139,249 research outputs found
Three-dimensional theory for interaction between atomic ensembles and free-space light
Atomic ensembles have shown to be a promising candidate for implementations
of quantum information processing by many recently-discovered schemes. All
these schemes are based on the interaction between optical beams and atomic
ensembles. For description of these interactions, one assumed either a
cavity-QED model or a one-dimensional light propagation model, which is still
inadequate for a full prediction and understanding of most of the current
experimental efforts which are actually taken in the three-dimensional free
space. Here, we propose a perturbative theory to describe the three-dimensional
effects in interaction between atomic ensembles and free-space light with a
level configuration important for several applications. The calculations reveal
some significant effects which are not known before from the other approaches,
such as the inherent mode-mismatching noise and the optimal mode-matching
conditions. The three-dimensional theory confirms the collective enhancement of
the signal-to-noise ratio which is believed to be one of the main advantage of
the ensemble-based quantum information processing schemes, however, it also
shows that this enhancement need to be understood in a more subtle way with an
appropriate mode matching method.Comment: 16 pages, 9 figure
Phase-Matched Second-Harmonic Generation from Metasurfaces Inside Multipass Cells
We demonstrate a simple and scalable approach to increase conversion
efficiencies of nonlinear metasurfaces by incorporating them into multipass
cells and by letting the pump beam to interact with the metasurfaces multiple
times. We experimentally show that by metasurface design, the associated
phase-matching criteria can be fulfilled. As a proof of principle, we achieve
phase matching of second-harmonic generation (SHG) using a metasurface
consisting of aluminium nanoparticles deposited on a glass substrate. The
phase-matching condition is verified to be achieved by measuring superlinear
dependence of the detected SHG as a function of number of passes. We measure an
order of magnitude enhancement in the SHG signal when the incident pump
traverses the metasurface up to 9 passes. Results are found to agree well with
a simple model developed to estimate the generated SHG signals. We also discuss
strategies to further scale-up the nonlinear signal generation. Our approach
provides a clear pathway to enhance nonlinear optical responses of
metasurface-based devices. The generic nature of our approach holds promise for
diverse applications in nonlinear optics and photonics
Revealing modified gravity signal in matter and halo hierarchical clustering
We use a set of N-body simulations employing a modified gravity (MG) model
with Vainshtein screening to study matter and halo hierarchical clustering. As
test-case scenarios we consider two normal branch Dvali-Gabadadze-Porrati
(nDGP) gravity models with mild and strong growth rate enhancement. We study
higher-order correlation functions up to and associated
hierarchical amplitudes . We find that
the matter PDFs are strongly affected by the fifth-force on scales up to
Mpc, and the deviations from GR are maximised at . For reduced
cumulants , we find that at small scales Mpc the MG is
characterised by lower values, with the deviation growing from in the
reduced skewness up to even in . To study the halo clustering we
use a simple abundance matching and divide haloes into thee fixed number
density samples. The halo two-point functions are weakly affected, with a
relative boost of the order of a few percent appearing only at the smallest
pair separations (Mpc). In contrast, we find a strong MG signal
in 's, which are enhanced compared to GR. The strong model exhibits a
level signal at various scales for all halo samples and in all
cumulants. In this context, we find that the reduced kurtosis to be an
especially promising cosmological probe of MG. Even the mild nDGP model leaves
a imprint at small scales Mpc, while the stronger model
deviates from a GR-signature at nearly all scales with a significance of
. Since the signal is persistent in all halo samples and over a range
of scales, we advocate that the reduced kurtosis estimated from galaxy
catalogues can potentially constitute a strong MG-model discriminatory as well
as GR self-consistency test.Comment: 19 pages, 11 figures, comments are welcom
Purcell Enhancement of Parametric Luminescence: Bright and Broadband Nonlinear Light Emission in Metamaterials
Single-photon and correlated two-photon sources are important elements for
optical information systems. Nonlinear downconversion light sources are robust
and stable emitters of single photons and entangled photon pairs. However, the
rate of downconverted light emission, dictated by the properties of
low-symmetry nonlinear crystals, is typically very small, leading to
significant constrains in device design and integration. In this paper, we show
that the principles for spontaneous emission control (i.e. Purcell effect) of
isolated emitters in nanoscale structures, such as metamaterials, can be
generalized to describe the enhancement of nonlinear light generation processes
such as parametric down conversion. We develop a novel theoretical framework
for quantum nonlinear emission in a general anisotropic, dispersive and lossy
media. We further find that spontaneous parametric downconversion in media with
hyperbolic dispersion is broadband and phase-mismatch-free. We predict a
1000-fold enhancement of the downconverted emission rate with up to 105 photon
pairs per second in experimentally realistic nanostructures. Our theoretical
formalism and approach to Purcell enhancement of nonlinear optical processes,
provides a framework for description of quantum nonlinear optical phenomena in
complex nanophotonic structures.Comment: 29 pages, 10 figure
Binocular contrast discrimination needs monocular multiplicative noise.
The effects of signal and noise on contrast discrimination are difficult to separate because of a singularity in the signal-detection-theory model of two-alternative forced-choice contrast discrimination (Katkov, Tsodyks, & Sagi, 2006). In this article, we show that it is possible to eliminate the singularity by combining that model with a binocular combination model to fit monocular, dichoptic, and binocular contrast discrimination. We performed three experiments using identical stimuli to measure the perceived phase, perceived contrast, and contrast discrimination of a cyclopean sine wave. In the absence of a fixation point, we found a binocular advantage in contrast discrimination both at low contrasts (<4%), consistent with previous studies, and at high contrasts (≥34%), which has not been previously reported. However, control experiments showed no binocular advantage at high contrasts in the presence of a fixation point or for observers without accommodation. We evaluated two putative contrast-discrimination mechanisms: a nonlinear contrast transducer and multiplicative noise (MN). A binocular combination model (the DSKL model; Ding, Klein, & Levi, 2013b) was first fitted to both the perceived-phase and the perceived-contrast data sets, then combined with either the nonlinear contrast transducer or the MN mechanism to fit the contrast-discrimination data. We found that the best model combined the DSKL model with early MN. Model simulations showed that, after going through interocular suppression, the uncorrelated noise in the two eyes became anticorrelated, resulting in less binocular noise and therefore a binocular advantage in the discrimination task. Combining a nonlinear contrast transducer or MN with a binocular combination model (DSKL) provides a powerful method for evaluating the two putative contrast-discrimination mechanisms
Optimisation of Quantum Trajectories Driven by Strong-field Waveforms
Quasi-free field-driven electron trajectories are a key element of
strong-field dynamics. Upon recollision with the parent ion, the energy
transferred from the field to the electron may be released as attosecond
duration XUV emission in the process of high harmonic generation (HHG). The
conventional sinusoidal driver fields set limitations on the maximum value of
this energy transfer, and it has been predicted that this limit can be
significantly exceeded by an appropriately ramped-up cycleshape. Here, we
present an experimental realization of such cycle-shaped waveforms and
demonstrate control of the HHG process on the single-atom quantum level via
attosecond steering of the electron trajectories. With our optimized optical
cycles, we boost the field-ionization launching the electron trajectories,
increase the subsequent field-to-electron energy transfer, and reduce the
trajectory duration. We demonstrate, in realistic experimental conditions, two
orders of magnitude enhancement of the generated XUV flux together with an
increased spectral cutoff. This application, which is only one example of what
can be achieved with cycle-shaped high-field light-waves, has farreaching
implications for attosecond spectroscopy and molecular self-probing
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