Physics

Physicists Harnessed Thousands of Molecules Into a Single Quantum State

In a major milestone for quantum physics, thousands of molecules have been induced to share the same quantum state, dancing together in unison like one huge super molecule.

This is a goal long-sought by physicists, who hope to harness complex quantum systems for technological applications – but getting a bunch of unruly molecules to work together is on a difficulty par with herding cats.

“People have been trying to do this for decades, so we’re very excited,” said physicist Cheng Chin from the University of Chicago.

“I hope this can open new fields in many-body quantum chemistry. There’s evidence that there are a lot of discoveries waiting out there.”

The concept of many particles acting together as one big particle – sharing their quantum states – is not a new one. We’ve achieved it and experimented with it for decades with clouds of single atoms in a state of matter called a Bose-Einstein condensate.

These are formed from atoms cooled to just a fraction above absolute zero (but not reaching absolute zero, at which point atoms stop moving). This causes them to sink to their lowest-energy state, moving extremely slowly so that their energy differences disappear, leading them to overlap in quantum superposition.

The result is a high-density cloud of atoms that acts like one ‘super atom’ or matter wave.

Molecules, however, are made up of multiple atoms bound together, and therefore are a lot more difficult to tame in this way.

“Atoms are simple spherical objects, whereas molecules can vibrate, rotate, carry small magnets,” Chin explained. “Because molecules can do so many different things, it makes them more useful, and at the same time much harder to control.”

To create their molecular Bose-Einstein condensate, the team, led by physicist Zhendong Zhang from the University of Chicago, started with an atomic Bose-Einstein condensate, using a gas of 60,000 cesium atoms.

Next, they cooled the condensate even further and ramped the magnetic field so that around 15 percent of the cesium atoms collided and bound together in pairs to form dicesium molecules. The unbound atoms were ejected from the trap, and a magnetic field gradient was applied to levitate and constrain the remaining molecules in a two-dimensional configuration.

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

The resulting gas was made up of molecules that the scientists found were all occupying the same quantum state, with the same spins, orientation, and vibration.

We’re yet to explore what a molecular Bose-Einstein condensate can do – but this is a significant step in that direction, providing an empty canvas for future experiments.

Not just for the molecular condensate itself, either, but for the transition between atomic and molecular Bose-Einstein condensates. Exploring how this works will help scientists streamline the process, so we can develop condensates with other molecules that may be easier to maintain or more efficient for different technological applications.

“In the traditional way to think about chemistry, you think about a few atoms and molecules colliding and forming a new molecule,” Chin said.

“But in the quantum regime, all molecules act together, in collective behavior. This opens a whole new way to explore how molecules can all react together to become a new kind of molecule.”

The team’s research has been published in Nature.

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