4,324 research outputs found
Momentum space metric, non-local operator, and topological insulators
Momentum space of a gapped quantum system is a metric space: it admits a
notion of distance reflecting properties of its quantum ground state. By using
this quantum metric, we investigate geometric properties of momentum space. In
particular, we introduce a non-local operator which represents distance square
in real space and show that this corresponds to the Laplacian in curved
momentum space, and also derive its path integral representation in momentum
space. The quantum metric itself measures the second cumulant of the position
operator in real space, much like the Berry gauge potential measures the first
cumulant or the electric polarization in real space. By using the non-local
operator and the metric, we study some aspects of topological phases such as
topological invariants, the cumulants and topological phase transitions. The
effect of interactions to the momentum space geometry is also discussed.Comment: 13 pages, 4 figure
VLT observations of the asymmetric Etched Hourglass Nebula, MyCn 18
Context. The mechanisms that form extreme bipolar planetary nebulae remain
unclear. Aims. The physical properties, structure, and dynamics of the bipolar
planetary nebula, MyCn 18, are investigated in detail with the aim of
understanding the shaping mechanism and evolutionary history of this object.
Methods. VLT infrared images, VLT ISAAC infrared spectra, and long-slit optical
Echelle spectra are used to investigate MyCn 18. Morpho-kinematic modelling was
used to firmly constrain the structure and kinematics of the source. A
timescale analysis was used to determine the kinematical age of the nebula and
its main components. Results. A spectroscopic study of MyCn 18's central and
offset region reveals the detailed make-up of its nebular composition.
Molecular hydrogen, atomic helium, and Bracket gamma emission are detected from
the central regions of MyCn 18. ISAAC spectra from a slit position along the
narrow waist of the nebula demonstrate that the ionised gas resides closer to
the centre of the nebula than the molecular emission. A kinematical age of the
nebula and its components were obtained by the P-V arrays and timescale
analysis. Conclusions. The structure and kinematics of MyCn 18 are better
understood using an interactive 3-D modelling tool called shape. A dimensional
and timescale analysis of MyCn 18's major components provides a possible
mechanism for the nebula's asymmetry. The putative central star is somewhat
offset from the geometric centre of the nebula, which is thought to be the
result of a binary system. We speculate that the engulfing and destruction of
an exoplanet during the AGB phase may have been a key event in shaping MyCn 18
and generating of its hypersonic knotty outflow.Comment: 15 pages, 3 tables, 13 figures. Accepted for publication by A&
Anisotropic magnetic diffuse scattering in an easy-plane type antiferromagnet ErNiGe
We report on neutron scattering studies of a rare earth intermetallic
compound ErNiGe. Polarized neutron scattering experiments revealed
that the magnetic ordered moment lies in ab-plane. Taking account of
a lack of the third higher harmonic reflection, ErNiGe is
considered to have a helical magnetic structure. The magnetic scattering
profiles along the - and the -directions are well
described by the sum of Gaussian and modified-Lorentzian terms, even far below
, indicating that short-range orders coexist with a
long-range order. Interestingly, the modified-Lorentzian-type diffuse
scattering is not present in the profiles along the -direction.
The anisotropy of the diffuse scattering suggests that the short-range-order
consists of one dimensional long-range helices along the c-axis.Comment: 4 pages, to be published in J. Phys.: Condens. Matter (HFM2008
Engineering the Cost Function of a Variational Quantum Algorithm for Implementation on Near-Term Devices
Variational hybrid quantum-classical algorithms are some of the most
promising workloads for near-term quantum computers without error correction.
