to exploit Bose-Einstein condensation, quantum
degenerate Fermi gases,
and cold molecules for
metrology and ultralow-temperature
physics.
INTENDED OUTCOME AND BACKGROUND
The Quantum Physics Division and
JILA are world renown for studies of
Bose-Einstein condensates and quantum
degenerate Fermi gases. The exemplary
JILA collaboration between NIST and
the University of Colorado (CU) led to
the achievement of the first Bose-Einstein condensate by Eric Cornell
(NIST) and Carl Wieman (CU), who
together received the 2001 Nobel Prize
in Physics. This achievement, coupled
with the creation of the first quantum
degenerate Fermi gas and the first Fermi
condensate by MacArthur Prize-winner
Deborah Jin (NIST), places the
Quantum Physics Division and JILA at
the forefront of studies of macroscopic
quantum mechanical systems.
A better understanding of these systems
is critical as the miniaturization of electronic
components pushes into the size
region where quantum mechanical
effects play a significant role in their
operation. Additionally, these systems
provide unique opportunities for
metrology and for gaining insights into
analogous transitions in technologically
important solid-state systems. We plan
to continue to explore the new quantum
mechanical systems that these discoveries
have made accessible and to maintain
our leadership position. The development
of techniques to produce ultracold
molecules also promises important
advances in chemical physics.
Accomplishments
Casimir-Polder Forces
Figure 2. The ultrahigh vacuum chamber used for Casimir force measurements. |
When two solid objects are brought very close together but not quite into physical contact, they experience a mutual attraction called the Casimir force. While this force is very small by most measures, it has technological
significance because in nanoscale mechanical
devices, the force can dominate the behavior. Generated by quantum mechanical
fluctuations in the vacuum separating the two objects, the Casimir force has long been predicted to depend on temperature, although this dependence has never been conclusively demonstrated.
Division scientists have been studying an effect that is simpler than, but related to, the Casimir force—the Casmir-Polder force. This force between a solid surface and a nearby atom is so small that it is most readily studied using an ultracold sample of atoms, a Bose-Einstein condensate (BEC).
Division scientists have recently succeeded in observing for the first time the temperature
dependence of a Casimir-type force. A BEC of rubidium atoms was suspended magnetically about 6 μm above a surface of fused quartz. Then, as the surface temperature
was increased from 300 K to 570 K, the attractive pull on the BEC was seen to more than double.
Kosterlitz-Thouless Transition on a Lattice
Figure 3. Top view of a quasi-2D Bose-Einstein condensate as the effective temperature gradually increases from left to right.
The randomly ordered dots that appear at higher temperature are the cores of thermally generated vortices and are the signature of a
Berezinskii-Kosterlitz-Thouless transition.
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The nature of a phase transition (such as melting, ferromagnetism, superconductivity,
or superfluidity) can be strongly affected by the dimensionality of the system. For instance, it’s widely believed that high-Tc superconductivity is at least in part a consequence of the two-dimensional nature of the layered structure of copper oxide crystals. In gases of ultracold atoms, the now familiar Bose-Einstein phase transition
(between a normal and a superfluid gas) that occurs in three dimensions is predicted
to be suppressed in two dimensions in favor of a more exotic transition known as the Berezinskii-Kosterlitz-Thouiless (BKT) transition.
Division scientists have been studying BKT physics in a system of ultracold rubidium atoms confined in a two-dimensional optical lattice. Atoms can move from one well of the lattice only by tunneling; atoms within each well make up a conventional, if small, 3-D BEC, with a well-defined quantum phase. Initially, the phases in all the wells are the same; the phase coherence of the system extends across the entire sample. As the temperature is increased, thermally driven phase fluctuations increase and eventually pairs of vortices are spawned and wander randomly through the sample, destroying
the phase coherent state. The sudden appearance of vortices is the microscopic mechanism for the BKT transition. While the BKT transition has been studied in other condensed-matter systems, this is the first time the appearance of vortices has been quantitatively documented.
