Spin Zone: Physicists Get 1st Look at Strange Quantum Magnetism

Illustration of ultracold fermionic atoms in an optical lattice potential. The atoms tended to tunnel into wells with others that had opposite spins. After a while, a line of atoms spontaneously organized itself, with the spins in a non-random pattern, revealing a signature of quantum magnetism

Using super-chilled atoms, physicists have for the first time observed a weird phenomenon called quantum magnetism, which describes the behavior of single atoms as they act like tiny bar magnets.

Quantum magnetism is a bit different from classical magnetism, the kind you see when you stick a magnet to a fridge, because individual atoms have a quality called spin, which is quantized, or in discrete states (usually called up or down). Seeing the behavior of individual atoms has been hard to do, though, because it required cooling atoms to extremely cold temperatures and finding a way to "trap" them.

The new finding, detailed in the May 24 issue of the journal Science, also opens the door to better understanding physical phenomena, such as superconductivity, which seems to be connected to the collective quantum properties of some materials. 

Spin science

The research team at the Swiss Federal Institute of Technology (ETH) in Zurich focused on atoms' spin, because that's what makes magnets magnetic — all the spins of the atoms in a bar magnet are pointed the same way.

To get a clear view of atoms' spin behaviors, the researchers had to cool potassium atoms to near absolute zero. That way, the random thermal "noise" — basically background radiation and heat — didn't spoil the view by jostling the potassium atoms around.

The scientists then created an "optical lattice" — a crisscrossing set of laser beams. The beams interfere with each other and create regions of high and low potential energy. Neutral atoms with no charge will tend to sit in the lattice's "wells," which are regions of low energy.

Once the lattice is built, the atoms will sometimes randomly "tunnel" through the sides of the wells, because the quantum nature of particles allows them to be in multiple places at the same time, or to have varying amounts of energy. [Quantum Physics: The Coolest Little Particles in Nature]

Another factor that determines where the atoms lie in the optical lattice is their up or down spin. Two atoms can't be in the same well if their spins are the same. That means atoms will have a tendency to tunnel into wells with others that have opposite spins. After a while, a line of atoms should spontaneously organize itself, with the spins in a non-random pattern. This kind of behavior is different from materials in the macroscopic world, whose orientations can have a wide range of in-between values; this behavior is also why most things aren't magnets — the spins of the electrons in the atoms are oriented randomly and cancel each other out.

And that's exactly what the researchers found. The spins of atoms do organize, at least on the scale the experiment examined.

"The question is, what are the magnetic properties of these one-dimensional chains?" said Tilman Esslinger, a professor of physics at ETH whose lab did the experiments. "Do I have materials with these properties? How can these properties be useful?"

Quantum magnetism

This experiment opens up possibilities for increasing the number of atoms in a lattice, and even creating two-dimensional, gridlike arrangements of atoms, and possibly triangular lattices as well.

One debate among experts is whether at larger scales the spontaneous ordering of atoms would happen in the same way. A random pattern would mean that in a block of iron atoms, for instance, one is just as likely to see a spin up or down atom in any direction. The spin states are in what is called a "spin liquid" — a mishmash of states. But it could be that atoms spontaneously arrange themselves at larger scales.

"They've put the foundation on various theoretical matters," said Jong Han, a professor of condensed matter physics theory at the State University of New York at Buffalo, who was not involved in the research. "They don't really establish the long-range order, rather they wanted to establish that they have observed a local magnetic order."

Whether the order the scientists found extends to larger scales is an important question, because magnetism itself arises from the spins of atoms when they all line up. Usually those spins are randomly aligned. But at very low temperatures and small scales, that changes, and such quantum magnets behave differently.

Han noted that such lattices, especially configurations where the potential wells connect to three others, rather than two or four, would be especially interesting. Esslinger's lab showed that atoms tend to jump to potential wells where the spins are opposite; but if the wells are arranged so that the atom can jump to two other atoms, it can't "choose" which well to go to because one of the two atoms will always be in the same spin state.

Esslinger said his lab wants to try building two-dimensional lattices and explore that very question. "What happens to magnetism if I change the geometry? It's no longer clear if spins should be up or down."