Probing causality with tunable Fermi gas condensates
Oct 23, 2013
Quantum mechanical effects normally occur on atomic or molecular scales. However in some circumstances, as can happen for very cold gases, we find quantum coherence over macroscopic condensates of many thousands of atoms. In J. Phys.: Condens. Matter 25 404211, we develop a new formalism to describe the hydrodynamic properties of the condensates of cold Fermi atoms. This is used to study the spontaneous creation of quantum excitations that mimics spontaneous particle creation in curved space-time in model Universes.
We study very cold Fermi gases of alkali atoms. Due to the Fermi exclusion principle, which prevents two alkali atoms having the same state, a condensate can only be developed if the atoms pair up to form bosons, either as Cooper pairs or as molecules. The strength of this pairing can be controlled by applying an external magnetic field. The quantum excitations of the diatoms are the phonons, the particles of sound. The speed of the phonons varies with the external field and can be tuned by means of a narrow Feshbach resonance.
Despite the complicated nature of the microscopic atomic interactions, we have derived a simple theory of phonons and their interactions in such tunable Fermi gases. Although the gas is non-relativistic, for a varying background magnetic field the dynamical equation of long-wavelength phonons can be cast as an equation of a quantum field in a relativistic curved space-time, in which sound plays the role of light. In principle this makes it possible to devise laboratory experiments to mimic tunable model Universes.
Unfortunately, for shorter wavelengths the underlying non-relativistic microscopic structure is reflected in the distortion of the sound-cone. We have calculated this distortion and find that its nature is to make spontaneous phonon production in a changing field, the simplest test of the picture, too inefficient to be observed experimentally. More positively, but only mentioned briefly here, not only is the sound-cone distorted, it is also fuzzy. This fuzziness, perhaps analogous to fluctuations of the light cone expected in quantum gravity, may be testable in time-of-flight experiments.
About the author
The study was carried out by Jen-Tsung Hsiang, Chi-Yong Lin and Da-Shin Lee at the Department of Physics, National Dong Hwa University in Taiwan, in collaboration with Ray J Rivers at the Blackett Laboratory, Imperial College London, UK. Work at National Dong Hwa University was supported in part by the National Science Council, Taiwan. This work first appeared in Journal of Physics: Condensed Matter