Negative temperatures are even “hotter” (Image: Croisy/Shutterstock

ABSOLUTE zero sounds like an unbreachable limit beyond which it is impossible to explore. In fact there is a weird realm of negative temperatures that not only exists in theory, but has also proved accessible in practice. An improved way of getting there, outlined last week, could reveal new states of matter.

Temperature is defined by how the addition or removal of energy affects the amount of disorder, or entropy, in a system. For systems at familiar, positive temperatures, adding energy increases disorder: heating up an ice crystal makes it melt into a more disordered liquid, for example. Keep removing energy, and you will get closer and closer to zero on the absolute or kelvin scale (-273.15 °C), where the system’s energy and entropy are at a minimum.

Negative-temperature systems have the opposite behaviour. Adding energy reduces their disorder. But they are not cold in the conventional sense that heat will flow into them from systems at positive temperatures. In fact, systems with negative absolute temperatures contain more atoms in high-energy states than is possible even at the hottest positive temperatures, so heat should always flow from them to systems above zero kelvin.

Creating negative-temperature systems to see what other “bizarro world” properties they might have is tricky. It is certainly not done by cooling an object down to absolute zero. It is, however, possible to leap straight from positive to negative absolute temperatures.


“Objects can’t be cooled to absolute zero, but you can leap straight to negative temperatures”

This has already been done in experiments in which atomic nuclei were placed in a magnetic field, where they act like tiny bar magnets and line up with the field. The field was then suddenly reversed, leaving the nuclei briefly aligned opposite to the direction in which they would have the lowest energy. While they were in this state they fleetingly behaved in a way consistent with them having negative absolute temperatures, before they too flipped over to line up with the field.

Because the nuclei can only flip between two possible states – parallel to the field or opposite to it – this set-up offered only limited possibilities for investigation. In 2005 Allard Mosk, now at the University of Twente in the Netherlands, devised a scheme for an experiment that would offer more knobs to turn to explore the negative temperature regime.

First, lasers are used to herd the atoms into a tight ball, which is in a highly ordered or low-entropy state. Other lasers are then trained on them to create a matrix of light called an optical lattice, which surrounds the ball of atoms with a series of low-energy “wells”.

The first set of lasers is then adjusted so that they try to push the ball of atoms apart. This leaves the atoms in an unstable state, as if they were balanced on a mountain peak, poised to roll downhill.

The optical lattice acts like a series of crevices along the mountainside, however, halting their progress. In this state, removing some of the atoms’ potential energy, letting them roll away from each other, would lead to greater disorder – the very definition of a negative temperature system (see graph).

Mosk’s ideas have now been refined by Achim Rosch of the University of Cologne, Germany, and colleagues. Their proposed experimental set-up is essentially the same, but Rosch and his team’s calculations bolster the case that it is feasible.

Crucially, they also suggest a way to test that the experiment would create negative temperatures. Since the atoms in the negative-temperature state have relatively high energies, they should move faster when released from the lattice than would a cloud of atoms with a positive temperature (Physical Review Letters, DOI: 10.1103/PhysRevLett.105.220405).

“The new work shows that achieving negative temperatures in this new way in the laboratory is realistic,” says Mosk, who was not involved in the new study. “That is something I would be very excited to see.”

Rosch and his colleagues are theorists, not geared up to perform the experiment, but they think a team of experimentalists could test their proposal within a year or so.

Using a combination of lasers and magnetic fields, the atoms in the set-up could be made to attract or repel one another at a range of different strengths. “One can use this to explore and create new states of matter and play with them in regimes we are not used to,” says Rosch. This is uncharted territory, he says, and it may hold some surprises.