Systems of cold atoms can sometimes give rise to behavior surprisingly like free particles moving close to the speed of light. However, unlike the kind of physics you see in experiments such as the Large Hadron Collider (LHC), the "particles" are actually collective phenomena, arising out of strong interactions among the components of the system. By manipulating the properties of the material, researchers can produce behavior analogous to many interesting systems in high energy physics—only at very low temperatures and with a "speed of light" dictated by the material's characteristics.

A new experiment by Manuel Endres and colleagues has achieved a Higgs-like excitation in a system composed of ultracold rubidium atoms. By pushing the atoms to a quantum critical point, where they change from an insulator to a superfluid, they were able to generate a transition that was analogous to the break in symmetry that gives rise to the Higgs field.

One of the cornerstones of quantum field theory is that each particle's properties depends on its interactions. This is true whether the particle is on its own, in an atom, or part of a larger material. The Higgs field is just one of a number of these interactions.

In materials, it is possible to adjust the types of interactions—and thus the properties of the quantum excitations that are produced. These particle-like excitations are known collectively as quasiparticles. Some of these quasiparticles behave like free relativistic particles, which may move close to a "speed of light" that is also set by the interactions. This "speed of light" is much smaller than the real speed of light in vacuum, but the physical behavior of the quasiparticles is the same as in high energy situations.

In this research, the experimenters started with an optical lattice of rubidium atoms. Rubidium atoms are bosons, particles that can all occupy the same quantum state. This simplified interactions so that they are dictated by a single tunable parameter: the "depth" of the lattice, which determines how easy it is for an atom to move from one site to another. For a shallow lattice depth, the researchers could make the rubidium flow as a superfluid, since nothing prevented atoms from moving from one site to another; for deeper values, the atoms were confined in place, creating an insulator.

Superfluids have a high degree of order: the atoms move in concert. On the other hand, insulators are disordered, because the atoms are isolated from each other, acting independently. Thus, the transition from superfluid to insulator behavior is a symmetry breaking operation; in quantum field theory, a symmetry breaking is associated with a Higgs mode, which is a separate and distinct excitation.

The early Universe, when everything was extremely hot, corresponds to the superfluid state. During the Universe's expansion, things cooled off, separating the different types of interactions; the Higgs field in particle physics is the means by which the superfluid state broke down. (The Higgs boson is a manifestation of the Higgs field, and only appears if enough energy is present to create one; the current experiment doesn't produce a Higgs boson-like quasiparticle.)

A Higgs excitation had been predicted to be present in cold atomic systems, but that doesn't mean one would be detected; it could be swamped by other effects, or it could be a feature not found in real systems, due to the necessary simplifications required in the theoretical models. However, if the Higgs excitation was present, it should have a particular energy signature, which the researchers were able to detect by measuring the spectrum of each individual atom in the system.

The optical lattice the researchers used was two-dimensional, which is intriguing because it wasn't certain that Higgs-like excitations could occur in lower-dimensional systems. Their results may have implications for theories with higher dimensions, such as string theory, in which the Higgs field manifests itself in the four dimensions we observe directly.

Nature, 2012. DOI: 10.1038/nature11255 (About DOIs).