We routinely encounter solids, liquids, gases, and even liquid crystals, in our everyday lives, but these are not the only phases of matter known to exist. Exotic phases are created in laboratories around the world, and their study has led to breakthroughs in technology as well as in our understanding of fundamental science. Famous examples include superconductors, in which electric currents flow without resistance and whose discovery stimulated the development of the standard model of particle physics, and the related phenomenon of superfluidity, in which neutral matter flows without viscosity. However, not all exotic phases that have been predicted theoretically have been conclusively observed experimentally, and one of particular interest to physicists is a phase of matter that combines properties of both solids and superfluids: the supersolid.
While many in the physics community are racing to establish the existence of supersolidity in solid helium, others are searching for independent confirmation of supersolids in exotic physical systems composed of atomic gases cooled to near the absolute zero of temperature. In an article published in this month’s edition of the journal Nature Physics, physicists Sarang Gopalakrishnan, Benjamin Lev, and Paul Goldbart at the University of Illinois propose a new method to trap ultracold atoms in a manner that could generate experimentally detectable supersolidity. The group's work is also the subject of a "News & Views" article in this month's Nature Physics.
Over the last two decades, atomic physicists have learned to refrigerate and trap gases to within a millionth of a degree above absolute zero by shining carefully tuned laser beams onto the atoms. By further chilling the atoms through the technique of evaporative cooling—similar to the cooling of coffee by the evaporation of hot liquid from a cup—the atoms are forced closer and closer together until the “quantum fuzziness” of each confined atom overlaps with its neighbor’s. When this overlap occurs, the atomic gas becomes a single quantum object, and the gas undergoes a phase transition to a superfluid. First produced in 1995, these so-called Bose–Einstein condensates (BECs) of atoms are now readily produced in atomic physics laboratories worldwide, including at Illinois.
The Illinois researchers’ new work predicts that inserting this BEC between two mirrors that form an optical cavity will cause the otherwise homogeneous superfluid BEC to form a crystalline pattern (i.e., to “solidify”) without losing its superfluidity. The idea is as follows: atoms in the BEC scatter light from the laser into the region between the mirrors, where it bounces back and forth forming a “standing wave” of regularly spaced regions of high- and low-intensity light, rather like the oscillations of a plucked string. The light exerts a force on the atoms, drawing them into the high-intensity regions, or “optical lattice.”
In the figure, an ultracold cloud of atoms is situated between two optical cavity mirrrors. Lasers scatter photons off the atoms, thereby filling the cavity with light. In response, the atoms spontaneously crystalize in the emergent light crystal.
The crucial extension of this scheme required to achieve real solids—liable to sound waves and cracks and other defects—is the use of a special kind of optical cavity called a multimode cavity. Multimode cavities can support multiple standing wave patterns at once. Shining a laser on a BEC placed inside such a multimode cavity will generate a complex standing wave pattern of light. In general, atoms in different parts of the cavity will conform to standing-wave patterns in different directions, i.e., by adopting distinct crystalline arrangements. Where such arrangements meet, there will be cracks and flaws in the resulting crystal, as there are in real solids. It appears plausible, too, that by rapidly turning on the laser (which is the analog of rapidly cooling a liquid) a solid could be created with sufficiently many imperfections that it would behave as a quantum glass. Observations of such a system would provide much valuable experimental input to theoretical attempts to elucidate the elusive quantum glass state of matter.
The system composed of a BEC confined in a multimode cavity constitutes a new paradigm for creating and studying novel phases of matter, and research at Illinois is currently underway to realize such systems.
This work is supported by the National Science Foundation, the Air Force Office of Scientific Research, and the U.S. Department of Energy. The conclusions presented are those of the authors and not necessarily those of the funding agencies.