Quantum magnonics seeks ways to make collective excitations of magnetic materials work as qubits. Most schemes use propagating magnons, but localized spin textures such as domain walls are easier to pin, observe, and move. However, stable entanglement between such macroscopic objects had not been demonstrated so far. This work fills that gap by proposing a realistic scheme based on optomechanical interaction in a cavity.
Two ferromagnetic strips with pinned Bloch domain walls are placed inside a single-mode chiral cavity. Circularly polarized light exerts radiation pressure on the walls via the inverse Faraday effect, giving an effective optomechanical interaction. The domain walls are modeled as quantum harmonic oscillators with frequencies set by the pinning potential. The system is described by a linearized Hamiltonian and Langevin equations, and stable phases are found from the eigenvalues of the drift matrix. Entanglement is computed via the logarithmic negativity of the covariance matrix.
In the parameter space (detuning and effective coupling), there are regions with one or three stable solutions. Near the line of transcritical bifurcation, where the stability of the roots changes, strong entanglement emerges. When the domain walls have the same frequency, the photon is first entangled with both walls, but as the detuning increases, entanglement shifts to the wall-wall pair. For different frequencies, the photon entangles alternately with one of the walls. Numerically, entanglement persists up to temperatures of about 26 mK at a frequency of 1 GHz, and up to 2.6 K at 100 GHz. The maximum logarithmic negativity reaches about 0.3.
This work demonstrates that entanglement between macroscopic spin textures can be created and controlled without strong nonlinearity, using only dissipative dynamics. This places domain walls on par with other candidates for quantum computing and memory. Unlike previous proposals with propagating magnons, here the objects are stationary and can be addressed individually.
The scheme can be extended to other chiral textures such as skyrmions, and to ferroelectric domain walls, where photostriction provides strong interaction. Using materials with high domain-wall oscillation frequencies (up to 100 GHz) would raise the operating temperature to a few kelvin. Arrays of multiple cavities could be built for scalable quantum networks.
Quantum magnonics, spintronics, quantum optomechanics, quantum sensors.
Experimental implementation using yttrium iron garnet and chiral cavities; measuring entanglement via homodyne detection of reflected light.
The problem of scalability in quantum systems: domain walls as macroscopic qubits could be integrated into dense arrays. The problem of decoherence: the proposed scheme works at higher temperatures than traditional magnonic systems.
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What the work does
The paper proposes a scheme for entangling two domain walls in ferromagnetic strips via a chiral optical resonator. Modeling the walls as quantum oscillators and analyzing the stability of the linearized dynamics, the authors identify parameter regions with strong entanglement, including a switch between photon-wall and wall-wall correlations as the detuning changes. The effect is shown to be robust to temperatures up to a few kelvin at high frequencies.
Where its strength is
The strength of the work is that it demonstrates robust entanglement of macroscopic spin textures without strong nonlinearity, leveraging dissipative phase transitions. This paves the way for using domain walls as stationary qubits that can be individually addressed.
Where it could go next
Worth trying
Why this may matter
The proposed scheme could form the basis for quantum devices that use macroscopic magnetic textures as qubits operating at relatively high temperatures. This is important for scaling quantum technologies and integrating with optical systems.
Nearby works in our archive
The archive is sparse around this work: the closest review is only 0.54, against a usual 0.64. It looks like few have drilled nearby.
Domain walls in magnetic materials behave like particles with a mass of about 10⁻²⁷ kg, and their zero-point fluctuations are around 10 nm, which is orders of magnitude larger than those of a single atom.
In Greg Egan's novel 'Schild's Ladder', quantum states of macroscopic objects are used for computation; domain walls could be a step toward such technology.