The aim of these variational algorithms is to guide the quantum system to a
target state that minimizes a cost function, by varying certain parameters in a
quantum circuit. This paper proposes a new approach for engineering cost
functions to improve the performance of a certain class of these variational
algorithms on today's small qubit systems. We apply this approach to a
variational algorithm that generates thermofield double states of the
transverse field Ising model, which are relevant when studying phase
transitions in condensed matter systems. We discuss the benefits and drawbacks
of various cost functions, apply our new engineering approach, and show that it
yields good agreement across the full temperature range.Comment: 8 pages, 4 figure
Semi-superfluid strings in High Density QCD
We show that topological superfluid strings/vortices with flux tubes exist in
the color-flavor locked (CFL) phase of color superconductors. Using a
Ginzburg-Landau free energy we find the configurations of these strings. These
strings can form during the transition from the normal phase to the CFL phase
at the core of very dense stars. We discuss an interesting scenario for a
network of strings and its evolution at the core of dense stars.Comment: minor changes in the tex
Modeling and Analyzing Academic Researcher Behavior
. This paper suggests a theoretical framework for analyzing the mechanism of the behavior of academic researchers whose interests are tangled and vary widely in academic factors (the intrinsic satisfaction in conducting research, the improvement in individual research ability, etc.) or non-academic factors (career rewards, financial rewards, etc.). Furthermore, each researcher also has his/her different academic stances in their preferences about academic freedom and academic entrepreneurship. Understanding the behavior of academic researchers will contribute to nurture young researchers, to improve the standard of research and education as well as to boost collaboration in academia-industry. In particular, as open innovation is increasingly in need of the involvement of university researchers, to establish a successful approach to entice researchers into enterprises' research, companies must comprehend the behavior of university researchers who have multiple complex motivations. The paper explores academic researchers' behaviors through optimizing their utility functions, i.e. the satisfaction obtained by their research outputs. This paper characterizes these outputs as the results of researchers' 3C: Competence (the ability to implement the research), Commitment (the effort to do the research), and Contribution (finding meaning in the research). Most of the previous research utilized the empirical methods to study researcher's motivation. Without adopting economic theory into the analysis, the past literature could not offer a deeper understanding of researcher's behavior. Our contribution is important both conceptually and practically because it provides the first theoretical framework to study the mechanism of researcher's behavior
Flows along cometary tails in the Helix planetary nebula NGC 7293
Previous velocity images which reveal flows of ionized gas along the most
prominent cometary tail (from Knot 38) in the Helix planetary nebula are
compared with that taken at optical wavelengths with the Hubble Space Telescope
and with an image in the emission from molecular hydrogen. The flows from the
second most prominent tail from Knot 14 are also considered. The kinematics of
the tail from the more complex Knot 32, shown here for the first time, also
reveals an acceleration away from the central star. All of the tails are
explained as accelerating ionized flows of ablated material driven by the
previous, mildly supersonic, AGB wind from the central star. The longest tail
of ionized gas, even though formed by this mechanism in a very clumpy medium,
as revealed by the emission from molecular hydrogen, appears to be a coherent
outflowing feature.Comment: 8 pages, 4 figures, accepted for publication in MNRA
Development of the analog ASIC for multi-channel readout X-ray CCD camera
We report on the performance of an analog application-specific integrated
circuit (ASIC) developed aiming for the front-end electronics of the X-ray
CCDcamera system onboard the next X-ray astronomical satellite, ASTRO-H. It has
four identical channels that simultaneously process the CCD signals.
Distinctive capability of analog-to-digital conversion enables us to construct
a CCD camera body that outputs only digital signals. As the result of the
front-end electronics test, it works properly with low input noise of =<30 uV
at the pixel rate below 100 kHz. The power consumption is sufficiently low of
about 150 mW/chip. The input signal range of 720 mV covers the effective energy
range of the typical X-ray photon counting CCD (up to 20 keV). The integrated
non-linearity is 0.2% that is similar as those of the conventional CCDs in
orbit. We also performed a radiation tolerance test against the total ionizing
dose (TID) effect and the single event effect. The irradiation test using 60Co
and proton beam showed that the ASIC has the sufficient tolerance against TID
up to 200 krad, which absolutely exceeds the expected amount of dose during the
period of operating in a low-inclination low-earth orbit. The irradiation of Fe
ions with the fluence of 5.2x10^8 Ion/cm2 resulted in no single event latchup
(SEL), although there were some possible single event upsets. The threshold
against SEL is higher than 1.68 MeV cm^2/mg, which is sufficiently high enough
that the SEL event should not be one of major causes of instrument downtime in
orbit.Comment: 16 pages, 6 figure
Designing high-fidelity multi-qubit gates for semiconductor quantum dots through deep reinforcement learning
In this paper, we present a machine learning framework to design
high-fidelity multi-qubit gates for quantum processors based on quantum dots in
silicon, with qubits encoded in the spin of single electrons. In this hardware
architecture, the control landscape is vast and complex, so we use the deep
reinforcement learning method to design optimal control pulses to achieve high
fidelity multi-qubit gates. In our learning model, a simulator models the
physical system of quantum dots and performs the time evolution of the system,
and a deep neural network serves as the function approximator to learn the
control policy. We evolve the Hamiltonian in the full state-space of the
system, and enforce realistic constraints to ensure experimental feasibility
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