783 research outputs found
Spatial encoding in primate hippocampus during free navigation.
The hippocampus comprises two neural signals-place cells and θ oscillations-that contribute to facets of spatial navigation. Although their complementary relationship has been well established in rodents, their respective contributions in the primate brain during free navigation remains unclear. Here, we recorded neural activity in the hippocampus of freely moving marmosets as they naturally explored a spatial environment to more explicitly investigate this issue. We report place cells in marmoset hippocampus during free navigation that exhibit remarkable parallels to analogous neurons in other mammalian species. Although θ oscillations were prevalent in the marmoset hippocampus, the patterns of activity were notably different than in other taxa. This local field potential oscillation occurred in short bouts (approximately .4 s)-rather than continuously-and was neither significantly modulated by locomotion nor consistently coupled to place-cell activity. These findings suggest that the relationship between place-cell activity and θ oscillations in primate hippocampus during free navigation differs substantially from rodents and paint an intriguing comparative picture regarding the neural basis of spatial navigation across mammals
Fidelity enhancement by logical qubit encoding
We demonstrate coherent control of two logical qubits encoded in a
decoherence free subspace (DFS) of four dipolar-coupled protons in an NMR
quantum information processor. A pseudo-pure fiducial state is created in the
DFS, and a unitary logical qubit entangling operator evolves the system to a
logical Bell state. The four-spin molecule is partially aligned by a liquid
crystal solvent, which introduces strong dipolar couplings among the spins.
Although the system Hamiltonian is never fully specified, we demonstrate high
fidelity control over the logical degrees of freedom. In fact, the DFS encoding
leads to higher fidelity control than is available in the full four-spin
Hilbert space.Comment: 10 pages, 2 figure
Scaling for Interfacial Tensions near Critical Endpoints
Parametric scaling representations are obtained and studied for the
asymptotic behavior of interfacial tensions in the \textit{full} neighborhood
of a fluid (or Ising-type) critical endpoint, i.e., as a function \textit{both}
of temperature \textit{and} of density/order parameter \textit{or} chemical
potential/ordering field. Accurate \textit{nonclassical critical exponents} and
reliable estimates for the \textit{universal amplitude ratios} are included
naturally on the basis of the ``extended de Gennes-Fisher'' local-functional
theory. Serious defects in previous scaling treatments are rectified and
complete wetting behavior is represented; however, quantitatively small, but
unphysical residual nonanalyticities on the wetting side of the critical
isotherm are smoothed out ``manually.'' Comparisons with the limited available
observations are presented elsewhere but the theory invites new, searching
experiments and simulations, e.g., for the vapor-liquid interfacial tension on
the two sides of the critical endpoint isotherm for which an amplitude ratio
is predicted.Comment: 42 pages, 6 figures, to appear in Physical Review
A Chandra study of the large-scale shock and cool filaments in Hydra A: Evidence for substantial gas dredge-up by the central outburst
We present the results of a Chandra study of the Hydra A galaxy cluster,
where a powerful AGN outburst created a large-scale cocoon shock. We
investigated possible azimuthal variations in shock strength and shape, finding
indications for a weak shock with a Mach number in the range ~1.2-1.3. We
measured the temperature change across the shock front. However, the detection
of a temperature rise in the regions immediately inside of the front is
complicated by the underlying temperature profile of the cluster atmosphere. We
measured the global temperature profile of the cluster up to 700 kpc, which
represents the farthest measurement obtained with Chandra for this cluster. A
"plateau" in the temperature profile in the range ~70-150 kpc indicates the
presence of cool gas, which is likely the result of uplift of material by the
AGN outburst. After masking the cool filaments visible in the hardness ratio
map, the plateau disappears and the temperature profile recovers a typical
shape with a peak around 190 kpc, just inside the shock front. However, it is
unlikely that such a temperature feature is produced by the shock as it is
consistent with the general shape of the temperature profiles observed for
relaxed galaxy clusters. We studied the spectral properties of the cool
filaments finding evidence that ~10^11 M_sun of low-entropy material has been
dredged up by the rising lobes from the central 30 kpc to the observed current
position of 75-150 kpc. The energy required to lift the cool gas is >~2.2 x
10^60 erg, which is comparable to the work required to inflate the cavities and
is ~25% of the total energy of the large-scale shock. Our results show that the
AGN feedback in Hydra A is acting not only by directly heating the gas, but
also by removing a substantial amount of potential fuel for the SMBH.Comment: 11 pages, 9 figures, accepted for publication in ApJ (version with
full resolution figures available at
http://www.bo.astro.it/~myriam/files/papers/gitti-hydra.pdf
Interfacial Tensions near Critical Endpoints: Experimental Checks of EdGF Theory
Predictions of the extended de Gennes-Fisher local-functional theory for the
universal scaling functions of interfacial tensions near critical endpoints are
compared with experimental data. Various observations of the binary mixture
isobutyric acid water are correlated to facilitate an analysis of the
experiments of Nagarajan, Webb and Widom who observed the vapor-liquid
interfacial tension as a function of {\it both} temperature and density.
