Integrating multiple quantum devices is crucial not only for scaling up computational power but also for connecting diverse functionalities in a single system. This integration requires quantum channels that can couple to a variety of quantum devices and transport information coherently across adjacent devices. While optical photons are essential for long-distance communication (e.g., over the internet), they couple weakly with solid-state systems typically used for computation. Therefore, alternatives to photons are needed for short-range, near-device communication.

Magnons, the elementary excitations of magnetization, are promising as near-device quantum information carriers. They interact with a wide range of quantum technologies, including superconducting transmons, optical photons, and nitrogen-vacancy (NV) centers. Their potential is particularly strong in ferromagnetic insulators, where magnons are expected to have macroscopic quantum coherence lengths required for transport. My proposal aims to develop the theoretical foundations to bring forth magnons as reliable quantum information carriers.

The length of a magnon-based communication channel is limited by the quantum coherence length of magnons. My initial objective is to calculate this coherence length by applying Keldysh formalism on a microscopic model of a ferromagnet involving spin-spin interactions and impurities. This theory will be applied to materials promising for quantum magnonics, such as Yttrium Iron Garnet and Mott insulator Cu2OSeO3.

Building on this theory, I will model the motion of a quantum wavepacket of a magnon. A traveling wave, that may consist of a superposition of semi-classical magnons, will experience distortion due to magnon dispersion and dissipation by magnetic impurities. I will analyze these effects to find the typical errors a magnon wavepacket will encounter and their probability of occurrence. Using this analysis, I will derive an effective stochastic model of wavepacket motion within a magnet, which will be suitable for studying abstract quantum communication protocols, thereby bridging solid-state theory and quantum information.

This stochastic model will be applied to a key application in quantum communication: remote entanglement generation between qubits mediated by magnons. I will focus specifically on the regime of weak magnon-qubit coupling to facilitate experimental feasibility. In this regime, I will adapt protocols originally developed for communication via light, tailoring them to be applicable for magnons. This analysis will yield the efficiency of magnon-mediated entanglement generation and methods to improve it.

This project will significantly advance the field of quantum magnonics, adding quantum transport to existing theoretical methods of magnon state generation and measurement. This will also advance solid-state quantum computing, enabling distributed algorithms and quantum networks that integrate diverse quantum technologies.
