Layered magnetic semiconductor aids research on microwave-to-light conversion in quantum networks

A layered magnetic semiconductor could help solve one of the most stubborn problems in quantum networks. Researchers want to be able to shuttle information between microwave hardware and optical systems. Physicists at the City College of New York have demonstrated microwave-to-optical conversion using chromium sulfide bromide (CrSBr). Their experiments show that magnetic waves inside the material can imprint microwave signals onto a laser beam. The result could support interfaces between future quantum processors and long-distance optical networks.

Many quantum processors operate on signals at microwave frequencies. Fiber optics, however, offer a practical channel for carrying information over long distances. This mismatch creates a major engineering challenge for connecting quantum systems. A transducer must convert information between the different frequencies without destroying the signal. The CCNY team used CrSBr to perform that conversion. The material contains magnetic moments that respond collectively when a microwave signal is applied. These collective motions generate quasiparticles known as magnons. The magnetic waves then interact with excitons inside the semiconductor.

Formation and role of excitons

Excitons form when an electron and a hole bind together. They also interact strongly with light near specific optical resonances. That interaction allows microwave-driven magnetic motion to influence the reflected laser beam, so the optical signal can follow the original microwave excitation faithfully.

The researchers measured the conversion over a microwave bandwidth of roughly 300 megahertz. They also tuned the operating frequency by applying an external magnetic field. One detail of the demonstration that stands out from an engineering perspective is that the team used a large crystal without a resonator to achieve the effect. Optical and microwave resonators can enhance interactions in frequency-conversion systems. Removing those components can provide greater flexibility in early device development. CrSBr also has a layered structure that could help engineers shrink future devices.

The researchers can thin the material down to just a few layers while retaining its magnetic and optical properties.

Challenges ahead for quantum interfaces

The work was led by postdoctoral researcher Pratap Chandra Adak from Vinod M. Menon's group. He noted that the material's structure creates opportunities for stronger interactions and tighter integration. The experiments were carried out in CCNY's Nanophotonics and Microphotonics Laboratory under Menon's direction. Although the experiment demonstrated the physical conversion mechanism, it has not yet transferred individual quantum states. That distinction matters, because quantum communication demands extremely high efficiency. The conversion process must also introduce very little additional noise.

The researchers identified several ways to improve the system. Thinner layers of CrSBr could boost interactions inside small devices. Microwave resonators and high-quality optical cavities could offer another route to enhancing conversion. The team also pointed to exciton-polaritons as a possible way to manage optical losses.

Menon said CrSBr combines strong optical interactions with magnetism at microwave frequencies within a single crystal. Expanding the study of layered magnetic materials may reveal other combinations suited to photonic-magnetic devices. The next engineering challenge will be turning the laboratory demonstration into an efficient quantum interface. That step will determine the material's practical value in future quantum networks. The research was published in the journal Nature Materials.