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A qubit is the delicate, information-processing heart of a quantum device. In the coming decades, advances in quantum information are expected to give us computers with new, powerful capabilities and detectors that can pick up atomic-scale signals in medicine, navigation and more. The realization of such technologies depends on having reliable, long-lasting qubits.

Now, researchers have taken an important step in understanding the rules necessary for the design of useful, efficient qubits.

Using advanced computer modeling, the researchers came up with a way to accurately predict and fine-tune key magnetic properties of a type of device called a molecular qubit. They also figured out which factors in the material that the qubit sits in affect this tuning the most and calculated how long the qubits can live.

Their predictions matched what experiments see.

"I think this work will open new venues for the simulations of molecular qubits from first principles, and I see it as a real starting point for many new investigations to come, especially on the assembly of molecular qubits," said Giulia Galli, who led the team.

Galli is a senior scientist at the U.S. Department of Energy's (DOE) Argonne National Laboratory and the Liew Family professor of electronic structure and simulations in the Pritzker School of Molecular Engineering and the Department of Chemistry at the University of Chicago.

The group's work is in the Journal of the American Chemical Society.

Designing qubits to spec

A molecular qubit is made of a molecule sitting inside a larger crystal. Galli's team focused on chromium-based systems.

Traditionally, scientists build molecular qubits by creating different materials, testing them and measuring their performance—like raising buildings of different materials and, once built, testing their stamina in different weather conditions.

It's a valid approach. But the team wanted to give direction on how to design molecular qubits to spec. Chromium-based qubits gave the research team an opportunity to develop a computational method that could predict how key qubit features would respond to different design choices.

"From a design perspective, we wanted to come up with rules to engineer different properties of qubits that are beneficial to our specific application, whether that's quantum communication, quantum sensing or quantum computing," said Argonne postdoctoral researcher Michael Toriyama.

"Through our work, we developed a fully computational method to figure out these engineering principles."

Split and spin

The star of the molecular qubit is something called "spin." It's a feature of every atom. Just as Morse code uses dots and dashes to carry messages, a molecular qubit uses spin to encode quantum information.

The spin of a chromium center can split into three magnetic energy levels. It's a phenomenon called "zero-field splitting," or ZFS. The energy levels change depending on how the atom is situated in the crystal. (The "zero" refers to the absence of outside electromagnetic fields.)

Scientists need to know the energies of each level to control the qubit precisely. Without knowing the ZFS values, controlling the qubit would be like trying to tune a radio without knowing a station's frequency.

The ability to set the ZFS is especially helpful in big quantum systems with many qubits, which need to have predictable, controllable energy differences to avoid unwanted interference. Controlling the ZFS also enables longer qubit lifetimes, or coherence times—more time for the qubit to process information before it disintegrates.

"We can predict the coherence time from the ZFS using our methods, enabling better design principles to extend the coherence of a qubit," Toriyama said. "It's like we're figuring out how to build better armor around the qubit to protect it."

The group's computational protocol for predicting the ZFS gives scientists a way to take full advantage of the molecular qubit's best asset: its tunability.

"In other qubit types, like diamond, for example, there are limited possibilities for modifications, whereas with molecules there is a lot you can do. You can tune properties to the application you need," said Diego Sorbelli, an assistant professor at the University of Perugia in Italy and a former postdoctoral researcher at the University of Chicago.

"It's kind of like using Lego blocks: Figure out which blocks go together and then get the final product with properties that you want," Toriyama said.

Qubit collaboration

How do you tune the ZFS of a molecular qubit? The Galli team highlighted two important dials for setting the ZFS just where it's needed: the geometry of the crystal surrounding the chromium center and the electric fields that arise from the crystal's chemical makeup.

The team's work is the first not only to provide a computational method for accurately predicting ZFS in chromium molecular qubits, but also the first to identify that ZFS can be controlled by manipulating the host crystal's electric fields.

"We give new design rules for modifying the composition of the environment to actively manipulate these spin structures, which we can accurately predict," said Lorenzo Baldinelli, first author of the paper, a graduate student at the University of Perugia, and a former visiting graduate student at the University of Chicago.

"So now, using our protocol, we can account not only for the electronic and spin properties of the qubit, but also of its surroundings."

It wasn't easy to do.

"These properties are extremely complicated to predict from first principle," Sorbelli said.

But the strong cross-disciplinary collaboration within Galli's group—chemists, materials scientists and physicists—helped tease out the most important dials in the chromium qubit's complex chemistry.

"I remember this was bugging me. How do we predict the zero-field splitting? What are the ingredients we need to be able to do this? Can we do this? I was very stubborn about it. And then Lorenzo came along, and we teamed up, eventually bringing Michael into the fold. From that point it was a steep learning curve, but it was also pretty smooth," Sorbelli said.

"Not too many groups are equipped to compute coherence properties of qubits. We leveraged the tools that our group has developed through years and years of research," Toriyama said.

"This was really a testament to how successful collaborations can be and how versatile our group is."