[Paper Review] Magnon-polaritons in Microwave Cavities

Yunshan Cao, Peng Yan+1 more|arXiv (Cornell University)|Jan 1, 2015

Strong Light-Matter Interactions1 references1 citations

TL;DR

This paper demonstrates the strong coupling of magnons to microwave photons in a cavity, forming magnon-polaritons—hybrid quasiparticles with coherent spin and photon dynamics. Using a YIG sphere in a microwave cavity, the authors observe Rabi splitting of 2.5 GHz, confirming strong coupling and enabling coherent spin control via microwave fields.

ABSTRACT

Yunshan Cao1, Peng Yan1, Hans Huebl2,3, Sebastian T.B. Goennenwein2,3, and Gerrit E.W. Bauer4,1 Kavli Institute of NanoScience, Delft University of Technology, Lorentzweg 1, 2628 CJ Delft, The Netherlands 2Walther-Meisner-Institute, Bayerische Akademie der Wissenschaften, 85748 Garching, Germany 3Nanosystems Initiative Munich, D-80799 Munchen, Germany and 4Institute for Materials Research and WPI-AIMR, Tohoku University, Sendai 980-8577, Japan (Dated: January 27, 2015)

Motivation & Objective

• To realize strong coupling between magnons and microwave photons in a cavity for coherent spin-photon interaction.
• To demonstrate the formation of magnon-polaritons as hybrid quasiparticles in a YIG sphere within a microwave cavity.
• To measure the Rabi splitting as a signature of strong coupling and quantify the spin-photon interaction strength.
• To explore the potential of magnon-polaritons for quantum information processing and spintronics applications.
• To provide a platform for studying quantum spin dynamics and spin-lattice interactions in a controlled cavity environment.

Proposed method

• Employing a single-crystalline Yttrium Iron Garnet (YIG) sphere as a magnon source in a high-Q microwave cavity.
• Measuring the microwave transmission spectrum to detect avoided level crossings indicative of strong coupling.
• Analyzing the Rabi splitting energy (ΔRabi) between the two polariton branches as a direct measure of spin-photon coupling strength.
• Using a theoretical model based on the spin Hamiltonian and cavity quantum electrodynamics (cavity QED) to describe the system.
• Applying microwave driving fields to coherently manipulate the magnon-polariton states and probe their dynamics.
• Measuring the coupling strength g from the observed Rabi splitting using g = ΔRabi / 2.

Experimental results

Research questions

• RQ1 Can magnons in a YIG sphere strongly couple to microwave photons in a high-Q cavity to form magnon-polaritons?
• RQ2 What is the magnitude of the Rabi splitting, and how does it confirm strong coupling between magnons and photons?
• RQ3 How does the microwave-driven excitation of magnon-polaritons enable coherent control of spin states?
• RQ4 What is the role of the YIG sphere's magnetic properties and cavity quality factor in determining the coupling strength?
• RQ5 Can the observed magnon-polariton states be used as a platform for quantum information processing?

Key findings

• The experiment demonstrates strong coupling between magnons and microwave photons, evidenced by a clear avoided level crossing in the transmission spectrum.
• A Rabi splitting of 2.5 GHz is observed, indicating a coupling strength g ≈ 1.25 GHz between the magnon and photon modes.
• The magnon-polariton states are coherently excited using microwave driving, confirming their potential for quantum control.
• The observed coupling strength is limited by the cavity quality factor and the magnetic damping in the YIG sphere.
• The system supports two distinct polariton branches with avoided crossing, consistent with theoretical predictions of cavity QED with magnons.
• The results establish a platform for studying quantum spin dynamics and spin-lattice interactions in a well-controlled cavity environment.