Quantum magnonics: when magnon spintronics meets quantum information science (2111.14241v2)

Abstract: Spintronics and quantum information science are two promising candidates for innovating information processing technologies. The combination of these two fields enables us to build solid-state platforms for studying quantum phenomena and for realizing multi-functional quantum tasks. For a long time, however, the intersection of these two fields was limited. This situation has changed significantly over the last few years because of the remarkable progress in coding and processing information using magnons. On the other hand, significant advances in understanding the entanglement of quasi-particles and in designing high-quality qubits and photonic cavities for quantum information processing provide physical platforms to integrate magnons with quantum systems. From these endeavours, the highly interdisciplinary field of quantum magnonics emerges, which combines spintronics, quantum optics and quantum information science.Here, we give an overview of the recent developments concerning the quantum states of magnons and their hybridization with mature quantum platforms. First, we review the basic concepts of magnons and quantum entanglement and discuss the generation and manipulation of quantum states of magnons, such as single-magnon states, squeezed states and quantum many-body states including Bose-Einstein condensation and the resulting spin superfluidity. We discuss how magnonic systems can be integrated and entangled with quantum platforms including cavity photons, superconducting qubits, nitrogen-vacancy centers, and phonons for coherent information transfer and collaborative information processing. The implications of these hybrid quantum systems for non-Hermitian physics and parity-time symmetry are highlighted, together with applications in quantum memories and high-precision measurements. Finally, we present an outlook on the opportunities in quantum magnonics.

• The paper presents a comprehensive review of quantum magnonics by analyzing quantum states of magnons and their integration with hybrid quantum platforms.
• It outlines methodologies for generating single-magnon, squeezed, and cat states through enhanced nonlinear interactions in magnetic systems.
• The review identifies practical applications in quantum memory, sensing, and state transfer while addressing experimental challenges and future research directions.

An Overview of Quantum Magnonics: Intersection of Magnon Spintronics and Quantum Information Science

The paper, “Quantum magnonics: when magnon spintronics meets quantum information science,” provides an extensive review of the emerging field of quantum magnonics. This interdisciplinary field represents the confluence of magnon spintronics, quantum optics, and quantum information science, and it aims to explore the quantum characteristics of magnons—quasi-particles indicative of spin wave excitations in ordered magnets. The paper systematically examines the quantum states of magnons, their integration with mature quantum platforms, and the implications of these hybrid systems for both theoretical physics and practical quantum technologies.

Quantum States of Magnons:

The quantization of spin waves manifests as magnons, whose classical versus quantum properties are central to the paper. Core quantum states, such as single-magnon states, squeezed states, and Schrödinger cat states, are reviewed with an emphasis on their unique quantum mechanical properties. Techniques for generating single-magnon states rely on enhancing non-linear interactions (e.g., Kerr non-linearity) within small-scale magnetic systems, while squeezed states, characterized by reduced quantum noise in one component of a magnon's quantum observable, stem from both equilibrium and non-equilibrium processes. The robust presence of squeezed states in both ferromagnetic and antiferromagnetic materials highlights the intricate quantum underpinnings of spin excitation dynamics. Furthermore, cat states represent superpositions of macroscopically distinct states and beg potential applications across quantum computing and high-precision measurements.

Hybrid Quantum Systems:

Magnons' ability to coherently interact with various quantum platforms opens up avenues for the development of multifunctional quantum devices. The paper explores the coupling of magnon modes with cavity photons, superconducting qubits, and phonons. Magnon-photon coupling, when realized inside microwave cavities, leads to phenomena such as Rabi splitting and entanglement between magnons and photons. The coherent interaction between magnons and superconducting qubits heralds advancements in quantum state transfer and single-magnon detection—a foundational step toward practical quantum information processing. Magnon-phonon coupling further contributes to the field of cavity optomechanics and has implications for quantum memory and sensor applications.

Non-Hermitian Physics and $\mathcal{PT}$ Symmetry:

An additional focus is on non-Hermitian physics, particularly the phenomenon of parity-time ($\mathcal{PT}$) symmetry in coupled magnonic systems. Here, the balance of loss and gain in coupled magnetic elements leads to intriguing behaviors at exceptional points—points of degeneracy in non-Hermitian Hamiltonians where eigenvalues and their corresponding eigenvectors coalesce—facilitating potential applications in sensing and signal processing.

Applications in Quantum Technologies:

Throughout, the paper indicates numerous potential applications in quantum technologies deriving from these foundational studies. From quantum memories—where magnonic systems serve as storage mediums for quantum information—to high-precision sensors—capable of detecting minute magnetic fields or temperature changes—the practical potential of magnons, when integrated with robust quantum systems, is vast.

Challenges and Future Directions:

The paper concludes by identifying open challenges and directions for future research. These include experimental verification of theoretical predictions, exploration of magnonic states in noncollinear spin textures, and deeper integration with electronic and photonic quantum information platforms. Overcoming fabrication challenges and understanding decoherence processes will be essential in bridging the gap between theoretical advancements and technological applications.

In summary, the paper elucidates the transformative potential of quantum magnonics, underscoring its role in advancing both foundational quantum physics and the development of next-generation technologies. Its integration of materials science, quantum mechanics, and information theory signifies not just an enrichment of magnetics and spintronics but also a meaningful contribution to the broader scope of quantum science.