Researchers develop molecular qubits that communicate at telecom frequencies

(Press-News.org) A team of scientists from the University of Chicago, the University of California Berkeley, Argonne National Laboratory, and Lawrence Berkeley National Laboratory has developed molecular qubits that bridge the gap between light and magnetism—and operate at the same frequencies as telecommunications technology. The advance, published today in Science, establishes a promising new building block for scalable quantum technologies that can integrate seamlessly with existing fiber-optic networks.

Because the new molecular qubits can interact at telecom-band frequencies, the work points toward future quantum networks—sometimes called the “quantum internet.” Such networks could enable ultra-secure communication channels, connect quantum computers across long distances, and distribute quantum sensors with unprecedented precision. Molecular qubits could also serve as highly sensitive quantum sensors; their tiny size and chemical flexibility mean they could be embedded in unusual environments—such as biological systems—to measure magnetic fields, temperature, or pressure at the nanoscale. And because they are compatible with silicon photonics, these molecules could be integrated directly into chips, paving the way for compact quantum devices that could be used for computing, communication, or sensing.

The new molecular qubit contains erbium, a rare-earth element. Rare earths are used in classical technologies as well as emerging quantum technologies because they absorb and emit light very “cleanly” relative to other elements, but they also interact strongly with magnetic fields.

“These molecules can act as a nanoscale bridge between the world of magnetism and the world of optics,” said Leah Weiss, postdoctoral scholar at the University of Chicago Pritzker School of Molecular Engineering (UChicago PME) and co-first author on the paper. “Information could be encoded in the magnetic state of a molecule and then accessed with light at wavelengths compatible with well-developed technologies underlying optical fiber networks and silicon photonic circuits.”

On a quantum level, the relationship between light and magnetism is subtle and complex. Light is often how quantum information is transmitted and read; magnetism is deeply connected to “spin”, a unique quantum property that underlies a variety of quantum technologies such as sensors and certain types of quantum computers. This work builds on a foundation of two fields, quantum optics—with applications in lasers and quantum networks—and synthetic chemistry—which is responsible for the contrast agents used in magnetic resonance imaging (MRI) machines—to establish a molecular building block that can bridge the divide between them. 

“Rare-earth chemistry provided a fortuitous combination of properties that allowed us to bring these capabilities to a molecular system,” said Grant Smith, graduate student at PME and another first author on the paper. “There were a lot of things pointing toward this as an exciting platform to advance the use of optical degrees of freedom in molecular spin qubits. One of the central focuses of this work, and the work in the lab more broadly, is that we want to really expand the gamut of quantum systems and materials that we can control and interact with.”  By doing this, he says, “you can begin to think about new and unconventional ways to utilize and integrate them into technologies.”

Using optical spectroscopy and microwave techniques, the team demonstrated that the erbium molecular qubits used frequencies compatible with silicon photonics, which are used in telecommunications, high-performance computing, and advanced sensors. The researchers say this compatibility with mature technologies could accelerate the development of hybrid molecular–photonic platforms for quantum networks.

“By demonstrating the versatility of these erbium molecular qubits, we’re taking another step toward scalable quantum networks that can plug directly into today’s optical infrastructure,” said David Awschalom, the Liew Family Professor of Molecular Engineering and Physics at the University of Chicago and principal investigator of the study. “We’ve also demonstrated that these atomically engineered qubits have the capabilities necessary for multi-qubit architectures, which opens the door to a wide spectrum of applications, including quantum sensing and hybrid organic-inorganic quantum systems.” 

Both Weiss and Smith emphasized the importance of their collaboration with chemists at UC Berkeley, especially their co-first author Ryan Murphy in the research group of Jeffrey Long, calling it “absolutely critical” to the work and “a privilege.” 

“Synthetic molecular chemistry provides an opportunity for optimizing the electronic and optical properties of rare earth ions in ways that can be difficult to access in conventional solid-state substrates,” said Murphy. “This study is just scratching the surface of what we think we can accomplish.”

“Our work shows that synthetic chemistry can be used to design and control quantum materials at the molecular level,” said Long, Professor of Chemistry at UC Berkeley and co-principal investigator. “This points to a powerful route for creating tailor-made quantum systems with applications in networking, sensing, and computation.”

The study was supported by the U.S. Department of Energy’s Office of Science and Q-NEXT, a DOE National Quantum Information Science Research Center. 

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