Molecules work as quantum bits

Quantum bits, or qubits, made from organometallic molecules could create tunable, highly sensitive sensors and maybe one day function as logic circuits in quantum computers, says Danna E. Freedman, a chemist at the Massachusetts Institute of Technology, who presented the work Monday in a Division of Physical Chemistry session at ACS Fall 2021.

Freedman, molecular engineer David D. Awschalom of the University of Chicago, and their colleagues built their molecule with a chromium atom linked to four hydrocarbon rings (Science, 2020, 10.1126/science.abb9352).

At a quantum level, the chromium atom, like everything else, has a property called spin, which can be imagined as a magnet with its north pole pointing up, down, or anywhere in between. Its spin state can be used as the 1s and 0s of computing, but because of its quantum nature the states can have more than one value at a time, leading to much higher performance computing than is possible with existing machines.

When the researchers shine a laser light through the qubit, the molecule emits infrared photoluminescence with brightness determined by the spin. The team can set the initial spin state by applying microwaves; changes in spin are measured by changes in the light output.

In addition to applications in quantum computing, the molecules could detect the action potential of a neuron or temperature changes related to cell reproduction, Freedman says, since both properties can affect the chromium atom’s spin. This molecular approach provides better control over the placement and orientation of the qubit than do alternatives made of semiconductors or diamonds with defects engineered in, Freedman says. The molecules’ properties can also be tuned by attaching or rearranging methyl groups on the molecule; coupling two molecules that emit at different wavelengths could allow the team to detect two analytes at once. “You can control, design, and predict these systems. And that is beautiful,” she says.