Using advanced computer modeling, researchers at the U.S. Department of Energy's Argonne National Laboratory have developed a way to accurately predict the behavior of devices called molecular qubits.

A qubit is the information-processing heart of quantum technologies. With high-performing qubits, we can expect to have computers with even more powerful capabilities than what we enjoy today, hackerproof information networks, and detectors that can pick up atomic-scale signals in medicine and navigation. And for that, we need to understand the ingredients for what makes a good qubit.

Qubits comes in numerous flavors. A molecular qubit is made of a molecule sitting inside a larger crystal.

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

It’s a valid approach. But the team wanted to provide directions on how to design molecular qubits to spec.

They came up with a method to accurately predict and tune the spin of chromium-based molecular qubits. (Spin is an atomic property. Just as Morse code uses dots and dashes to carry messages, a molecular qubit uses spin to encode quantum information.) The team also figured out which factors in the qubit material 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, Argonne senior scientist and University of Chicago professor, who led the team.

The spin of a chromium center can split into three magnetic energy levels, a phenomenon called zero-field splitting, or ZFS. Controlling the ZFS also enables longer qubit lifetimes — more time for the qubit to process information before it disintegrates.

The team’s method prescribes how to tune the ZFS. By adjusting the geometry of the crystal surrounding the chromium center or the electric fields arising from the crystal’s chemical makeup, one can set the ZFS just where it’s wanted.

The team’s work is the first not only to provide a 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.

“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.”

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