Investigating mysterious data in ultracold gases of rubidium atoms, scientists
at the Joint Quantum Institute of the National
Institute of Standards and Technology (NIST) and the University of Maryland
and their collaborators have found that properly tuned radio-frequency waves
can influence how much the atoms attract or repel one another, opening up new
ways to control their interactions.
In the sequence of green arrows, a pair of ultracold gas atoms collides, briefly forms a molecule, and flies apart, in the presence of an external magnetic field (not shown) that influences this process. By adding RF radiation (lightning bolts) of the right frequency, the atoms can experience being in many different molecular states (red arrows), providing even more extensive and detailed control of the collision. The size of the yellow bursts indicate the amount of absorption/emission of RF radiation. Credit: Eite Tiesinga, NIST/JQI
As the authors report* in an upcoming issue of Physical Review A, the radio-frequency
(RF) radiation could serve as a second "knob," in addition to the
more traditionally used magnetic fields, for controlling how atoms in an ultracold
gas interact. Just as it is easier to improve reception on a home radio by both
electronically tuning the frequency on the receiver and mechanically moving
the antenna, having two independent knobs for influencing the interactions in
atomic gases could produce richer and more exotic arrangements of ultracold
atoms than ever before.
Previous experiments with ultracold gases, including the creation of Bose-Einstein
condensates, have controlled atoms by using a single knob—traditionally,
magnetic fields. These fields can tune atoms to interact strongly or weakly
with their neighbors, pair up into molecules, or even switch the interactions
from attractive to repulsive. Adding a second control makes it possible to independently
tune the interactions between atoms in different states or even between different
types of atoms. Such greater control could lead to even more exotic states of
matter. A second knob, for example, may make it easier to create a weird three-atom
arrangement known as an Efimov state, whereby two neutral atoms that ordinarily
do not interact strongly with one another join together with a third atom under
the right conditions.
For many years, researchers had hoped to use RF radiation as a second knob
for atoms, but were limited by the high power required. The new work shows that,
near magnetic field values that have a big effect on the interactions, significantly
less RF power is required, and useful control is possible.
In the new work, the JQI/NIST team examined intriguing experimental data of
trapped rubidium atoms taken by the group of David Hall at Amherst College in
Massachusetts. This data showed that the RF radiation was an important factor
in tuning the atomic collisions. To explain the complicated way in which the
collisions varied with RF frequency and magnetic field, NIST theorist Thomas
Hanna developed a simple model of the experimental arrangement. The model reconstructed
the energy landscape of the rubidium atoms and explained how RF radiation was
changing the atoms' interactions with one another. In addition to providing
a roadmap for rubidium, this simplified theoretical approach could reveal how
to use RF to control ultracold gases consisting of other atomic elements, Hanna
says.
* A.M. Kaufman, R.P. Anderson, T.M. Hanna, E. Tiesinga, P.S. Julienne, and
D.S. Hall, Radiofrequency dressing of multiple Feshbach resonances, to appear
in Physical Review A.