Quantum pieces made from “design molecules” are coming into vogue. By carefully adapting the composition of molecules, researchers are creating chemical systems suitable for a variety of quantum tasks.
“The ability to control molecules … makes them a beautiful and wonderful system to work with,” said Danna Freedman, a chemist at Northwestern University in Evanston, Island. "Molecules are the best." Freedman described his research on Feb. 8 at the annual meeting of the American Association for the Advancement of Science, held online.
Quantum bits, or qubits, are analogous to the bits found in conventional computers. But instead of existing in a state of 0 or 1, as standard bits do, qubits can possess both values simultaneously, allowing new types of calculations impossible for conventional computers.
In addition to their potential use in quantum computers, molecules can also serve as quantum sensors, devices that can make extremely sensitive measurements, such as the scattering of tiny electromagnetic forces (SN: 23/3/18).
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In the qubits of Freedman and colleagues, a single chromium ion, an electrically charged atom, is at the center of the molecule. The value of the qubit is represented by the electronic rotation of that chromium ion, a measure of the angular momentum of its electrons. Additional groups of atoms are attached to chromium; by swapping some of the atoms in those groups, researchers can change the properties of the qubit to alter its functioning.
Recently, Freedman and colleagues developed molecules to suit a particular need: light-responding molecular qubits. Lasers can set the values of qubits and help read the results of calculations, researchers reported in the December 11 issue of the journal Science. Another possibility could be the creation of biocompatible molecules, Freedman claims, so that they can be used to detect conditions within living tissue.
Molecules have another special appeal: all types are exactly the same. Many types of qubits are made from pieces of metal or other material deposited on a surface, resulting in slight differences between qubits at the atomic level. But the use of chemical techniques to accumulate molecules atom by atom means that the qubits are identical, becoming better-performing devices. “That’s something really powerful in the bottom-up approach that chemistry offers,” Freedman said.
Scientists are already using individual atoms and ions in quantum devices (SN: 29/06/17), but molecules are more complicated to work thanks to their multiple components. As a result, molecules are a relatively new quantum resource, Caltech physicist Nick Hutzler said at the meeting. "People don't even know what to do with (molecules) yet … But people are discovering new things every day."