Physicists have coaxed a cloud of atoms into having a temperature beyond absolute zero and placed them in a geometric structure that could produce an unknown form of matter
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| The temperature of atoms is determined by their energy and entropy (Credit: agsandrew/Shutterstock) |
A cloud of atoms with a temperature beyond absolute zero – which is also bizarrely hotter than any positive temperature imaginable – could be a mysterious new quantum state of matter.
Luca Donini at the University of Cambridge and his colleagues have put thousands of potassium atoms into this seemingly paradoxical situation by precisely manipulating their energy levels and quantum states. “Quantum mechanics allows you to do this, while classical thermodynamics would never allow it,” he says.
Normally, if you have a collection of atoms that are moving around quickly, with lots of kinetic energy, then the temperature is high. In contrast, slow-moving atoms with little kinetic energy have a low temperature. When all atoms are perfectly still, the Kelvin scale – used by scientists to measure temperature – hits absolute zero, the lowest value possible.
But physicists also have a more precise definition of temperature that reflects how energy is distributed among the atoms, a property related to a system’s entropy, or how disordered it is.
For a collection of atoms with a positive absolute temperature, only a few of them have lots of energy, while most have lower energy levels. At absolute zero, all atoms have the same, minimal energy. But at negative absolute temperatures the distribution flips so that higher-energy atoms become the majority, and those with lower energy become outliers.
This leads to one of the biggest oddities about negative absolute temperatures – they are technically hotter than all positive temperatures. This is because a negative absolute temperature object will always have more energy, so putting it together with a positive temperature object would make heat flow into the latter, as from a warm room into a cold glass of water.
Donini and his colleagues achieved this unusual temperature by putting potassium atoms into a vacuum chamber and cooling them very close to absolute zero using lasers and magnetic fields. This allowed them to control the quantum states and energies of the atoms, ultimately coaxing them into a negative absolute temperature. They presented the work at the annual meeting of the American Physical Society Division of Atomic, Molecular and Optical Physics in Fort Worth, Texas, on 5 June.
This kind of experiment was first performed in 2013, but now the researchers have carried out a more advanced version that allows them to arrange the negative temperature atoms into a pattern of hexagons and triangles called the Kagome lattice.
In this arrangement, the atoms can be pushed into a quantum state where all of their energy comes from interactions with other atoms, but they have no kinetic energy. Physicists are unsure what kind of matter those properties will create, says team member Ulrich Schneider, also at the University of Cambridge, and they now face the challenge of pinning down the properties of this new quantum substance.
“We’re going into unknown territory, and we are getting measurements, but interpreting them and knowing what they mean is not trivial,” says Schneider.
Achim Rosch at the University of Cologne, Germany, says that exotic and rich quantum behaviours tend to emerge from particles arranged in Kagome lattices because of the very specific energies they can have in this arrangement and because of its intricate geometry. For instance, some theorists predict that the atoms may form a fluid that flows with zero viscosity, an idea that seems contradictory when none of the atoms have any kinetic energy, and so should be unable to move. “There is a slew of possibilities of what could happen in this context,” says Rosch.
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