Molecules brought into a single quantum state

Quantum technology has a lot of promise, but several research barriers need to be overcome before it can be widely used. A team of US researchers has advanced the field another step, by bringing multiple molecules into a single quantum state at the same time.

A Bose-Einstein condensate is a state of matter that only occurs at very low temperatures – close to absolute zero. At this temperature, multiple particles can clump together and behave as though they were a single atom – something that could be useful in quantum technology. But while scientists have been able to get single atoms into this state for decades, they hadn’t yet achieved it with molecules.

“Atoms are simple spherical objects, whereas molecules can vibrate, rotate, carry small magnets,” says Cheng Chin, a professor of physics at the University of Chicago, US. “Because molecules can do so many different things, it makes them more useful, and at the same time much harder to control.”

Chin’s team has now brought molecules of caesium (Cs₂) into the Bose-Einstein state. “People have been trying to do this for decades, so we’re very excited,” he says.

The team used a low temperature of 10 nanokelvins to reach this point. A nanokelvin is a billionth of a kelvin, or a billionth of one degree Celsius, making this temperature just fractionally above absolute zero. They also packed the caesium molecules tightly to limit their movement.

“Typically, molecules want to move in all directions, and if you allow that, they are much less stable,” says Chin. “We confined the molecules so that they are on a 2D surface and can only move in two directions.”

These conditions made the molecules effectively identical: lined up in the same orientation, with the same vibrational frequency and in the same quantum state. The team was able to link up several thousand molecules in this condensate.

Chin says this achievement has implications for quantum engineering. “It’s the absolute ideal starting point. For example, if you want to build quantum systems to hold information, you need a clean slate to write on before you can format and store that information.”

Chin is the senior author on a paper describing the research, published in Nature.

“In the traditional way to think about chemistry, you think about a few atoms and molecules colliding and forming a new molecule,” he says. “But in the quantum regime, all molecules act together, in collective behaviour. This opens a whole new way to explore how molecules can all react together to become a new kind of molecule.”

More on quantum physics:

• Quantum physics for the terminally confused
• First entanglement-based quantum network
• In search of that quantum advantage