13,250 research outputs found
The Ecosystem Approach to Fisheries: Issues, Terminology, Principles, Institutional Foundations, Implementation and Outlook
Ecosystems are complex and dynamic natural units that produce goods and services beyond those of benefit to fisheries. Because fisheries have a direct impact on the ecosystem, which is also impacted by other human activities, they need to be managed in an ecosystem context. The meaning of the terms 'ecosystem management', 'ecosystem based management', 'ecosystem approach to fisheries'(EAF), etc., are still not universally defined and progressively evolving. The justification of EAF is evident in the characteristics of an exploited ecosystem and the impacts resulting from fisheries and other activities. The rich set of international agreements of relevance to EAF contains a large number of principles and conceptual objectives. Both provide a fundamental guidance and a significant challenge for the implementation of EAF. The available international instruments also provide the institutional foundations for EAF. The FAO Code of Conduct for Responsible Fisheries is particularly important in this respect and contains provisions for practically all aspects of the approach. One major difficulty in defining EAF lies precisely in turning the available concepts and principles into operational objectives from which an EAF management plan would more easily be developed. The paper discusses these together with the types of action needed to achieve them. Experience in EAF implementation is still limited but some issues are already apparent, e.g. in added complexity, insufficient capacity, slow implementation, need for a pragmatic approach, etc. It is argued, in conclusion, that the future of EAF and fisheries depends on the way in which the two fundamental concepts of fisheries management and ecosystem management, and their respective stakeholders, will join efforts or collide
Concentration for Trotter error
Quantum simulation is expected to be one of the key applications of future
quantum computers. Product formulas, or Trotterization, are the oldest and,
still today, an appealing method for quantum simulation. For an accurate
product formula approximation in the spectral norm, the state-of-the-art gate
complexity depends on the number of Hamiltonian terms and a certain 1-norm of
its local terms. This work studies the concentration aspects of Trotter error:
we prove that, typically, the Trotter error exhibits 2-norm (i.e., incoherent)
scaling; the current estimate with 1-norm (i.e., coherent) scaling is for the
worst cases. For k-local Hamiltonians and higher-order product formulas, we
obtain gate count estimates for input states drawn from a 1-design ensemble
(e.g., computational basis states). Our gate count depends on the number of
Hamiltonian terms but replaces the 1-norm quantity by its analog in 2-norm,
giving significant speedup for systems with large connectivity. Our results
generalize to Hamiltonians with Fermionic terms and when the input state is
drawn from a low-particle number subspace. Further, when the Hamiltonian itself
has Gaussian coefficients (e.g., the SYK models), we show the stronger result
that the 2-norm behavior persists even for the worst input state. Our main
technical tool is a family of simple but versatile inequalities from
non-commutative martingales called uniform smoothness. We use them to derive
Hypercontractivity, namely p-norm estimates for low-degree polynomials, which
implies concentration via Markov's inequality. In terms of optimality, we give
examples that simultaneously match our p-norm bounds and the spectral norm
bounds. Therefore, our improvement is due to asking a qualitatively different
question from the spectral norm bounds. Our results give evidence that product
formulas in practice may generically work much better than expected.Comment: 43 pages, 1 figur
Fast Thermalization from the Eigenstate Thermalization Hypothesis
The Eigenstate Thermalization Hypothesis (ETH) has played a major role in
explaining thermodynamic phenomena in closed quantum systems. However, no
connection has been known between ETH and the timescale of thermalization for
open system dynamics. This paper rigorously shows that ETH indeed implies fast
thermalization to the global Gibbs state. We show fast convergence for two
models of thermalization. In the first, the system is weakly coupled to a bath
of quasi-free Fermions that we routinely refresh. We derive a finite-time
version of Davies' generator, with explicit error bounds and resource
estimates, that describes the joint evolution. The second is Quantum Metropolis
Sampling, a quantum algorithm for preparing Gibbs states on a quantum computer.
