In a physics lab in New York City, a crystal thinner than a human hair just helped scientists pull off a trick that could one day connect the world's most powerful computers through beams of light.
Physicists at The City College of New York found a way to turn microwave signals into light using magnetic waves inside a layered crystal called chromium sulfide bromide, or CrSBr. The work, published in the journal Nature Materials, was led by the Laboratory for Nano and Micro Photonics (LaNMP) under physics professor Vinod M. Menon, with postdoctoral researcher Pratap Chandra Adak leading the study.
Here's why it matters. Many of today's leading quantum computers — machines that solve problems far beyond ordinary computers — do their thinking using microwave signals. But microwaves are terrible at traveling long distances. Light, on the other hand, can speed through optical fiber cables for miles with barely any loss. The challenge has always been in between: to link a microwave-based quantum computer to a light-based network, you need a transducer, a kind of translator that converts one signal into another without scrambling the information it carries.
The CCNY team found their translator in CrSBr, a layered magnetic semiconductor. When driven by microwaves, the crystal's atomic magnetic moments move together in waves called magnons. These magnons nudge the energies of excitons — pairs of electrons and holes that interact strongly with light. As a result, laser light bouncing off the crystal picks up a clean optical signal that faithfully tracks the microwave drive. The trick works because light and matter interact with unusual strength near these exciton states.
The conversion worked across a microwave window of about 300 megahertz, and its operating frequency could be tuned simply by adjusting a magnetic field. Strikingly, the team saw the effect in a plain bulk crystal — no fancy resonators or cavities needed to boost the interaction. That's a big deal for building simple, compact devices.
Converting ordinary telecom signals is routine. Converting fragile quantum signals is far harder, because a useful quantum interface must work efficiently while adding almost no noise. "A particular advantage of CrSBr is its layered structure, which gives us considerable freedom in device design and integration," said Adak. "These materials can be thinned down to just a few layers while retaining their key magnetic and optical properties."
The experiment proves the mechanism works. The longer-term dream — transferring individual quantum states — needs big gains in efficiency and tighter control of noise. The paper points to promising paths: thinner magnetic flakes, optical cavities, and hybrid light-matter states called exciton–polaritons. "CrSBr brings strong optical interactions and microwave-frequency magnetism together in the same crystal," Menon said. The collaboration spanned 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. One day, this tiny crystal could be the bridge that lets the world's quantum computers talk to each other through light.