Designing Tunable Molecular Qubits

Researchers bring together the tools of chemistry and physics to the develop rules for designing tunable molecular qubits. Credit: Image by University of Chicago.

A ground-up approach to qubit design leads to a new framework for creating versatile, highly tailored quantum devices.

Advances in quantum science have the potential to revolutionize the way we live. Quantum computers hold promise for solving problems that are intractable today, and we may one day use quantum networks as hackerproof information highways.

The realization of such forward-looking technologies hinges in large part on the qubit — the fundamental component of quantum systems. A major challenge of qubit research is designing them to be customizable, tailored to work with all kinds of sensing, communication, and computational devices.

Scientists have taken a major step in the development of tailored qubits. In a paper published in the Journal of the American Chemical Society, the team, which includes researchers at MIT, the University of Chicago, and Columbia University, demonstrates how a particular molecular family of qubits can be finely tuned over a broad spectrum, like turning a sensitive dial on a wideband radio.

The team also outlines the underlying design features that enable exquisite control over these quantum bits.

“This is a new platform for qubit design. We can use our predictable, controllable, tunable design strategy to create a new quantum system,” said Danna Freedman, MIT professor of chemistry and a co-author of the study. “We’ve demonstrated the broad range of tunability over which these design principles work.”

The work is partially supported by Q-NEXT, a U.S. Department of Energy (DOE) National Quantum Information Science Research Center led by Argonne National Laboratory.

The researchers’ work focuses on a specific group of molecules: those with a central chromium atom surrounded by four hydrocarbon molecules to form a pyramidlike structure.

The molecular qubit advantage

The qubit is the quantum equivalent of the traditional computing bit. Physically, it may take any of several forms, such as a specially prepared atom inside a crystal or an electrical circuit. It can also be a lab-made molecule.

One advantage of a molecular qubit is that, like a tiny 3D-printed gadget, it can be engineered from the bottom up, giving the scientist freedom to tune the qubit for different functions.

“We’re working to change the atomic structure through synthetic chemistry and then learning­ how those changes modify the physics of the qubit,” said Leah Weiss, a University of Chicago postdoctoral researcher and study co-author.

A molecular qubit’s information is stored in its spin, a property of atomic-level materials. Scientists engineer the spin by adjusting — tuning — the arrangement of the molecule’s electrons, its electronic structure. The information enters the qubit as particles of light, or photons, and is encoded in the qubit’s spin. The spin-encoded information is then translated again into photons, to be read out.

Different photon wavelengths are more suitable for different applications. One wavelength may work better for biosensing applications, another for quantum communication.

The ligand’s the thing

One of the molecular qubit’s key tuning dials is the ligand field strength, the strength of the bonds connecting the central metal atom to the surrounding hydrocarbons.

“The ligand is fundamentally everything. We can intentionally control the way in which the ligand environment influences the spin and rationally control where the emitted photons end up,” said Dan Laorenza, MIT graduate student and lead author of the paper.

Researchers demonstrated that they could exercise remarkably fine tuning over these bonds. Not only that, but they also showed that the ligand field strengths are adjustable over a relatively broad spectrum, while computational simulations performed by researchers at Columbia provided quantum mechanical insight into the ligands’ role in controlling the molecule’s electronic properties.

The light emitted by their chromium qubits spanned an impressive 100 nanometers.

“This is an unprecedented range of tunability for qubits targeting designer applications,” Freedman said.

“Just by keeping the central metal ion the same, which is doing the hard work of the quantum information processing, but tuning the surrounding environment through ligands, you can play around with the properties,” said University of Glasgow’s Sam Bayliss, who co-authored the study while a postdoctoral researcher at the University of Chicago. “That’s very hard to do with other systems, like solid-state systems, where you’re essentially fixed at whatever the elemental properties give you.”

A solid-state qubit is created by scooping out a tiny, atom-sized bit of matter from a crystal, and the resulting vacancy is where quantum information is stored and processed. While they have their advantages, solid-state qubits can’t be tuned with the same chemical precision, for example.

“With those, effectively, you get no tuning,” Freedman said. “You’re really going from zero to 100 there.”

Laying out the design rules

Approaching the molecule’s design by focusing on its electronic structure — the molecule’s energy levels — rather than its physical structure was key to the team’s discovery.

“Throwing the physical structure out the window and focusing entirely on the electronic structure, which is something that can be achieved across a range of molecular platforms, is really the key innovative detail,” Freedman said.

The researchers spell out the design criteria for building similar molecules in their paper, laying the groundwork for creating new tunable molecular qubits that can be designed toward a future application.

“Having demonstrated the accuracy of our computational methods on these chromium qubits, we can now use the same methods to simplify the screening process,” said Arailym Kairalapova, one of the Columbia researchers who performed the calculations.

“By bringing together the tools of chemistry and physics, it’s possible to start to understand the design rules that will guide the continued improvement of this class of qubits,” Weiss said.

One could custom-design qubits that attach to a biological system and use them for quantum biosensing. Or researchers could architect a qubit to be water-soluble so that it could detect signals in an aqueous environment.

“One of the terrific things about this platform is that, if the molecule doesn’t emit at a certain wavelength, it’s easy for us to go back in the lab, make a new material at a low cost, and see which one gives us the appropriate feature we want,” Laorenza said. “We can do this in a few days. It’s not something that takes a really intense, high amount of fabrication.”

The team attributes its success also to innovations in studies of light-matter interactions.

“A few years ago, this was just a dream — to have a set of molecular systems be a novel platform for quantum information science,” Bayliss said. “Seeing where we are now is really exciting.”

The team plans to explore different ligand environments to widen the range of photon emission.

“This is now a jumping off point that we hope allows many more chemists to be invited into this space, opening up the work to a much broader range of chemists who could contribute quite a bit to quantum information science,” Laorenza said.

Reference: “Tunable Cr4+ Molecular Color Centers” by Daniel W. Laorenza, Arailym Kairalapova, Sam L. Bayliss, Tamar Goldzak, Samuel M. Greene, Leah R. Weiss, Pratiti Deb, Peter J. Mintun, Kelsey A. Collins, David D. Awschalom, Timothy C. Berkelbach and Danna E. Freedman, 24 November 2021, Journal of the American Chemical Society.
DOI: 10.1021/jacs.1c10145

This work was supported by the U.S. Department of Energy Office of Science National Quantum Information Science Research Centers.

About Q-NEXT

Q-NEXT, a U.S. Department of Energy (DOE) National Quantum Information Science Research Center led by Argonne National Laboratory, brings together roughly 100 world-class researchers from national laboratories, universities and leading U.S. technology companies to develop the science and technology to control and distribute quantum information. Q-NEXT collaborators and institutions will create two national foundries for quantum materials and devices, develop networks of sensors and secure communications systems, establish simulation and network testbeds, and train a next-generation quantum-ready workforce to ensure continued U.S. scientific and economic leadership in this rapidly advancing field.


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