Outer space

Particles near absolute zero help probe quantum magnetism

Physicists have cooled particles to within one-billionth of a degree above absolute zero.

They wanted to gain insight into the zany realm of quantum mechanics.

The Japanese and US physicists used lasers to cool ytterbium atoms to investigate the unexplored effects of quantum magnetism. Their findings are published in Nature Physics.

“Unless an alien civilization is doing experiments like these right now, anytime this experiment is running at Kyoto University it is making the coldest fermions in the universe,” says corresponding author Kaden Hazzard, from Rice University in Texas. “Fermions are not rare particles. They include things like electrons and are one of two types of particles that all matter is made of.”

A billionth of a degree above absolute zero sounds impressive, but what does that actually mean?

Absolute zero is -273.15 °C or 0 K (where K stands for Kelvin – the standard scientific measurement of temperature). By comparison, even the cold, dead, empty vacuum of space is a balmy 2.7 °C above absolute zero. This is because, even 13.7 billion years later, the residual “afterglow” of the Big Bang continues to give even the most remote parts of space a bit of warmth.

Nothing can get colder than absolute zero. At 0 K, no energy from molecular motion (known to us common folk as “heat”) can be transferred from one system to another. The only motion at absolute zero is the quantum vibration of particles known as “zero-point energy.” But let’s not get ahead of ourselves.

Suffice it to say that energy can never get lower than the energy at 0 K – the point at which heat transfer ceases.

Surely, if things are really that close to absolute zero, there can’t be many interesting things happening. On the contrary…

“The payoff of getting this cold is that the physics really changes,” says Hazzard. “The physics starts to become more quantum mechanical, and it lets you see new phenomena.”

Particles like photons and electrons are subject to the laws of quantum dynamics.

Read more: “Microfridge” capable of cooling cell-sized objects to be used in investigations of quantum entanglement

Laser cooling has been used by physicists to cool atoms to ridiculously low temperatures for a quarter century now. By forcing the atom to absorb and re-emit a photon (a particle of light), physicists can change and manipulate the momentum (vibrations) of the atoms.

Temperature of atoms relates to their movements relative to each other. But if they are all moving at the same speed and in the same direction, the lower their temperature.

Using the array of laser-cooled ytterbium atoms, the physicists were able to simulate the behavior of magnetic and superconducting materials. One such behavior involves the collective behavior of electrons similar to a crowd of people performing “the wave” in a stadium.

Rice University theoretical physicists (from left) Eduardo Ibarra-García-Padilla, Kaden Hazzard and Hao-Tian Wei are collaborating with experimental physicists at Kyoto University in Japan to study unexplored quantum magnets using the universe’s coldest fermions. Credit: Jeff Fitlow/Rice University.

They compared their experimental simulation to the theoretical results of such quantum magnetic materials as described by the mathematically complex Hubbard model created in 1963 by theoretical physicist John Hubbard.

Their analysis showed that they had achieved record-breaking temperatures.

“Comparing their measurements to our calculations, we can determine the temperature. The record-setting temperature is achieved thanks to fun new physics that has to do with the very high symmetry of the system,” says Hazzard.

When physicists talk of symmetry, they are referring to the transformations you can perform on a given system (for example, 90-degree rotation) which exhibit no change in the physical characteristics of the system.

The Hubbard model simulated in Kyoto utilized “spin” symmetry in ytterbium.

Spin is one of those strange quantum qualities that has no real analogue in our everyday experience. Quantum mechanics tells us that there are a number of different types of angular momentum. Orbital angular momentum is simple – this is when a particle, like a negatively-charged electron, orbits a central force like the central force provided by the positive charge on an atomic nucleus.

Spin angular momentum, however, is a little more complex. It is best described as the particles “spinning” on their own axis. But, given particles don’t have a “shape” that we can really imagine (no, they’re not spheres), it’s hard to imagine what that really means.

There are six possible arrangements of the outer-shell electrons’ spin states, meaning that ytterbium has six spin states – referred to in mathematics as a “special unitary” group 6, or SU(6). The Kyoto simulator is the first in the world to reveal magnetic correlations in an SU(6) Hubbard model. This means that the quantum magnetic alignment of one atom is affecting others.

As the number, N, in SU(N) gets higher, so does the complexity of the system.

“That’s the real reason to do this experiment,” Hazzard says. “Because we’re dying to know the physics of this SU(N) Hubbard model.”

Co-author Eduardo Ibarra-García-Padilla, a graduate student at Rice University, says the Hubbard model aims to capture the fundamental reasons why materials become metals, insulators, magnets or superconductors.

“One of the fascinating questions that experiments can explore is the role of symmetry,” Ibarra-García-Padilla adds. “To have the ability to engineer it in a laboratory is extraordinary. If we can understand this, it may guide us to making real materials with new, desired properties.”

The Kyoto experiments give a glimpse of how these complex quantum systems operate by allowing physicists to watch them in action.

“Right now, this coordination is short-ranged, but as the particles are cooled even further, subtler and more exotic phases of matter can appear,” Hazzard explains. “One of the interesting things about some of these exotic phases is that they are not ordered in an obvious pattern, and they are also not random. There are correlations, but if you look at two atoms and ask, ‘Are they correlated?’ you won’t see them. They are much more subtle. You can’t look at two or three or even 100 atoms. You kind of have to look at the whole system.”

“These systems are pretty exotic and special, but the hope is that by studying and understanding them, we can identify the key ingredients that need to be there in real materials,” says Hazzard.

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