Two pendulums linked by a spring of light begin to swing in unison without ever touching. In the quantum world, such synchronization means something more: particles lose their individuality and become a single object. This is exactly what happens to two domain walls, the boundaries between regions of different magnetization, inside a chiral resonator, a cavity that transmits light of only one polarization.

Light in the resonator exerts pressure on the walls via the inverse Faraday effect, where photons create a magnetic field in the medium. This optomechanical interaction turns light into a spring: oscillations of one wall are transmitted to the other. The physicists built a theoretical model in which the walls behave as quantum oscillators and found regions of parameters where a dissipative phase transition occurs, a change of stable state due to losses. Near such points, entanglement is maximal.

Previously, it was unclear whether two macroscopic magnetic defects could be entangled without strong nonlinearity and at temperatures above millikelvin. Now it has been shown that two pinned walls in a chiral resonator can be brought into a stable entangled state by controlling only the laser and the pinning potential. At equal frequencies, the photon first entangles with both walls, and as the detuning increases, entanglement shifts to the wall-wall pair. At different frequencies, the photon entangles alternately with one of the walls.

This scheme does not require strong nonlinearity and relies on dissipative dynamics, making it practical. Domain walls as macroscopic qubits could be integrated into dense arrays for quantum computing and memory. Operation at temperatures up to a few kelvin solves the decoherence problem that plagues traditional magnonic systems. In the future, such entangled walls could become sensitive magnetic field probes and building blocks for quantum networks.

The numbers are impressive: entanglement persists up to 2.6 K at a frequency of 100 GHz, and the laser power needed is only a few milliwatts. The logarithmic negativity, a measure of entanglement, reaches 0.3, a notable value for macroscopic objects. The scheme can be extended to other chiral textures, such as skyrmions, and to ferroelectric domain walls, where photostriction provides strong interaction. It is possible to create arrays of several resonators for scalable quantum networks.