Measuring Nonadiabatic Interactions in Cold Gases: The Atomic Hot Potato

van Fa Frank Abeelen

K. Li, Hong-Fang Song, H. Xu +2 more

Azimuthal anisotropy has been ubiquitously observed in high-energy nuclear (heavy-ion) collisions, perceived to require hydrodynamic interactions. This work reports a study of anisotropic expansion of cold 6 Li Fermi gases, released from anisotropic potential traps, under tunable interaction strength by a magnetic field. A universal scaling of the momentum anisotropy response to geometry is observed for the first time between cold-atom and heavy-ion systems as a function of opacity—the average number of collisions per particle ( n coll ), despite their vast differences in scale and physics. The anisotropy response increases quickly at small opacity, without the need of a large amount of interactions, and shows no sign of saturation in the observed range, with an approximate power-law dependence of n coll , characteristic of random walks. This universality potentially unifies a variety of vastly different physical systems, from dilute atomic gases to the hot and dense quark-gluon plasma of the early universe.

John E. Thomas

Kurt Gibble

We theoretically analyze frequency shifts of cold-atom clocks due to background gas collisions. Because nearly all collisions with room-temperature background gases that transfer momentum eject cold atoms from a clock, the frequency shifts for cold atoms are significantly different than those of room-temperature clocks. From general arguments, measurements of the loss of Ramsey fringe amplitude can usefully bound this source of systematic error.

N. Balakrishnan

Kurt Gibble

Frequency shifts from background-gas collisions currently contribute significantly to the inaccuracy of atomic clocks. Because nearly all collisions with room-temperature background gases that transfer momentum eject the cold atoms from the clock, the interference between the scattered and unscattered waves in the forward direction dominates these frequency shifts. We show they are ≈ 10 times smaller than in room-temperature clocks and that van der Waals interactions produce the cold-atom background-gas shift. General considerations allow the loss of the Ramsey fringe amplitude to bound this frequency shift.