‘Designer molecules’ could create tailor-made quantum devices

 

A molecule with a core chromium ion (purple) can act as a quantum bit, recording data in the spin direction (indicated by its arrow in this illustration). The characteristics of the ion are altered by attached atoms (grey), allowing it to be handled by a laser (purple squiggle) and emit light in response (red squiggle).

DANIEL LAORENZA/NORTHWESTERN UNIVERSITY

Quantum bits built from so-called "designer molecules" are becoming popular. Researchers are constructing chemical systems that are suitable for a variety of quantum jobs by carefully adjusting the composition of molecules.

"The capacity to regulate molecules... makes them just a lovely and fantastic system to deal with," Danna Freedman, a chemist at Northwestern University in Evanston, Illinois, said. "Molecules are the most amazing." Freedman presented her findings at the American Association for the Advancement of Science's annual meeting, which was held online.

Qubits, or quantum bits, are similar to the bits used in conventional computers. Qubits, on the other hand, can have both values at the same time, unlike regular bits, which can only have one. This allows for new forms of calculations that are unachievable with traditional computers.

Molecules, in addition to their potential application in quantum computers, can also be used as quantum sensors, which are devices that can make incredibly sensitive measurements, such as detecting miniscule electromagnetic forces.

A single chromium ion, an electrically charged atom, resides at the core of the molecule in Freedman and colleagues' qubits. The electronic spin of that chromium ion, which is a measure of the angular momentum of its electrons, is used to represent the qubit's value. The chromium is bonded to further groups of atoms, and the researchers can adjust the qubit's capabilities by changing out some of the atoms in those groups.

Freedman and colleagues have created molecules to meet a specific need: light-responsive molecular qubits. According to the researchers, lasers can set the values of qubits and aid in reading out the outcomes of calculations. Another option, according to Freedman, is to develop biocompatible chemicals that may be used to detect conditions inside live tissue.

Molecules also have a unique attractiveness in that all of the same sort are identical. Many forms of qubits are constructed from metal or other material put on a surface, resulting in minor atomic variations between qubits. However, because chemical processes are used to construct molecules atom by atom, the qubits are identical, resulting in higher-performing devices. "There's something incredibly strong about chemistry's bottom-up approach," said Freedman.

Individual atoms and ions are already used in quantum devices by scientists, but molecules are more difficult to work with due to their many constituents. As a result, according to Caltech physicist Nick Hutzler, molecules constitute a relatively new quantum resource. "People don't truly understand what [molecules] can do yet.... People, on the other hand, are constantly learning new things."

References:

• S.L. Bayliss et al. Optically addressable molecular spins for quantum information processing. Science, Vol. 370, December 11, 2020, p. 1309. doi:10.1126/science.abb9352.
• D. Freedman. Molecular quantum information technology; a new way to access quantumcomputers. American Association for the Advancement of Science annual meeting, February 8, 2021.