Bring water slowly to the boil and an exciting battle of energies takes place. The interaction energy tries to hold the water molecules together through mutual attraction; but the motional energy, increasing as the heat turns up, tries to scatter the molecules. The water boils at the point where the motional energy wins the contest, overcoming the interaction energy and allowing the liquid to turn into vapour. This process is known as a phase transition. But such transitions aren’t just confined to water.
Members of the TherMiQ team in ETH Zürich have been able to observe novel phase transitions at the quantum level by carefully manipulating a tiny cloud of rubidium atoms in a crystal-like lattice.
The interaction energy stems from collisions between atoms that move back and forth between lattice sites. The motional energy of the atoms, on the other hand, can be controlled through the intensity of the laser beams, which determines how easily the atoms can move inside the lattice.
Then the team introduced long-range interaction energy by building a resonator with highly reflecting mirrors. This allowed light particles, known as photons, scattered by one of the atoms to cross the rubidium cloud several times. Thus the atoms could ‘feel’ the presence of the original atom that first deviated the photon. Renate Landig, a PhD student in the group, said: “Using this trick we now have three competing energy scales in our system: besides the motional and interaction energies there is, in addition, the energy associated with the long-range interaction. By varying the motional energy and the long-range interaction energy, we are able to study a number of novel quantum phase transitions.”
By changing the long-range interaction energy, the team were able to induce the atoms to arrange themselves in a ‘checkerboard’ pattern. Tobias Donner explains: “The peculiarity of this phase transition, which is similar to that between water and water vapour, is that it’s a first order transition”. In such phase transitions a particular property of the substance changes suddenly, whereas in second order phase transitions of the sort that have been detected in artificial quantum systems up to now, the change is gradual.
Indeed the team were also able to induce another unusual phase transition by making both the motional energy and the long-range interaction energy very large. In that case, too, a checkerboard pattern appeared inside the lattice, but this time there was phase coherence between the atoms – in other words, their quantum mechanical wave functions were synchronised.
Phase coherence is usually only observed when the atoms are relatively free to roam. The coexistence of a checkerboard pattern and phase coherence at the same time indicates a supersolid phase.
The hybrid state of supersolidity was theoretically predicted as much as fifty years ago, but thus far unambiguously detecting it has proved difficult.
The paper setting out the results, ‘Quantum phases from competing short- and long-range interactions in an optical lattice’, R. Landig et al., was published on 11 April 2016 on the Nature website, DOI: 10.1038/nature17409
13 April 2016