[Paper Review] Coherent coupling between ferromagnetic magnon and superconducting qubit

Yutaka Tabuchi, Seiichiro Ishino+5 more|arXiv (Cornell University)|Oct 14, 2014

Mechanical and Optical Resonators3 citations

TL;DR

This paper demonstrates strong, coherent coupling between a magnon excitation in a millimeter-sized yttrium iron garnet (YIG) sphere and a superconducting transmon qubit via a microwave cavity, achieving a coupling strength of 21.0 MHz—exceeding both the magnon and cavity decay rates. The system operates in the strong coupling regime, enabling quantum control of magnons through tunable parametric driving, establishing a foundation for quantum magnonics.

ABSTRACT

Rigidity of an ordered phase in condensed matter results in collective excitation modes spatially extending in macroscopic dimensions. Magnon is a quantum of an elementary excitation in the ordered spin system, such as ferromagnet. Being low dissipative, dynamics of magnons in ferromagnetic insulators has been extensively studied and widely applied for decades in the contexts of ferromagnetic resonance, and more recently of Bose-Einstein condensation as well as spintronics. Moreover, towards hybrid systems for quantum memories and transducers, coupling of magnons and microwave photons in a resonator have been investigated. However, quantum-state manipulation at the single-magnon level has remained elusive because of the lack of anharmonic element in the system. Here we demonstrate coherent coupling between a magnon excitation in a millimetre-sized ferromagnetic sphere and a superconducting qubit, where the interaction is mediated by the virtual photon excitation in a microwave cavity. We obtain the coupling strength far exceeding the damping rates, thus bringing the hybrid system into the strong coupling regime. Furthermore, we find a tunable magnon-qubit coupling scheme utilising a parametric drive with a microwave. Our approach provides a versatile tool for quantum control and measurement of the magnon excitations and thus opens a new discipline of quantum magnonics.

Motivation & Objective

• To achieve coherent, strong coupling between a magnon mode in a ferromagnetic insulator and a superconducting qubit for quantum information applications.
• To overcome the challenge of weak single-magnon coupling by leveraging collective magnon excitations and qubit nonlinearity.
• To demonstrate tunable coupling via parametric driving, enabling dynamic control of magnon-qubit interactions.
• To establish a hybrid quantum system combining macroscopic spin excitations and superconducting circuits for quantum state manipulation.

Proposed method

• A transmon-type superconducting qubit is coupled to a microwave cavity mode (TE102), which mediates interaction between the qubit and the Kittel-mode magnon in a YIG sphere.
• The YIG sphere (0.5 mm diameter) supports a coherent, low-dissipation magnon mode with a large spin density (N ≈ 1.4×10^18), enhancing coupling strength.
• Coupling is mediated via virtual photon exchange in the cavity, with the qubit and magnon interacting indirectly through the cavity mode, avoiding direct qubit-magnon coupling.
• Tunable coupling is achieved using a parametric microwave drive, enabling dynamic control of the effective magnon-qubit interaction strength.
• Theoretical modeling uses a Hamiltonian including qubit-cavity, cavity-magnon, and qubit-magnon interactions, with secular approximation and perturbative treatment of residual coupling.
• Numerical simulations of the Kittel-mode spectrum under parametric drive reproduce observed doublets and Rabi splitting, validating the model.

Experimental results

Research questions

• RQ1 Can a magnon mode in a macroscopic ferromagnetic insulator be coherently coupled to a superconducting qubit in the quantum regime?
• RQ2 What is the achievable coupling strength between a magnon and a superconducting qubit, and does it exceed decoherence rates to enter the strong coupling regime?
• RQ3 Can the magnon-qubit coupling be dynamically tuned using a parametric microwave drive?
• RQ4 What is the role of residual coupling in the system, and how does it affect the observed spectral features?

Key findings

• The magnon-qubit coupling strength is measured at 21.0 MHz, exceeding both the magnon linewidth (1.4 MHz) and cavity linewidth (2.5 MHz), confirming operation in the strong coupling regime.
• The system exhibits clear anticrossing in the spectrum, indicating coherent energy exchange between the magnon and cavity modes.
• A second magnetostatic mode, detuned by 4.3 MHz, is observed to couple to the cavity with a strength of 4.2 MHz, contributing to spectral splitting.
• The parametric drive enables tunable coupling, with a conversion ratio of 4.0 ± 0.4 MHz/pW between drive power and effective coupling strength.
• Residual coupling of -2.5 MHz (measured) is observed, shifting the Kittel-mode frequency depending on the qubit state, consistent with numerical simulations.
• Numerical simulations reproduce the experimental doublet and two-photon Rabi splitting under parametric drive, validating the theoretical model.