Cold Fermions and
Resonance Superfluidity
Figure 4. Measured Fermi superfluid phase diagram compared with BCS-BEC crossover theory (red line).
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Several studies of the behavior of Fermi condensates in the region of the BCS-BEC crossover have been completed. Working
with condensed matter theorists with expertise in high-Tc superconductivity, the measured phase diagram for Fermi superfluidity
was compared with a BCS-BEC crossover theory. While the good qualitative
agreement seen in this comparison is very encouraging, quantitative differences reveal the need for more advanced theoretical
understanding of the crossover.
We have also quantitatively studied the thermodynamics of the Fermi gas in the region of the BCS-BEC crossover. We measured the momentum distribution of the Fermi gas, which shows dramatic effects due to the pairing of atoms. We also measured the energy of the gas at unitarity
(strongest possible interactions). This measurement demonstrated that ultracold Fermi gases are in the regime of “universality,”
where the physics does not depend on the details of the interatomic potentials.
In related work, a new experimental probe of the gas was introduced by demonstrating
that information can be extracted from noise in absorption images of the ultracold cloud. In particular, by looking for correlations in the noise we were able to detect atom-pair correlations, including both spatial correlation and momentum correlations. In the future, this new technique
could be used to detect the pairing of atoms in a Fermi superfluid.
Another exciting direction for future work is exploring the possibility of creating new types of Fermi superfluidity. To this end, we have explored the possibility of creating a new type of atom pairs using a p-wave Feshbach resonance in the ultracold Fermi gas. We successfully created and detected p-wave molecules, as well as p-wave quasi-bound states. We measured both the energy of these pairs and their lifetime as a function of magnetic field. Unfortunately, the measured lifetime of the molecules is relatively short and appears to be limited by molecule-atom collisions.
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Figure 5. P-wave molecules, seen after dissociation.
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Cold Molecules
Figure 6. Absorption images of a gas of ultracold heteronuclear molecules expanding after release from the trap. Elapsed time on top frame is 1 ms, middle 3 ms, and bottom 6 ms.
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We have performed the first experimental and theoretical investigations of ground state polar molecules confined within a potential surface defined by combined electric and magnetic fields. This type of trap is novel as the molecule has both electric dipole and magnetic dipole moments,
causing complex dynamics within the spatially inhomogeneous electric and magnetic fields. Unlike previous work where ground state polar molecules were trapped inside an electric trap, the new trap configuration gives the freedom to apply an external electric field to control dipole-dipole interactions while keeping the molecules trapped in an inhomogeneous
magnetic field.
This capability will be important for precision measurements and for study of dipolar gas physics and cold dipolar collisions and reactions. We have already used this trap to determine the collision cross section between cold molecules and hot background gas. In addition, we made clear observations of single particle dynamics
inside the trap that are in excellent agreement with theory.
We see a possibility of producing ultracold polar molecules from ultracold gases, using Feshbach resonances and optical transitions.
For this experiment we have built an apparatus that is capable of producing ultracold-atom gas mixtures and accessing magnetic-field Feshbach resonances.
Rapid progress has been made in this experiment.
Using an interspecies Feshbach resonance, we have created and detected heteronuclear Feshbach molecules, measured
their binding energy as a function of magnetic field, and studied various molecule-atom collisions that limit the molecule lifetime. These results are very promising in that we have been able to realize
relatively long lifetimes for ultracold, trapped molecules.
With these molecules we have begun optical
spectroscopic measurements, exploring
transitions to electronically excited molecule states. We have seen a dramatic enhancement in these excitation rates when starting with Feshbach molecules, compared to photoassociation of ultracold atoms. Future work will employ multiple optical frequencies, referenced to an optical
frequency comb, to drive these weakly bound molecules to more deeply bound states.
First strategic focus |
Second strategic focus |
Third strategic focus |
Fourth strategic focus
"Technical Activities 2005-2007" - Table of Contents |
