Quantum computers and sensors operate at microwave frequencies, but transmitting their signals over long distances is conveniently done via optical fibers, which use light. To bridge these two worlds, we need an intermediary — a device that converts microwaves into optical photons and vice versa without destroying fragile quantum information.

In the new work, the layered crystal CrSBr serves as such an intermediary. Inside it, microwaves induce collective oscillations of magnetic moments — magnons. These oscillations affect electronic excitations, excitons, which interact strongly with light. As a result, the laser beam reflected from the crystal acquires modulation: additional sidebands appear near the main frequency. Enhancing this effect near exciton resonances allowed coherent conversion without complex resonators.

This approach promises a broad frequency band, about 300 MHz, and scalability. The authors showed that several exciton-polariton resonances inherit the magnon coupling, which could expand the optical tuning range and reduce losses. This is a step toward practical quantum networks where microwave qubits are connected via optical channels.