298 research outputs found
Dephasing of a Qubit due to Quantum and Classical Noise
The qubit (or a system of two quantum dots) has become a standard paradigm
for studying quantum information processes. Our focus is Decoherence due to
interaction of the qubit with its environment, leading to noise. We consider
quantum noise generated by a dissipative quantum bath. A detailed comparative
study with the results for a classical noise source such as generated by a
telegraph process, enables us to set limits on the applicability of this
process vis a vis its quantum counterpart, as well as lend handle on the
parameters that can be tuned for analyzing decoherence. Both Ohmic and
non-Ohmic dissipations are treated and appropriate limits are analyzed for
facilitating comparison with the telegraph process.Comment: 12 pages, 8 figure
Modal beam splitter:Determination of the transversal components of an electromagnetic light field
The transversal profile of beams can always be defined as a superposition of orthogonal fields, such as optical eigenmodes. Here, we describe a generic method to separate the individual components in a laser beam and map each mode onto its designated detector with low crosstalk. We demonstrate this with the decomposition into Laguerre-Gaussian beams and introduce a distribution over the integer numbers corresponding to the discrete orbital and radial momentum components of the light field. The method is based on determining an eigenmask filter transforming the incident optical eigenmodes to position eigenmodes enabling the detection of the state of the light field using single detectors while minimizing cross talk with respect to the set of filter masks considered.UK Engineering and Physical Sciences Research Council [EP/J01771X/1]This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]
The Coulomb phase shift revisited
We investigate the Coulomb phase shift, and derive and analyze new and more
precise analytical formulae. We consider next to leading order terms to the
Stirling approximation, and show that they are important at small values of the
angular momentum and other regimes. We employ the uniform approximation.
The use of our expressions in low energy scattering of charged particles is
discussed and some comparisons are made with other approximation methods.Comment: 13 pages, 5 figures, 1 tabl
Physics of Ultra-Peripheral Nuclear Collisions
Moving highly-charged ions carry strong electromagnetic fields that act as a
field of photons. In collisions at large impact parameters, hadronic
interactions are not possible, and the ions interact through photon-ion and
photon-photon collisions known as {\it ultra-peripheral collisions} (UPC).
Hadron colliders like the Relativistic Heavy Ion Collider (RHIC), the Tevatron
and the Large Hadron Collider (LHC) produce photonuclear and two-photon
interactions at luminosities and energies beyond that accessible elsewhere; the
LHC will reach a energy ten times that of the Hadron-Electron Ring
Accelerator (HERA). Reactions as diverse as the production of anti-hydrogen,
photoproduction of the , transmutation of lead into bismuth and
excitation of collective nuclear resonances have already been studied. At the
LHC, UPCs can study many types of `new physics.'Comment: 47 pages, to appear in Annual Review of Nuclear and Particle Scienc
Lepton Scattering off Few-Nucleon Systems at Medium and High Energies
The interpretation of recent Jlab experimental data on the exclusive process
A(e,e'p)B off few-nucleon systems are analyzed in terms of realistic nuclear
wave functions and Glauber multiple scattering theory, both in its original
form and within a generalized eikonal approximation. The relevance of the
exclusive process 4He(e,e'p)^3H for possible investigations of QCD effects is
illustrated.Comment: 6 pages, 3 figures. Plenary talk given by C. Ciofi degli Atti at the
XX European Conference "Few Body Problems in Physics", Pisa, Italy, September
2007. To appear in Few-Body System
Quantum Imaging with Incoherently Scattered Light from a Free-Electron Laser
The advent of accelerator-driven free-electron lasers (FEL) has opened new
avenues for high-resolution structure determination via diffraction methods
that go far beyond conventional x-ray crystallography methods. These techniques
rely on coherent scattering processes that require the maintenance of
first-order coherence of the radiation field throughout the imaging procedure.
Here we show that higher-order degrees of coherence, displayed in the intensity
correlations of incoherently scattered x-rays from an FEL, can be used to image
two-dimensional objects with a spatial resolution close to or even below the
Abbe limit. This constitutes a new approach towards structure determination
based on incoherent processes, including Compton scattering, fluorescence
emission or wavefront distortions, generally considered detrimental for imaging
applications. Our method is an extension of the landmark intensity correlation
measurements of Hanbury Brown and Twiss to higher than second-order paving the
way towards determination of structure and dynamics of matter in regimes where
coherent imaging methods have intrinsic limitations
Full quantum distribution of contrast in interference experiments between interacting one dimensional Bose liquids
We analyze interference experiments for a pair of independent one dimensional
condensates of interacting bosonic atoms at zero temperature. We show that the
distribution function of fringe amplitudes contains non-trivial information
about non-local correlations within individual condensates and can be
calculated explicitly using methods of conformal field theory. We point out
interesting relations between these distribution functions, the partition
function for a quantum impurity in a one-dimensional Luttinger liquid, and
transfer matrices of conformal field theories. We demonstrate the connection
between interference experiments in cold atoms and a variety of statistical
models ranging from stochastic growth models to two dimensional quantum
gravity. Such connection can be used to design a quantum simulator of unusual
two-dimensional models described by nonunitary conformal field theories with
negative central charges.Comment: 9 pages, 5 figures; Accepted for publication in Nature Physic
Photon statistics of a random laser
A general relationship is presented between the statistics of thermal
radiation from a random medium and its scattering matrix S. Familiar results
for black-body radiation are recovered in the limit S to 0. The mean photocount
is proportional to the trace of 1-SS^dagger, in accordance with Kirchhoff's law
relating emissivity and absorptivity. Higher moments of the photocount
distribution are related to traces of powers of 1-SS^dagger, a generalization
of Kirchhoff's law. The theory can be applied to a random amplifying medium (or
"random laser") below the laser threshold, by evaluating the Bose-Einstein
function at a negative temperature. Anomalously large fluctuations are
predicted in the photocount upon approaching the laser threshold, as a
consequence of overlapping cavity modes with a broad distribution of spectral
widths.Comment: 26 pages, including 9 figure
Observation of a red-blue detuning asymmetry in matter-wave superradiance
We report the first experimental observations of strong suppression of
matter-wave superradiance using blue-detuned pump light and demonstrate a
pump-laser detuning asymmetry in the collective atomic recoil motion. In
contrast to all previous theoretical frameworks, which predict that the process
should be symmetric with respect to the sign of the pump-laser detuning, we
find that for condensates the symmetry is broken. With high condensate
densities and red-detuned light, the familiar distinctive multi-order,
matter-wave scattering pattern is clearly visible, whereas with blue-detuned
light superradiance is strongly suppressed. In the limit of a dilute atomic
gas, however, symmetry is restored.Comment: Accepted by Phys. Rev. Let
Quantum nature of laser light
All compositions of a mixed-state density operator are equivalent for the
prediction of the probabilities of future outcomes of measurements. For
retrodiction, however, this is not the case. The retrodictive formalism of
quantum mechanics provides a criterion for deciding that some compositions are
fictional. Fictional compositions do not contain preparation device operators,
that is operators corresponding to states that could have been prepared. We
apply this to Molmer's controversial conjecture that optical coherences in
laser light are a fiction and find agreement with his conjecture. We generalise
Molmer's derivation of the interference between two lasers to avoid the use of
any fictional states. We also examine another possible method for
discriminating between conerent states and photon number states in laser light
and find that it does not work, with the equivalence for prediction saved by
entanglement
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