Unique image of atoms that behave like quantum waves

In 1920, physicist Erwin Schrödinger wrote an equation that predicts how ‘wave particles’ should behave. Now researchers can perfectly recreate these predictions in the lab.

This is the clearest picture ever of individual atoms behaving like a wave, as predicted by quantum mechanics. These types of images can help researchers study this strange and poorly understood quantum behavior.

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An important insight of quantum theory is that particles such as atoms can behave like waves. The waveform that an atom can take is known as a ‘wave packet’. You can imagine this as a series of ripples on a water surface, but more bundled and compressed.

Physicists can predict exactly how a wave packet will change over time. For this they use the Schrödinger equation, developed by the Austrian physicist Erwin Schrödinger. That makes analyzing wave packets a great way to test how well we can manipulate and image an atom in the quantum world, says physicist Tarik Yefsah of the French National Center for Scientific Research and the Ecole Normale Supérieure in Paris. He and his colleagues did this in an experiment with extremely cold lithium atoms.

Cooling lasers

To image the atoms, the researchers had to cool them to almost absolute zero (-273.15 °C). To do this, they placed lithium atoms in a small, vacuum chamber and bombarded them with lasers and magnetic fields. As a result, their energy decreased and they became cooler.

The researchers used the same instruments to monitor the quantum states of the atoms – and thus their waveform. They arranged the atoms so that they were not too close together and ensured that the quantum state of each atom corresponded to a wave packet. Then they released the atoms to see how the wave packets changed.

According to the rules of quantum physics, you cannot photograph a wave packet. If you try to image a wave function, the waveform collapses and you only ‘see’ a particle in a certain place. The researchers got around this by taking multiple images and taking the average. In doing so, they reconstructed a so-called probability density of where the atom is located, which corresponds to a wave packet.

In the images, each atom started as a small dot. The Schrödinger equation predicts that a wave packet that is free to move will spread out as time passes. The images of the lithium atoms that Yefsah and his colleagues took showed exactly that. They were also able to show how changing the initial properties of the wave packet, such as its width, changed the way the packet spread. In all cases, the observations on their images matched the famous equation.

Precise control

Physicist Peter Schauss of the University of Virginia in the United States says the wave packet is such a well-understood part of quantum theory that these findings are not surprising. But it does show that the researchers had a high degree of control over the processes by which they cooled and accurately imaged the atoms.

This check offers the opportunity for further research. The atoms used in the experiment belong to a type of particle called fermions. According to Schauss, fermions are very interesting for physicists, especially if there is mutual interaction. Theorists, for example, think that ultra-cold fermions, with strong mutual interactions, could form new quantum phases of matter. However, the mathematics that can predict exactly how this happens is too difficult. Experiments could provide more clarity, says Schauss. But until now it has been difficult to cool fermions to extremely low temperatures.

Yefsah and his colleagues now hope to use their controlled method for such systems with strongly interacting atoms. In the most extreme cases, their images could reveal quantum wave behavior similar to that of the quantum matter inside incredibly compact neutron stars. Or in the dense ‘soup’ of particles that existed just after the Big Bang.