A new bridge for quantum networks: Physicists convert microwaves to light using 2D magnets

Physicists at The City College of New York have developed a novel method to transfer microwave signals into light by utilizing magnetic waves within a two-dimensional semiconductor. This breakthrough, detailed in a study titled "Microwave-to-optical transduction using magnon–exciton coupling" published in Nature Materials, paves the way for constructing interfaces that could potentially link quantum computers via optical networks.

Current quantum processors rely on microwave signals, whereas light is more efficient for long-distance data transmission through optical fibers. To bridge this gap, the researchers employed chromium sulfide bromide (CrSBr), a layered magnetic semiconductor. When exposed to microwaves, the atomic magnetic moments in the crystal align collectively, creating magnons—collective atomic magnetic oscillations.

These magnons alter the energies of excitons, electron-hole pairs with strong light-matter interactions. Consequently, laser light reflected off the crystal carries a coherent optical signal that mirrors the original microwave drive. The conversion process is effective across a microwave frequency range of about 300 megahertz and can be finely tuned using an applied magnetic field.

Impressively, the researchers achieved this frequency conversion in a bulk crystal without the need for any optical or microwave resonators to enhance the interaction. While frequency conversion is commonplace in modern telecommunications, converting quantum signals presents additional challenges. A high-performance quantum interface must exhibit high efficiency while introducing minimal noise.

The layered structure of CrSBr offers significant flexibility in device design and integration, allowing for the thinning of the material to retain its essential magnetic and optical properties. This discovery establishes the microwave-to-optical conversion mechanism, but further advancements are necessary to achieve efficient quantum state transfer.

The research outlines potential strategies to enhance interactions and efficiency, such as utilizing thinner magnetic flakes, incorporating microwave resonators, and developing high-quality optical cavities. Additionally, engineering exciton-polaritons—hybrid light-matter states—could help manage optical losses. Vinod M. Menon, the physics professor leading the Laboratory for Nano and Micro Photonics at CCNY, expressed optimism about the implications of these findings, noting the unique combination of strong optical interactions and microwave-frequency magnetism within a single CrSBr crystal.

The collaborative effort involved researchers from the CUNY Advanced Science Research Center, Columbia University, the University of Chemistry and Technology Prague, the University of Chicago, and RPTU Kaiserslautern-Landau in Germany.