Synopsis: A Crystal of Light and Atoms

A predicted type of atom-light crystal could host phonon-like excitations, allowing for new ways to simulate the physics of solids.  
Synopsis figure
S. Ostermann et al., Phys. Rev. X (2016)

Using an optical lattice, researchers can corral cold atoms into a crystalline array to simulate phenomena found in solids, such as magnetism and superconductivity. A theoretical proposal now shows how a crystalline phase could emerge without the artificial order imposed by a lattice. The scheme, which involves a uniform gas and lasers, mimics the spontaneous breaking of symmetry that occurs when a liquid becomes a solid, and it hosts wave-like lattice vibrations akin to phonons. The atom-light crystal may therefore offer a new way to simulate materials.

Helmut Ritsch and colleagues from the University of Innsbruck, Austria, considered a long, cigar-shaped Bose-Einstein condensate lying along the path of two counterpropagating laser beams. This setting allows for a dynamical atom-light interaction, in which the atoms move to the strongest regions in the optical field, and the refractive index is modified wherever the atoms bunch. The team found that, above a threshold laser density, this interaction causes the atoms to cluster into regularly spaced peaks. These peaks act as a diffraction grating for the beams, producing a periodic optical field, or a “light crystal.”

Such self-ordering has been predicted when the atoms and light are bounded within a cavity, but cavity walls cut off phonon-like excitations. Based on numerical simulations, Ritsch and colleagues suggest that their atom-light crystal would host a spectrum of phonon modes. These could be used to investigate phonon-mediated pairing interactions, like those that drive superconductivity in metals.

This research is published in Physical Review X.

–Jessica Thomas

Jessica Thomas is the Editor of Physics.


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