Antonow's rule is confirmed and, with the aid of previously studied {\it
universal amplitude ratios}, the crucial analytic ``background'' contribution
to the surface tension near the endpoint is estimated. The residual singular
behavior thus uncovered is consistent with the theoretical scaling predictions
and confirms the expected lack of symmetry in . A searching test of
theory, however, demands more precise and extensive experiments; furthermore,
the analysis highlights, a previously noted but surprising, three-fold
discrepancy in the magnitude of the surface tension of isobutyric acid
water relative to other systems.Comment: 6 figure
Finite-size and Particle-number Effects in an Ultracold Fermi Gas at Unitarity
We investigate an ultracold Fermi gas at unitarity confined in a periodic box
using renormalization group (RG) techniques. Within this approach we
can quantitatively assess the long range bosonic order parameter fluctuations
which dominate finite-size effects. We determine the finite-size and
particle-number dependence of universal quantities, such as the Bertsch
parameter and the fermion gap. Moreover, we analyze how these universal
observables respond to the variation of an external pairing source. Our results
indicate that the Bertsch parameter saturates rather quickly to its value in
the thermodynamic limit as a function of increasing box size. On the other
hand, we observe that the fermion gap shows a significantly stronger dependence
on the box size, in particular for small values of the pairing source. Our
results may contribute to a better understanding of finite-size and
particle-number effects present in Monte-Carlo simulations of ultracold Fermi
gases.Comment: 13 pages, 7 figure
Casimir forces in binary liquid mixtures
If two ore more bodies are immersed in a critical fluid critical fluctuations
of the order parameter generate long ranged forces between these bodies. Due to
the underlying mechanism these forces are close analogues of the well known
Casimir forces in electromagnetism. For the special case of a binary liquid
mixture near its critical demixing transition confined to a simple parallel
plate geometry it is shown that the corresponding critical Casimir forces can
be of the same order of magnitude as the dispersion (van der Waals) forces
between the plates. In wetting experiments or by direct measurements with an
atomic force microscope the resulting modification of the usual dispersion
forces in the critical regime should therefore be easily detectable. Analytical
estimates for the Casimir amplitudes Delta in d=4-epsilon are compared with
corresponding Monte-Carlo results in d=3 and their quantitative effect on the
thickness of critical wetting layers and on force measurements is discussed.Comment: 34 pages LaTeX with revtex and epsf style, to appear in Phys. Rev.
Towards an Asymptotic-Safety Scenario for Chiral Yukawa Systems
We search for asymptotic safety in a Yukawa system with a chiral
symmetry, serving as a toy model for the
standard-model Higgs sector. Using the functional RG as a nonperturbative tool,
the leading-order derivative expansion exhibits admissible non-Ga\ssian
fixed-points for which arise from a conformal threshold
behavior induced by self-balanced boson-fermion fluctuations. If present in the
full theory, the fixed-point would solve the triviality problem. Moreover, as
one fixed point has only one relevant direction even with a reduced hierarchy
problem, the Higgs mass as well as the top mass are a prediction of the theory
in terms of the Higgs vacuum expectation value. In our toy model, the fixed
point is destabilized at higher order due to massless Goldstone and fermion
fluctuations, which are particular to our model and have no analogue in the
standard model.Comment: 16 pages, 8 figure
The Fermi GBM Gamma-Ray Burst Spectral Catalog: Four Years Of Data
In this catalog we present the updated set of spectral analyses of GRBs
detected by the Fermi Gamma-Ray Burst Monitor (GBM) during its first four years
of operation. It contains two types of spectra, time-integrated spectral fits
and spectral fits at the brightest time bin, from 943 triggered GRBs. Four
different spectral models were fitted to the data, resulting in a compendium of
more than 7500 spectra. The analysis was performed similarly, but not
identically to Goldstein et al. 2012. All 487 GRBs from the first two years
have been re-fitted using the same methodology as that of the 456 GRBs in years
three and four. We describe, in detail, our procedure and criteria for the
analysis, and present the results in the form of parameter distributions both
for the observer-frame and rest-frame quantities. The data files containing the
complete results are available from the High-Energy Astrophysics Science
Archive Research Center (HEASARC).Comment: Accepted for publication in ApJ
An Open-System Quantum Simulator with Trapped Ions
The control of quantum systems is of fundamental scientific interest and
promises powerful applications and technologies. Impressive progress has been
achieved in isolating the systems from the environment and coherently
controlling their dynamics, as demonstrated by the creation and manipulation of
entanglement in various physical systems. However, for open quantum systems,
engineering the dynamics of many particles by a controlled coupling to an
environment remains largely unexplored. Here we report the first realization of
a toolbox for simulating an open quantum system with up to five qubits. Using a
quantum computing architecture with trapped ions, we combine multi-qubit gates
with optical pumping to implement coherent operations and dissipative
processes. We illustrate this engineering by the dissipative preparation of
entangled states, the simulation of coherent many-body spin interactions and
the quantum non-demolition measurement of multi-qubit observables. By adding
controlled dissipation to coherent operations, this work offers novel prospects
for open-system quantum simulation and computation.Comment: Pre-review submission to Nature. For an updated and final version see
publication. Manuscript + Supplementary Informatio
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