4,078 research outputs found
Coherent Control of Stationary Light Pulses
We present a detailed analysis of the recently demonstrated technique to
generate quasi-stationary pulses of light [M. Bajcsy {\it et al.}, Nature
(London) \textbf{426}, 638 (2003)] based on electromagnetically induced
transparency. We show that the use of counter-propagating control fields to
retrieve a light pulse, previously stored in a collective atomic Raman
excitation, leads to quasi-stationary light field that undergoes a slow
diffusive spread. The underlying physics of this process is identified as pulse
matching of probe and control fields. We then show that spatially modulated
control-field amplitudes allow us to coherently manipulate and compress the
spatial shape of the stationary light pulse. These techniques can provide
valuable tools for quantum nonlinear optics and quantum information processing.Comment: 27 pages, 10 figure
Nonlinear optics with stationary pulses of light
We show that the recently demonstrated technique for generating stationary
pulses of light [Nature {\bf 426}, 638 (2003)] can be extended to localize
optical pulses in all three spatial dimensions in a resonant atomic medium.
This method can be used to dramatically enhance the nonlinear interaction
between weak optical pulses. In particular, we show that an efficient Kerr-like
interaction between two pulses can be implemented as a sequence of several
purely linear optical processes. The resulting process may enable coherent
interactions between single photon pulses.Comment: 4 pages, 2 figure
Manipulating Light Pulses via Dynamically Controlled Photonic Bandgap
When a resonance associated with electromagnetically induced transparency
(EIT) in an atomic ensemble is modulated by an off-resonant standing light
wave, a band of frequencies can appear for which light propagation is
forbidden. We show that dynamic control of such a bandgap can be used to
coherently convert a propagating light pulse into a stationary excitation with
non-vanishing photonic component. This can be accomplished with high efficiency
and negligble noise even at a level of few-photon quantum fields thereby
facilitating possible applications in quantum nonlinear optics and quantum
information.Comment: 4 pages, 3 figure
Shaping quantum pulses of light via coherent atomic memory
We describe a technique for generating pulses of light with controllable
photon numbers, propagation direction, timing, and pulse shapes. The technique
is based on preparation of an atomic ensemble in a state with a desired number
of atomic spin excitations, which is later converted into a photon pulse.
Spatio-temporal control over the pulses is obtained by exploiting long-lived
coherent memory for photon states and electromagnetically induced transparency
(EIT) in an optically dense atomic medium. Using photon counting experiments we
observe generation and shaping of few-photon sub-Poissonian light pulses. We
discuss prospects for controlled generation of high-purity n-photon Fock states
using this technique.Comment: 4 pages, 4 figure
Quantum Memory Process with a Four-Level Atomic Ensemble
We examine in detail the quantum memory technique for photons in a double
atomic ensemble in this work. The novel application of the present
technique to create two different quantum probe fields as well as entangled
states of them is proposed. A larger zero-degeneracy class besides dark-state
subspace is investigated and the adiabatic condition is confirmed in the
present model. We extend the single-mode quantum memory technique to the case
with multi-mode probe fields, and reveal the exact pulse matching phenomenon
between two quantized pulses in the present system.Comment: 7 pages, 1 figure, to appear in Euro. Phys. J.
Implementation of analytical Hartree-Fock gradients for periodic systems
We describe the implementation of analytical Hartree-Fock gradients for
periodic systems in the code CRYSTAL, emphasizing the technical aspects of this
task. The code is now capable of calculating analytical derivatives with
respect to nuclear coordinates for systems periodic in 0, 1, 2 and 3 dimensions
(i.e. molecules, polymers, slabs and solids). Both closed-shell restricted and
unrestricted Hartree-Fock gradients have been implemented. A comparison with
numerical derivatives shows that the forces are highly accurate.Comment: accepted by Comp. Phys. Com
Strong disorder renormalization group study of aperiodic quantum Ising chains
We employ an adaptation of a strong-disorder renormalization-group technique
in order to analyze the ferro-paramagnetic quantum phase transition of Ising
chains with aperiodic but deterministic couplings under the action of a
transverse field. In the presence of marginal or relevant geometric
fluctuations induced by aperiodicity, for which the critical behavior is
expected to depart from the Onsager universality class, we derive analytical
and asymptotically exact expressions for various critical exponents (including
the correlation-length and the magnetization exponents, which are not easily
obtainable by other methods), and shed light onto the nature of the ground
state structures in the neighborhood of the critical point. The main results
obtained by this approach are confirmed by finite-size scaling analyses of
numerical calculations based on the free-fermion method
Optical and IR Photometry of Globular Clusters in NGC1399: Evidence for Color-Metallicity Nonlinearity
We combine new Wide Field Camera~3 IR Channel (WFC3/IR) F160W (H) imaging
data for NGC1399, the central galaxy in the Fornax cluster, with archival F475W
(g), F606W (V), F814W (I), and F850LP (z) optical data from the Advanced Camera
for Surveys (ACS). The purely optical g-I, V-I, and g-z colors of NGC1399's
rich globular cluster (GC) system exhibit clear bimodality, at least for
magnitudes . The optical-IR I-H color distribution appears
unimodal, and this impression is confirmed by mixture modeling analysis. The
V-H colors show marginal evidence for bimodality, consistent with bimodality in
V-I and unimodality in I-H. If bimodality is imposed for I-H with a double
Gaussian model, the preferred blue/red split differs from that for optical
colors; these "differing bimodalities" mean that the optical and optical-IR
colors cannot both be linearly proportional to metallicity. Consistent with the
differing color distributions, the dependence of I-H on g-I for the matched GC
sample is significantly nonlinear, with an inflection point near the trough in
the g-I color distribution; the result is similar for the I-H dependence on g-z
colors taken from the ACS Fornax Cluster Survey. These g-z colors have been
calibrated empirically against metallicity; applying this calibration yields a
continuous, skewed, but single-peaked metallicity distribution. Taken together,
these results indicate that nonlinear color-metallicity relations play an
important role in shaping the observed bimodal distributions of optical colors
in extragalactic GC systems.Comment: 15 pages, 12 figures, accepted for publication in the Astrophysical
Journa
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