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INTENDED OUTCOME AND
BACKGROUND
This program provides measurements and data to enable the development
of ultracold atom technology, in particular the use of coherent matter
waves in sensors, atom interferometers, and quantum information
processing devices.
The Division maintains two efforts in this area, one theoretical and
one experimental. The theoretical program is focused on quantitative
modeling of degenerate quantum gases, with particular attention to the
dynamics of Bose-Einstein condensates subject to external forces, e.g.,
manipulation of condensates confined in an optical lattice. This program
is an outgrowth of extensive collaborations with experimental groups at
NIST, JILA, and elsewhere, begun in the mid-1990s.
The experimental program develops deterministic atom-delivery systems,
i.e., devices that can deliver precisely one atom to a predetermined
location, on demand. In addition, the Division is developing a testbed
for quantum communication systems, together with the Atomic Physics
Division and the Information Technology Laboratory.
Accomplishments
Characterization of
“Atom-on-Demand” Source
Recent developments in our laboratory have led to a new source of
atoms that is almost completely deterministic: single atoms can be
made available at a high rate with near unity certainty. Such a
source has applications not only in quantum information processing,
where single atoms can be used as qubits, but also in nanotechnology,
when doping of nanostructures reaches the level where each
nanostructure contains only a countable number of dopants.
In order to proceed to the next step in our research program,
which will be to extract atoms from our source and demonstrate
controlled deposition, we have conducted a number of studies to
characterize our source. These have involved a detailed analysis
of all the experimental parameters that determine the performance
of the source, and also real-time photography of single atoms.
The detailed analysis has allowed us to implement Monte Carlo
simulations that show extremely good agreement with experimental
observations of such parameters as the single-atom probability
and the single-atom extraction success rate. The photography,
using a high-sensitivity CCD camera (Fig. 9), has given us the
ability to investigate the spatial properties of our source,
which will prove crucial in extracting, transporting,
and depositing the single atoms with high spatial resolution.
Figure 9. Real-time images of single atoms in an
atom-on-demand source. Without feedback, the number of atoms is random,
fluctuating between one, two, and three. With feedback, there is always
just a single atom in the source. |
Resonant Atom-Molecule Mixtures in
Quantum-Degenerate Gases
Cooling fermionic, atomic gases to quantum degeneracy, in conjunction
with the ability to directly control the interatomic interactions
through magnetic-field-tunable Feshbach resonances, has opened the
door to a detailed study of the crossover between
Bardeen-Cooper-Schrieffer superfluidity and Bose-Einstein
condensation of diatomic molecules in an atomic Fermi gas.
A Feshbach resonance can be used to couple atom pairs into a bound
molecular state, thereby creating bosonic dimers in the gas.
Under suitable conditions, these dimers can undergo Bose-Einstein
condensation. The BCS-BEC crossover is a cornerstone problem of
condensed matter physics, due in large part to its importance in
the understanding of high-temperature superconductivity.
We have studied the equilibrium phase diagram of atom-molecule
mixtures in a simple model under experimentally relevant conditions.
A finite fraction of molecules persists for energies above
dissociation threshold. At high temperatures this is ascribable
to a detailed balance between atomic association and molecular
dissociation in the gas, while in the quantum-degenerate regime
Pauli blocking stabilizes molecular states embedded in the continuum.
Figure 10. Phase diagrams of atom-molecule mixture showing the
molecule fraction (A) and condensate fraction (B) as a
function of temperature and resonance energy, εRes,
in units of the Fermi temperature and Fermi energy, respectively. Lines
indicate contours of constant entropy, which are the trajectories followed
by the system under adiabatic variation of the threshold energy. |
Recent experiments found a significant discrepancy between theory
and experiment concerning the dependence of the dimer condensation
threshold on the temperature of the gas. We found that this
discrepancy disappeared when the thermodynamic evolution of the
system was taken into account. In particular, the temperatures
measured in the experiments are those of the initial state of
the Fermi gas; as one sweeps through the Feshbach resonance,
the temperature of the gas changes. The conditions of the
experiments are believed to be adiabatic, which means that the
system should follow a trajectory of constant entropy in the
thermodynamic phase diagram. Examples of such trajectories are
shown in Fig. 10. These demonstrate that assuming fixed
temperature of the gas would overestimate the condensate threshold
temperature.
First strategic focus |
Second strategic focus |
Third strategic focus
"Technical Activities 2004" - Table of Contents |
