Quantum Computers Talk in Microwaves. Fiber Optics Carry Light. City College Physicists Bridged the Two With a Layered Magnet, Chromium Sulfide Bromide, Whose Spin Waves Imprint a Microwave Signal Directly Onto Reflected Laser Light, With No Resonator at All, Across a 300-Megahertz Window That a Magnetic Field Can Tune.

Most of the world's leading quantum processors speak in microwaves, the gigahertz-frequency signals that drive and read out superconducting qubits. Optical fiber, the only practical way to carry quantum information across a city or between buildings, speaks in light at frequencies roughly 10,000 times higher. Linking the two requires a transducer, a device that shifts a signal between those frequency ranges without destroying the fragile information it carries. Physicists at the City College of New York report in Nature Materials that a layered magnetic semiconductor can do the conversion using nothing more than its own internal magnetism.

The material is chromium sulfide bromide, CrSBr, a crystal that stacks in sheets like graphite and is magnetic at low temperature. When the team, led by physics professor Vinod M. Menon at CCNY's Laboratory for Nano and Micro Photonics, drove the crystal with microwaves, the magnetic moments of its atoms began to move together in collective waves called magnons. Those magnons shift the energy of excitons, the bound electron-hole pairs in the same crystal that interact strongly with light. The result is that laser light reflected off the crystal comes back carrying a coherent optical signal that tracks the microwave drive. The microwave information has been imprinted on the light.

The conversion worked across a microwave window of roughly 300 megahertz, and the operating frequency could be tuned by applying a magnetic field. What surprised the researchers most was that the effect appeared in a bulk crystal with no optical cavity and no microwave resonator to amplify the interaction, the kind of engineering that nearly every competing transduction platform relies on. The strength comes from the unusually strong coupling between light and matter near the exciton resonance in CrSBr.

"A particular advantage of CrSBr is its layered structure, which gives us considerable freedom in device design and integration," said Pratap Chandra Adak, the postdoctoral researcher 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 team is explicit about what the experiment has and has not done. Frequency conversion is routine in classical telecommunications; converting a quantum signal is far harder, because a useful quantum interface has to operate at high efficiency while adding almost no noise. The present work establishes the physical mechanism. Transferring individual quantum states, which is what a network of quantum computers would actually need, will require large gains in efficiency and careful control of the noise added along the way. The paper lays out several routes: thinner magnetic flakes, microwave resonators, high-quality optical cavities, and engineering exciton-polaritons, hybrid states of light and matter, to manage optical loss.

"What excites me is the potential to build on these results," Menon said. "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." The result joins a fast-moving contest, with groups pursuing electro-optic crystals, mechanical resonators and trapped atoms as the bridge between quantum chips and the fiber that would connect them. A two-dimensional magnet that does the job in its bare form, before any of the usual amplifying tricks are applied, is a new entrant with room to grow.