Â鶹ÒùÔº

Quantum technologies, which leverage quantum mechanical effects to process information, could outperform their classical counterparts in some complex and advanced tasks. The development and real-world deployment of these technologies partly relies on the ability to transfer information between different types of quantum systems effectively.

A long-standing challenge in the field of quantum technology is converting quantum signals carried by microwave photons (i.e., particles of electromagnetic radiation in the microwave frequency range) into optical photons (i.e., visible or near visible light particles). Devices designed to perform this conversion are known as microwave-to-optical transducers.

Researchers at the California Institute of Technology recently developed a new microwave-to-optical transducer based on rare-earth ion-doped crystals. Their on-chip transducer, outlined in a paper in Nature Â鶹ÒùÔºics, was implemented using ytterbium-171 ions doped in a YVO₄ crystal.

"In the context of quantum technologies, the vision is that one day we will have quantum computers interconnected in a quantum internet, similar to the current classical and communication infrastructure," Andrei Faraon, senior author of the paper, told Â鶹ÒùÔº.

"One of the leading technologies for quantum computing is based on superconducting qubits, so there are efforts to try and connect these computers via optical fibers that can transmit quantum information at room temperature and over long distances."

The operation of superconducting qubits is supported by single photons with frequencies of a few gigahertz. Devising transducers that can reliably convert these photons into optical photons, which travel more easily in optical fibers, is a key challenge within quantum technology.

"Most technologies to achieve this rely on nonlinear optical systems that are coupled to both optical and microwave resonators," said Faraon. "Some previous works suggested that the atoms doped crystals could serve as the nonlinear system that performs this conversion, or transduction. Our lab had expertise in working with rare-earth atoms (or ions) doped in crystals, so we decided to see if indeed they can perform efficient transduction."

Faraon and his colleagues initially tried to develop microwave-to-optical transducers using erbium atoms, but the efficiencies they achieved were low. The researchers thus decided to switch to ytterbium 171 ions in yttrium orthovanadate, which ended up being highly effective at coupling microwave photons and optical photons, yielding higher efficiencies. The coupling they observed was so good that they could attain good efficiencies even without the need to engineer optical resonators.

"We started with a crystal substrate, about half a millimeter thick, doped with rare-earth ions," explained Tian Xie, co-first author of the paper. "On its top surface, we patterned a superconducting microwave resonator that talks to billions of dopant spins inside the crystal. The back surface gets a thin gold mirror to enhance the collection of optical photons."

The primary advantage of the transducer developed by Faraon, Xie and their colleagues is that its underlying large spin ensemble forms an intrinsic ultra-strong "nonlinearity," which is typically several orders of magnitude higher than that formed in other conventional materials. As this ensemble is built into the material itself, the device remains simple, compact and easy to cool, which minimizes added noise.

"Another feature is that the operation frequencies are set by the atomic energy levels, leading to automatically matched optical and microwave frequencies across devices," said Xie. "This uniformity is crucial for future quantum networks, where indistinguishable photons are needed to create entanglement between remote quantum nodes."

In initial experiments, the researchers were able to directly measure noise in their solid-state atomic transducer for the first time. Moreover, they found that this noise was surprisingly low.

"The transducer only adds about a single photon of noise, and we were able to trace every contribution to its source," said Rikuto Fukumori, co-first author of the paper. "What's exciting is that there's room to further push the noise lower, perhaps to be in the quantum regime. Hitting that level would remove a key obstacle for scalable quantum processors and long-distance quantum networks."

The new transducer developed by Faraon, Xie, Fukumori and their colleagues may soon be improved further and could be used to reliably connect future quantum networks. As part of their next studies, the researchers would also like to connect a single photon microwave source to their transducer, as this would allow them to demonstrate the transduction of single photons.

"This source is composed of some superconducting quantum bits and other superconducting devices," added Faraon. "Another direction for future research will be to improve the efficiency of the device and lowering the noise, which we are doing by investigating new materials with higher ytterbium concentration and by improving the design.

"We hope that in a few years, we will show remote entanglement of superconducting qubits using this transduction technology, and we will interconnect future superconducting quantum computers."

© 2025 Science X Network