155 research outputs found
Observation of coherence revival and fidelity saturation in a delta-kicked rotor potential
We experimentally investigate the effect of atomic -kicked rotor
potentials on the mutual coherence between wavepackets in an atom
interferometer. The differential action of the kicked rotor degrades the mutual
coherence, leading to a reduction of the interferometry fringe visibility;
however, when the repetition rate of the kicked rotor is at or near the quantum
resonance, we observe revival of matter-wave coherence as the number of kicks
increases, resulting in non-vanishing coherence in the large kick number limit.
This coherence saturation effect reflects a saturation of fidelity decay due to
momentum displacements in deep quantum regime. The saturation effect is
accompanied with an invariant distribution of matter-wave coherence under the
kicked rotor perturbations.Comment: 10 pages, 3 figures. Minor revision
Realization of Coherent Optically Dense Media via Buffer-Gas Cooling
We demonstrate that buffer-gas cooling combined with laser ablation can be
used to create coherent optical media with high optical depth and low Doppler
broadening that offers metastable states with low collisional and motional
decoherence. Demonstration of this generic technique opens pathways to coherent
optics with a large variety of atoms and molecules. We use helium buffer gas to
cool 87Rb atoms to below 7 K and slow atom diffusion to the walls.
Electromagnetically induced transparency (EIT) in this medium allows for 50%
transmission in a medium with initial OD >70 and for slow pulse propagation
with large delay-bandwidth products. In the high-OD regime, we observe
high-contrast spectrum oscillations due to efficient four-wave mixing.Comment: 4 pages, 4 figures. V2: modified title, abstract, introduction,
conclusion; added references; improved theoretical fit in figure 3(b);
shortened slow light theory description; clarified simplicity of apparatus.
Final version as published in Phys. Rev.
Long Phase Coherence Time and Number Squeezing of two Bose-Einstein Condensates on an Atom Chip
We measured the relative phase of two Bose-Einstein condensates confined in
an radio frequency induced double well potential on an atom chip. We observed
phase coherence between the separated condensates for times up to 200 ms after
splitting, a factor of 10 beyond the phase diffusion limit expected for a
coherent state in our experimental conditions (20 ms). The enhanced coherence
time is attributed to number squeezing of the initial state by a factor of 10.
In addition, we demonstrated a rotationally sensitive (Sagnac) geometry for a
guided atom interferometer by propagating the split condensates.Comment: 5 pages, 5 figure
The Energy Landscape, Folding Pathways and the Kinetics of a Knotted Protein
The folding pathway and rate coefficients of the folding of a knotted protein
are calculated for a potential energy function with minimal energetic
frustration. A kinetic transition network is constructed using the discrete
path sampling approach, and the resulting potential energy surface is
visualized by constructing disconnectivity graphs. Owing to topological
constraints, the low-lying portion of the landscape consists of three distinct
regions, corresponding to the native knotted state and to configurations where
either the N- or C-terminus is not yet folded into the knot. The fastest
folding pathways from denatured states exhibit early formation of the
N-terminus portion of the knot and a rate-determining step where the C-terminus
is incorporated. The low-lying minima with the N-terminus knotted and the
C-terminus free therefore constitute an off-pathway intermediate for this
model. The insertion of both the N- and C-termini into the knot occur late in
the folding process, creating large energy barriers that are the rate limiting
steps in the folding process. When compared to other protein folding proteins
of a similar length, this system folds over six orders of magnitude more
slowly.Comment: 19 page
Theory of an optical dipole trap for cold atoms
The theory of an atom dipole trap composed of a focused, far red-detuned, trapping laser beam, and a pair of red-detuned, counterpropagating, cooling beams is developed for the simplest realistic multilevel dipole interaction scheme based on a model of a (3+5)-level atom. The description of atomic motion in the trap is based on the quantum kinetic equations for the atomic density matrix and the reduced quasiclassical kinetic equation for atomic distribution function. It is shown that when the detuning of the trapping field is much larger than the detuning of the cooling field, and with low saturation, the one-photon absorption (emission) processes responsible for the trapping potential can be well separated from the two-photon processes responsible for sub-Doppler cooling atoms in the trap. Two conditions are derived that are necessary and sufficient for stable atomic trapping. The conditions show that stable atomic trapping in the optical dipole trap can be achieved when the trapping field has no effect on the two-photon cooling process and when the cooling field does not change the structure of the trapping potential but changes only the numerical value of the trapping potential well. It is concluded that the separation of the trapping and cooling processes in a pure optical dipole trap allows one to cool trapped atoms down to a minimum temperature close to the recoil temperature, keeping simultaneously a deep potential well
Quantum Optics and Photonics
Contains table of contents for Part II, table of contents for Section 1, and reports on six research projects.Charles S. Draper Laboratories Contract DL-H-418468U.S. Air Force - Electronic Systems Division Contract F19628-89-K-0300U.S. Navy - Office of Naval Research Grant N0014-91-J-1808U.S. Air Force - Electronic Systems Division Contract F19628-89-K-003
Atom focusing by far-detuned and resonant standing wave fields: Thin lens regime
The focusing of atoms interacting with both far-detuned and resonant standing
wave fields in the thin lens regime is considered. The thin lens approximation
is discussed quantitatively from a quantum perspective. Exact quantum
expressions for the Fourier components of the density (that include all
spherical aberration) are used to study the focusing numerically. The following
lens parameters and density profiles are calculated as functions of the pulsed
field area : the position of the focal plane, peak atomic density,
atomic density pattern at the focus, focal spot size, depth of focus, and
background density. The lens parameters are compared to asymptotic, analytical
results derived from a scalar diffraction theory for which spherical aberration
is small but non-negligible (). Within the diffraction theory
analytical expressions show that the focused atoms in the far detuned case have
an approximately constant background density
while the peak density behaves as , the focal distance or
time as , the focal spot size as
, and the depth of focus as .
Focusing by the resonant standing wave field leads to a new effect, a Rabi-
like oscillation of the atom density. For the far-detuned lens, chromatic
aberration is studied with the exact Fourier results. Similarly, the
degradation of the focus that results from angular divergence in beams or
thermal velocity distributions in traps is studied quantitatively with the
exact Fourier method and understood analytically using the asymptotic results.
Overall, we show that strong thin lens focusing is possible with modest laser
powers and with currently achievable atomic beam characteristics.Comment: 21 pages, 11 figure
Quantum Optics and Photonics
Contains table of contents for Part II, table of contents for Section 1 and reports on six research projects.U.S. Air Force - Electronic Systems Division Contract F19628-92-K-0013U.S. Navy - Office of Naval Research Grant N0014-91-J-1808Ballistic Missile Defense Organization Grant NG0921-94-C-0101MIT Plasma Fusion Cente
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