Researchers Trigger Long-Distance Phase Transitions

A team of researchers at the Institute for Quantum Electronics at ETH Zurich has shown that particles can be made to “feel” each other even over large distances, resulting in the observation of novel phase transitions that result from energetic three-way battles.

Researchers used a quantum simulator to create and monitor both interaction and motional energy, forces that respectively attempt to keep molecules together or break them apart. Researchers cooled a tiny cloud of rubidium atoms to temperatures just above absolute zero and then caught them in a crystal-like lattice made of laser beams. The interaction energy here stems from collisions between atoms that move back and forth between lattice sites, while the motional energy of the atoms can be controlled through the intensity of the laser beams, determining how easily the atoms can move inside the lattice.

The team then built a resonator using two highly reflecting mirrors to ensure that light particles scattered by one of the atoms would fly through the rubidium cloud several times. In that way, all the atoms in the cloud come into contact with the scattered photon sooner or later, therein “feeling” the presence of the original atom that first deviated the photon. This feeling over a distance is tantamount to an effective long-range interaction. How strongly the atoms interact in this way can be exactly controlled through the frequency of the laser beams.

“Using this trick we now have three competing energy scales in our system: besides the motional and interaction energies there is … the energy associated with the long-range interaction,” says Renate Landig, a PhD student working on the project. “By varying the motional energy and the long-range interaction energy, we are able to study a number of novel quantum phase transitions.”

The researchers were already familiar with some of the possible phase transitions. For instance, when the long-range interaction is very small and the motional energy is increased little by little, the phase of the rubidium cloud changes from a Mott insulator, with one immobile atom sitting on each lattice site, to a superfluid, in which atoms can move freely.

In contrast, something completely different happened when the researchers increased the long range interaction energy. At a particular strength of that interaction energy the atoms spontaneously arranged themselves in a checkerboard pattern, with one empty lattice site between two atoms. “The peculiarity of this phase transition, which is similar to that between water and water vapor, is that it’s a first order transition,” says Tobias Donner, a scientist in the group. In such phase transitions a particular property of a substance changes suddenly, whereas second order phase transitions, which are the type of transitions that have been detected in artificial quantum systems up to now, are characterized by a gradual change.

The physicists 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, a checkerboard pattern again appeared inside the lattice, but this time there was phase coherence between the atoms. In other words, the quantum mechanical wave functions were synchronized. Phase coherence is usually observed only when the atoms are relatively free to roam, as is the case, for instance, in the superfluid state. The coexistence of a checkerboard pattern and phase coherence at the same time indicates that one is dealing with 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.

In the future, the team will use its quantum simulator to study such exotic effects more closely, aiming to obtain a general idea of quantum phenomena in increasingly complex systems. This, in turn, goes hand in hand with the development and investigation of materials with special properties.