This is the clearest ever image of individual atoms behaving like a wave, as predicted by quantum mechanics. Such images could eventually be used to study this exotic and poorly understood quantum behaviour.
The fact that particles like atoms can behave like waves is a key insight of quantum theory. One specific shape of wave that an atom can adopt is known as a 鈥渨ave packet鈥�, which is like a series of ripples that you may see on water, but far more bunched up and compressed.
Physicists can predict exactly how a wave packet will change over time by using an equation developed by physicist Erwin Schr枚dinger. This makes analysing wave packets a great test object for how well an atom can be controlled and imaged in the quantum realm, says at the French National Centre for Scientific Research and the聽脡cole normale sup茅rieure in Paris. He and his colleagues did so in an experiment with extremely cold lithium atoms.
Advertisement
To put atoms in the quantum realm, the researchers had to make them nearly as cold as absolute zero. They placed lithium atoms in a small, airless chamber, then hit them with lasers and magnetic fields, which lowered their energy and made them cooler.
Additionally, the researchers could use the same tools to control the atoms鈥� quantum states, and this meant that they could control their shape as waves. Consequently, they arranged the atoms so as not to be too close to each other, made sure each atom鈥檚 quantum state corresponded to a wave packet and then loosened some of the forces that were keeping the atoms pinned in place so they could watch the wave packets change.
In images, each atom started as a tight dot. The Schr枚dinger equation predicts that a wave packet that is free to move rather than being held still will spread as time goes on 鈥� that is, the dot should become larger and fuzzier. The images of the lithium atoms that Yefsah and his colleagues took showed exactly that. He says they could also show how altering the initial properties of the wave packet, such as its width, changed the way it subsequently spread. In all cases, the observations they made from their images agreed with the famous equation.
at the University of Virginia says the wave packet is such a well-understood component of quantum theory that the findings of the new experiment are not surprising 鈥� but they do show that the researchers had a high degree of control over the processes used to cool and then precisely image the atoms.
He says that atoms of this type, which are called fermions, are very interesting to physicists, especially when they interact with each other, but remain difficult to cool down to extremely low temperatures. For instance, theorists think that strongly interacting ultracold fermions could form many new quantum phases of matter 鈥� but the mathematics is too difficult to predict exactly how and experiments could add clarity, says Schauss.
Yefsah and his colleagues are hoping to use their procedure on such systems of strongly interacting atoms next. In the most extreme cases, their images may reveal quantum wave behaviour similar to those in the exotic quantum matter inside incredibly dense neutron stars, or in the 鈥渟oup鈥� of highly interacting particles that existed right after the big bang, he says.
arXiv
