525 research outputs found
The role of preparation in quantum process tomography
In a recent letter one of us pointed out how differences in preparation
procedures for quantum experiments can lead to non-trivial differences in the
results of the experiment. The difference arise from the initial correlations
between the system and environment. Therefore, any quantum experiment that is
prone to the influences from the environment must be prepared carefully. In
this paper, we study quantum process tomography in light of this. We suggest
several experimental setups, where preparation of initial state plays a role on
the final outcome of the experiment. We show that by studying the linearity and
the positivity of the resulting maps the experimenter can determine the nature
of the initial correlations between the system and the environment.Comment: 11 pages, 4 fig
Topological Signal Processing on Quantum Computers for Higher-Order Network Analysis
Predicting and analyzing global behaviour of complex systems is challenging
due to the intricate nature of their component interactions. Recent work has
started modelling complex systems using networks endowed with multiway
interactions among nodes, known as higher-order networks. In this context,
simplicial complexes are a class of higher-order networks that have received
significant attention due to their topological structure and connections to
Hodge theory. Topological signal processing utilizes these connections to
analyze and manipulate signals defined on non-Euclidean domains such as
simplicial complexes. In this work, we present a general quantum algorithm for
implementing filtering processes in TSP and describe its application to
extracting network data based on the Hodge decomposition. We leverage
pre-existing tools introduced in recent quantum algorithms for topological data
analysis and combine them with spectral filtering techniques using the quantum
singular value transformation framework. While this paper serves as a
proof-of-concept, we obtain a super-polynomial improvement over the best known
classical algorithms for TSP filtering processes, modulo some important caveats
about encoding and retrieving the data from a quantum state. The proposed
algorithm generalizes the applicability of tools from quantum topological data
analysis to novel applications in analyzing high-dimensional complex systems
Probing high pressure properties of single wall carbon nanotubes through fullerene encapsulation
The high pressure behavior of bundled 1.35±0.1nm diameter single wall carbon nanotubes (SWNT) filled with C70 fullerenes (usually called peapods) has been investigated by Raman spectroscopy and compared with the corresponding behavior of the nonfilled SWNT. We show experimentally that two reversible pressure-induced transitions take place in the compressed bundle SWNT. The first transition, in the 2–2.5GPa range, is in good correspondence with predictions of the thermodynamic instability of the nanotube circular cross section for the studied tube diameter. An interaction between the fullerenes and the tube walls is then observed at about 3.5GPa, which evidences a progressive deformation of the tube cross section. The second transition takes place at pressures between 10 and 30GPa, and is evidenced by two effects by a strong frequency downshift of the Raman transverse modes and the concomitant disappearance of the fullerenes Raman modes in peapods. The pressure at which the second transition takes place is strongly dependent on the nature of the pressure transmitting medium. We also report irreversible effects at high pressure as the shortening of the tubes, the formation of nanostructures and the disappearance of the C70 Raman signal in some cases. Transmission electron microscopy studies are also reported supporting these transformations
Compact Nuclei in Galaxies at Moderate Redshift:II. Their Nature and Implications for the AGN Luminosity Function
This study explores the space density and properties of active galaxies to
z=0.8. We have investigated the frequency and nature of unresolved nuclei in
galaxies at moderate redshift as indicators of nuclear activity such as Active
Galactic Nuclei (AGN) or starbursts. Candidates are selected by fitting imaged
galaxies with multi-component models using maximum likelihood estimate
techniques to determine the best model fit. We select those galaxies requiring
an unresolved point-source component in the galaxy nucleus, in addition to a
disk and/or bulge component, to adequately model the galaxy light. We have
searched 70 WFPC2 images primarily from the Medium Deep Survey for galaxies
containing compact nuclei. In our survey of 1033 galaxies, the fraction
containing an unresolved nuclear component greater than 5% of the total galaxy
light is 9+/-1% corrected for incompleteness. In this second of two papers in
this series, we discuss the nature of the compact nuclei and their hosts.
We present the upper limit luminosity function (LF) for low-luminosity AGN
(LLAGN) in two redshift bins to z=0.8. Mild number density evolution is
detected for nuclei at -18 -16
and this flatness, combined with the increase in number density, is
inconsistent with pure luminosity evolution. Based on the amount of density
evolution observed for these objects, we find that almost all present-day
spiral galaxies could have hosted a LLAGN at some point in their lives. We also
comment on the likely contribution of these compact nuclei to the soft X-ray
background.Comment: 50 pages, 14 figures, to appear in ApJ, April 199
Completely Positive Maps and Classical Correlations
We expand the set of initial states of a system and its environment that are
known to guarantee completely positive reduced dynamics for the system when the
combined state evolves unitarily. We characterize the correlations in the
initial state in terms of its quantum discord [H. Ollivier and W. H. Zurek,
Phys. Rev. Lett. 88, 017901 (2001)]. We prove that initial states that have
only classical correlations lead to completely positive reduced dynamics. The
induced maps can be not completely positive when quantum correlations
including, but not limited to, entanglement are present. We outline the
implications of our results to quantum process tomography experiments.Comment: 4 pages, 1 figur
Orthorhombic distortion on Li intercalation in anatase
Published versio
Characterization of TiO2 nanoparticles in langmuir-blodgett films
In this work we have synthesized TiO2 nanoparticles, using either a sol–gel base catalysed process in the interior of CTAB reversed micelles (TiO2 CTAB sol), or the
neutralization of a TiO2/H2SO4 solution in the interior of AOT reversed micelles. From the absorption and emission data of the TiO2 nanoparticles it is possible to conclude that
in the sol–gel route there remains alkoxide groups in the structure, originating transitions lower than the energy gap of TiO2 semiconductor. These transitions disappear in the neutralization procedure, where the alkoxide groups are absent in the structure. We have assigned the observed indirect
and direct optical transitions according to the anatase band structure. TiO2 Langmuir-Blodgett (LB) films were prepared either by direct deposition of titanium isopropoxide or by deposition of the TiO2 CTAB sol. These films showed photoluminescence, which was attributed to band-gap emission and to surface recombination of defect states
Operational Significance of Discord Consumption: Theory and Experiment
Coherent interactions that generate negligible entanglement can still exhibit
unique quantum behaviour. This observation has motivated a search beyond
entanglement for a complete description of all quantum correlations. Quantum
discord is a promising candidate. Here, we demonstrate that under certain
measurement constraints, discord between bipartite systems can be consumed to
encode information that can only be accessed by coherent quantum interactions.
The inability to access this information by any other means allows us to use
discord to directly quantify this `quantum advantage'. We experimentally encode
information within the discordant correlations of two separable Gaussian
states. The amount of extra information recovered by coherent interaction is
quantified and directly linked with the discord consumed during encoding. No
entanglement exists at any point of this experiment. Thus we introduce and
demonstrate an operational method to use discord as a physical resource.Comment: 10 pages, 3 figures, updated with Nature Physics Reference,
simplified proof in Appendi
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