Image credit: ChatGPT · ccny.cuny.edu

Physicists at The City College of New York have demonstrated a new method for linking quantum computers by converting microwave signals into light using a layered semiconductor. The team focused on chromium sulfide bromide (CrSBr), a material specifically chosen for its ability to facilitate this process, rather than more commonly used semiconductors. When driven by microwaves, the material generates magnetic waves that shift the energies of electron-hole pairs, imprinting the microwave signal onto reflected laser light.

Pratap Chandra Adak explained that “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.” Published in Nature Materials, this research, led by a CCNY-led collaboration, establishes a materials platform for building interfaces that could one day enable optical networks for quantum information transfer.

CrSBr Semiconductor Enables Microwave-to-Optical Signal Conversion

The layered structure of chromium sulfide bromide allows for considerable freedom in device design and integration, according to the research team. This characteristic is particularly valuable as the material maintains its essential magnetic and optical properties even when reduced to just a few atomic layers, potentially leading to more compact and efficient devices. The conversion of microwave signals to optical signals occurs through the excitation of magnons, collective movements of atomic magnetic moments, which then influence excitons, bound pairs of electrons and holes.

This magnon-exciton coupling imprints the microwave information onto the energy levels of the excitons, and subsequently onto reflected laser light; the researchers detail this process in their publication, “Microwave-to-optical transduction using magnon–exciton coupling.” The experiment demonstrated successful conversion across a microwave frequency window of approximately 300 megahertz, with the operating frequency adjustable via an applied magnetic field. The observed effect occurred within a bulk crystal, eliminating the need for optical or microwave resonators typically used to amplify such interactions.

“What excites me is the potential to build on these results,” Vinod Menon said, suggesting future work will focus on increasing efficiency and minimizing noise to enable the transfer of individual quantum states. The present experiment confirms the mechanism for microwave-to-optical conversion.

A particular advantage of CrSBr is its layered structure, which gives us considerable freedom in device design and integration.
Pratap Chandra Adak, a postdoctoral researcher in Menon’s group who led the study

Magnon-Exciton Coupling Drives Quantum Network Interface

While the present experiment establishes the conversion mechanism, the researchers acknowledge that transferring individual quantum states requires further improvements in efficiency and noise control. The team identified several strategies for enhancing performance, including utilizing even thinner magnetic flakes, incorporating microwave resonators, and employing high-quality optical cavities. These advancements will allow for building robust interfaces capable of linking quantum computers via optical fiber networks, a critical step towards realizing a scalable quantum internet.

Tunable Frequency & Layered Structure Enhance Transduction Efficiency

This bandwidth stems from the material’s ability to sustain collective magnetic excitations, known as magnons, which modulate the energy of excitons, electron-hole pairs, within the semiconductor. The researchers observed that the strong interaction between light and matter near exciton resonances facilitated this efficient conversion. This level of control is important for building practical quantum networks, where maintaining the integrity of quantum information is paramount.

While frequency conversion is common in conventional telecommunications, achieving this with the high fidelity and low noise required for quantum systems presents a significant challenge. The current work confirms the underlying mechanism for this microwave-to-optical transduction, and lays the groundwork for further improvements in efficiency and scalability.

The team is now exploring strategies to utilize even thinner magnetic flakes to further enhance performance and build more robust interfaces for linking quantum computers.