Physicists have proposed a way to control quantum entanglement between light and microwave photons using nothing more exotic than a spinning magnetic material. In a theoretical study published in Applied Physics B, researchers describe a hybrid cavity optomagnonic system in which the Barnett effect—the magnetization produced when a magnetic substance rotates—shifts the frequency of a magnon mode in a way that makes the direction of entanglement between different parts of the system controllable, even perfectly switchable, by simply reversing an external magnetic field.
The scheme, proposed by MingCui Li and AiXi Chen, is a theoretical design rather than an experimental demonstration. It builds on a hybrid system containing a tunable Fabry-Perot optical cavity and a microwave cavity. Inside the microwave cavity sit two pieces of yttrium iron garnet, a magnetic insulator prized in quantum physics research: a stationary YIG slab and a YIG sphere that rotates around a fixed axis. The magnon modes—the quantized spin waves—in both pieces of YIG are strongly coupled to the microwave cavity mode through magnetic dipole interactions, while the magnons in the stationary slab also interact with optical modes in the Fabry-Perot cavity through a magnon-induced Brillouin scattering process.
The optical side of the system is deliberately engineered for precision. Because YIG is birefringent, the Fabry-Perot cavity supports two distinguishable sets of optical resonant modes, labeled transverse magnetic and transverse electric. A pump laser at 1550 nanometers drives one of these modes, and the system is tuned to a triple-resonance condition in which the frequencies of the two optical modes and the magnon mode satisfy a strict resonance relation. Under strong pumping, the driven optical mode can be treated as a classical field, which linearizes the dynamics and allows the team to analyze the quantum fluctuations of the remaining modes with a standard covariance-matrix treatment.
The crucial ingredient is the rotating YIG sphere. Rotation of a magnetic material generates magnetization through the Barnett effect, and in this system it produces a frequency shift of the magnon mode in the sphere. That shift, denoted by the researchers as a Barnett detuning, can be made positive or negative simply by reversing the direction of the external bias magnetic field applied to the sphere, or by reversing the rotation direction itself. This sign flip is what gives the scheme its nonreciprocal character: the entanglement structure of the system depends on the sign of the shift, not just its magnitude.
In the proposed configuration, the magnon mode in the stationary YIG slab serves as an intermediary that bridges optical and microwave photons, which otherwise have a frequency mismatch far too large for direct interaction. When the Barnett shift is positive, the calculations show that bipartite entanglement arises mainly between the second optical mode and the stationary magnon, and between that optical mode and the microwave cavity mode. When the shift is negative, entanglement instead appears between the optical mode and the magnon in the rotating sphere, while the optical-microwave entanglement vanishes in the relevant parameter region. In other words, flipping the magnetic field switches the optical mode’s entangled partner from one magnon to the other, or to the microwave field, even though neither of those latter modes interacts directly with the light.
To quantify how sharply this switching works, the authors introduce a bidirectional contrast ratio that compares the entanglement obtained for positive and negative values of the Barnett shift. The ratio ranges from zero, indicating no nonreciprocity, to one, indicating ideal nonreciprocity in which the entanglement can be turned fully on or off. In their numerical results, the contrast ratio for the optical-microwave pair reaches the ideal value of one, meaning that optical-microwave entanglement can be perfectly switched on and off by toggling the sign of the Barnett frequency shift. The contrast ratios for the optical-magnon pairs reach approximately 0.83 and 0.98 in the strongest configurations examined, with larger Barnett shifts producing stronger nonreciprocity.
The proposal extends beyond pairwise entanglement. Using a residual contangle measure designed for continuous-variable systems, the team also computed tripartite entanglement among combinations of the optical mode, the two magnon modes, and the microwave mode. Tripartite optical-magnon-magnon entanglement appears mainly when the Barnett shift is negative, while tripartite optical-magnon-microwave entanglement dominates when the shift is positive. With the effective optomagnonic coupling set high enough, the contrast ratio for the tripartite optical-magnon-microwave entanglement also approaches the ideal value, and the calculations show near-perfect on-off control of one tripartite configuration and perfect on-off control of another, along with complete transfer between them, achieved purely by reversing the bias field.
The strength of the entanglement depends on several tunable parameters. The effective optomagnonic coupling grows with the single-photon magnon coupling coefficient, the quality factor of the optical cavity, and the square root of the pump power, so raising any of these strengthens the entanglement. In the baseline calculation, the team assumed a 1550-nanometer laser at 10 milliwatts, a Fabry-Perot quality factor of 5 times 10 to the 7th, a microwave drive at 5 gigahertz, and electromagnonic couplings of 30 megahertz, with a bath temperature of 0.01 kelvin. Lowering the quality factor reduced both the entanglement and its contrast ratios, while increasing the pump power to 100 or 150 milliwatts allowed significant bipartite and tripartite entanglement to persist even at a reduced quality factor of 8 times 10 to the 6th.
The authors also address feasibility. The required rotational speeds fall within experimentally demonstrated ranges: nanoparticles around 100 nanometers have been spun at several gigahertz, and theoretical work on the magnonic Einstein-de Haas effect suggests a micron-sized YIG sphere could be accelerated to angular velocities exceeding 10 gigahertz. The analysis relies on a Markovian noise assumption, which the authors argue is justified because the relevant reservoir correlation times—including magnon-phonon thermalization times of 1 to 10 nanoseconds—are far shorter than the system’s microsecond-scale evolution time. They further suggest that electromagnetic noise from the rotation mechanism could be minimized with an airflow-driven turbine rather than an electric motor, and that thermal fluctuations could be suppressed by operating in a dilution refrigerator at millikelvin temperatures, where experiments have already shown YIG maintains extremely low magnon damping and strong coupling to microwave cavity photons.
If realized experimentally, the scheme would offer a route to directing quantum correlations among optical, magnonic, and microwave degrees of freedom in quantum nodes that link disparate physical systems. Because optical and microwave photons carry fundamentally different frequencies, interfaces that generate and route entanglement between them are of interest for hybrid quantum networks, and the ability to switch entanglement on and off with a magnetic field adds a control knob that does not require re-engineering the device itself. For now, the results remain a theoretical prediction based on a linearized Gaussian analysis under stated parameter assumptions; the authors note that the applicability of the Markovian assumption in the specific parameter regime still requires careful verification, and that maintaining low electromagnetic background noise and stable temperatures in a rotating YIG sphere remain significant experimental challenges.
Subject of Research: Theoretical proposal for nonreciprocal optical-microwave entanglement control via the Barnett effect in a hybrid cavity optomagnonic system
Article Title: Nonreciprocal optical-microwave entanglement induced by Barnett effect
Article References: Li, M., & Chen, A. (2026). Nonreciprocal optical-microwave entanglement induced by Barnett effect. Applied Physics B, 132(11), Article 114. https://doi.org/10.1007/s00340-026-08726-w
Image Credits: AI Generated
DOI: 10.1007/s00340-026-08726-w
Keywords: quantum entanglement, Barnett effect, cavity optomagnonics, yttrium iron garnet, magnons, microwave photons, nonreciprocity, Fabry-Perot cavity, quantum networks, logarithmic negativity, tripartite entanglement, theoretical physics
News Source: Katie Riggs. (October 28, 2026). Rotating Magnet Could Switch Quantum Entanglement On and Off. Scienmag.
