, and discover their persistence in mixed-quantum states of multiple photons. Finally, we propose a feasible experimental route to generate and measure such skyrmions using integrated photonic networks, and suggest avenues for similar implementations in other quantum systems. Our work paves the way for robustly encoding classical information on partially coherent light or on mixed quantum states and for encoding topological charges on many-body quantum systems.