In the quest to harness quantum technology, researchers at the leading edge of this field have created a pioneering transducer. It does so incredibly efficiently, converting microwave signals to optical signals with rare-earth ions. Senior author Andrei Faraon, along with co-first authors Tian Xie and Rikuto Fukumori, heads up a creative team. Their pioneering design is poised to greatly enhance the connectivity of coming quantum networks. The discovery, published today in Nature Physics, represents a major breakthrough towards the promise of quantum communication and computation.
This transducer employs ytterbium 171 ions, which are doped into yttrium orthovanadate. It builds upon the unique optical properties of these ions to achieve efficient quantum coupling between microwave and optical photons. The goal of this new approach is to avoid limitations previously experienced with erbium atoms, avoiding low efficiency rates. This device consists of a transducer with a superconducting microwave resonator embedded into its surface. This unique design facilitates its interaction with many dopant spins embedded within the crystal matrix.
Enhanced Signal Conversion
Faraon and his colleagues made an important tactical decision. They chose ytterbium 171 ions for their high efficiency coupling between microwave and optical signals. This decision distinguishes their transducer from previous prototypes that had difficulties with erbium atoms.
“We started with a crystal substrate, about half a millimeter thick, doped with rare-earth ions,” – Tian Xie
This innovative design enabled the researchers to create a superconducting microwave resonator on the top surface. This resonator then couples to the billions of dopant spins frozen in the crystal. A thin, gold mirror has recently been attached to the back surface. This inclusion greatly increases the collection area for optical photons and increases the efficiency of the optical transducer dramatically.
This results in an effective, ultra-strong nonlinearity internal to the large spin ensemble. This unique intrinsic property is the key to efficient quantum signal processing. This nonlinearity is orders of magnitude larger than those in traditional materials, allowing for much more efficient quantum operations.
Noise Reduction and Future Improvements
Perhaps the most impressive aspect of this transducer is its ability to reduce noise when converting analog signals to digital. For the first time in their experiments, the researchers directly measured the noise produced by their solid-state atomic transducer.
“The transducer only adds about a single photon of noise, and we were able to trace every contribution to its source,” – Rikuto Fukumori
Fukumori remained hopeful for additional improvement. He noted, “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.”
This new emphasis on making qubits quieter dovetails nicely with Faraon’s dream of what quantum tech should look like. He argues that the key to achieving quantum advantage lies in reducing noise, enabling a powerful link-up between quantum computers. This would enable a strong quantum internet, like today’s classical communication networks.
The Path Forward for Quantum Networks
While this research has clear, important implications in the near term, the possibilities extend even further. It provides a foundation for the next leap toward fully realized quantum computing and networking. The unique ability to tune operational frequencies through atomic energy levels guarantees universal compatibility between devices.
“Another feature is that the operation frequencies are set by the atomic energy levels, leading to automatically matched optical and microwave frequencies across devices,” – Tian Xie
Xie further highlighted the crucial role of this uniformity for producing indistinguishable photons that are required to entangle remote quantum nodes. This trait is imperative for developing useful and trustworthy links between quantum networks.
Faraon and his team are just getting started—and already looking ahead to the next phase of their research. They hope to improve device efficiency while making devices even quieter. They’re exploring novel materials with more ytterbium and optimizing their design to both maximize intensity and minimize noise.
“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,” – Andrei Faraon
In doing so, they have adopted an incredibly ambitious goal. They intend to show remote entanglement of superconducting qubits through this exciting transduction metamaterial in the next few years.
“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,” – Andrei Faraon