Physicists at The City College of New York have demonstrated a new way to transfer microwave signals onto light using magnetic waves inside a layered semiconductor. The work establishes a materials platform for building interfaces that could one day link quantum computers through optical networks. The research, entitled "Microwave-to-optical transduction using magnon–exciton coupling," was led by the Laboratory for Nano and Micro Photonics (LaNMP) at CCNY, headed by physics professor Vinod M. Menon. It appears in the journal Nature Materials.
Many of today's leading quantum processors operate using microwave signals, while light is far better suited to carrying information over long distances through optical fiber. Linking the two requires a transducer: a device that converts a signal from one frequency range to another while preserving the information it carries.
The team turned to chromium sulfide bromide (CrSBr), a layered magnetic semiconductor. When driven by microwaves, the atomic magnetic moments in the crystal move collectively, producing waves known as magnons. These waves shift the energies of excitons, bound pairs of electrons and holes that interact strongly with light. As a result, laser light reflected from the crystal picks up a coherent optical signal that tracks the microwave drive. The approach takes advantage of the especially strong interaction between light and matter near exciton resonances.
The conversion worked across a microwave frequency window of approximately 300 megahertz, and its operating frequency could be tuned with an applied magnetic field. Notably, the researchers observed the effect in a bulk crystal, without any optical or microwave resonator to boost the interaction.
Frequency conversion is routine in modern telecommunications. Converting quantum signals is far more demanding: a useful quantum interface must operate with high efficiency while adding almost no noise. Researchers across several fields are pursuing different physical mechanisms and material platforms to meet these requirements.
"A particular advantage of CrSBr is its layered structure, which gives us considerable freedom in device design and integration," said Pratap Chandra Adak, a postdoctoral researcher in Menon's group who led the study. "These materials can be thinned down to just a few layers while retaining their key magnetic and optical properties. That opens opportunities to strengthen the interactions and build more compact devices."
The present experiment establishes the microwave-to-optical conversion mechanism. The longer-term goal, transferring individual quantum states, will require substantial gains in efficiency and careful control of added noise. The paper identifies several routes to stronger interactions and higher efficiency, including thinner magnetic flakes, microwave resonators and high-quality optical cavities. Another promising direction is engineering exciton–polaritons, hybrid states of light and matter, to manage optical loss.
"What excites me is the potential to build on these results," said Menon. "CrSBr brings strong optical interactions and microwave-frequency magnetism together in the same crystal. As the family of layered magnetic materials expands, we have more opportunities to discover useful combinations of properties and engineer new opto-magnonic devices."
The CCNY-led collaboration included 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.
Work at CCNY was supported by the U.S. Department of Energy's Office of Science, Basic Energy Sciences; DARPA; and the Gordon and Betty Moore Foundation.