In both cases, no guarantee for fast convergence was previously known for
non-commuting Hamiltonians, partly due to technical issues with a finite energy
resolution. The critical feature of ETH we exploit is that operators in the
energy basis can be modeled by independent random matrices in a near-diagonal
band. We show this gives quantum expander at nearby eigenstates of the
Hamiltonian. This then implies fast convergence to the global Gibbs state by
mapping the problem to a one-dimensional classical random walk on the energy
eigenstates. Our results explain finite-time thermalization in chaotic open
quantum systems and suggest an alternative formulation of ETH in terms of
quantum expanders, which we investigate numerically for small systems.Comment: 76 pages, 14 figures. Corrections in v2 for the system-bath joint
evolutio
Signature of superconducting states in cubic crystal without inversion symmetry
The effects of absence of inversion symmetry on superconducting states are
investigated theoretically. In particular we focus on the noncentrosymmetric
compounds which have the cubic symmetry like LiPtB. An appropriate
and isotropic spin-orbital interaction is added in the Hamiltonian and it acts
like a magnetic monopole in the momentum space. The consequent pairing
wavefunction has an additional triplet component in the pseudospin space, and a
Zeeman magnetic field can induce a collinear supercurrent
with a coefficient . The effects of anisotropy embedded in the cubic
symmetry and the nodal superconducting gap function on are also
considered. From the macroscopic perspectives, the pair of mutually induced
and magnetization can affect the distribution of magnetic
field in such noncentrosymmetric superconductors, which is studied through
solving the Maxwell equation in the Meissner geometry as well as the case of a
single vortex line. In both cases, magnetic fields perpendicular to the
external ones emerge as a signature of the broken symmetry.Comment: 16 pages in pre-print forma
BL Lacertae are probable sources of the observed ultra-high energy cosmic rays
We calculate angular correlation function between ultra-high energy cosmic
rays (UHECR) observed by Yakutsk and AGASA experiments, and most powerful BL
Lacertae objects. We find significant correlations which correspond to the
probability of statistical fluctuation less than , including penatly
for selecting the subset of brightest BL Lacs. We conclude that some of BL Lacs
are sources of the observed UHECR and present a list of most probable
candidates.Comment: Replaced with the version accepted for publication in JETP Let
Direct strain and elastic energy evaluation in rolled-up semiconductor tubes by x-ray micro-diffraction
We depict the use of x-ray diffraction as a tool to directly probe the strain
status in rolled-up semiconductor tubes. By employing continuum elasticity
theory and a simple model we are able to simulate quantitatively the strain
relaxation in perfect crystalline III-V semiconductor bi- and multilayers as
well as in rolled-up layers with dislocations. The reduction in the local
elastic energy is evaluated for each case. Limitations of the technique and
theoretical model are discussed in detail.Comment: 32 pages (single column), 9 figures, 39 reference
Magnetic field tuning of antiferromagnetic YbPt
We present measurements of the specific heat, magnetization, magnetocaloric
effect and magnetic neutron diffraction carried out on single crystals of
antiferromagnetic YbPt, where highly localized Yb moments order at
K in zero field. The antiferromagnetic order was suppressed to
by applying a field of 1.85 T in the plane.
Magnetocaloric effect measurements show that the antiferromagnetic phase
transition is always continuous for , although a pronounced step
in the magnetization is observed at the critical field in both neutron
diffraction and magnetization measurements. These steps sharpen with decreasing
temperature, but the related divergences in the magnetic susceptibility are cut
off at the lowest temperatures, where the phase line itself becomes vertical in
the field-temperature plane. As , the antiferromagnetic
transition is increasingly influenced by a quantum critical endpoint, where
ultimately vanishes in a first order phase transition.Comment: 9 pages, 6 figure
Radiometric force in dusty plasmas
A radiofrequency glow discharge plasma, which is polluted with a certain
number of dusty grains, is studied. In addition to various dusty plasma
phenomena, several specific colloidal effects should be considered. We focus on
radiometric forces, which are caused by inhomogeneous temperature distribution.
Aside from thermophoresis, the role of temperature distribution in dusty
plasmas is an open question. It is shown that inhomogeneous heating of the
grain by ion flows results in a new photophoresis like force, which is specific
for dusty discharges. This radiometric force can be observable under conditions
of recent microgravity experiments.Comment: 4 pages, amsmat
Matrix Product Density Operators: when do they have a local parent Hamiltonian?
We study whether one can write a Matrix Product Density Operator (MPDO) as
the Gibbs state of a quasi-local parent Hamiltonian. We conjecture this is the
case for generic MPDO and give supporting evidences. To investigate the
locality of the parent Hamiltonian, we take the approach of checking whether
the quantum conditional mutual information decays exponentially. The MPDO we
consider are constructed from a chain of 1-input/2-output (`Y-shaped')
completely-positive maps, i.e. the MPDO have a local purification. We derive an
upper bound on the conditional mutual information for bistochastic channels and
strictly positive channels, and show that it decays exponentially if the
correctable algebra of the channel is trivial. We also introduce a conjecture
on a quantum data processing inequality that implies the exponential decay of
the conditional mutual information for every Y-shaped channel with trivial
correctable algebra. We additionally investigate a close but nonequivalent
cousin: MPDO measured in a local basis. We provide sufficient conditions for
the exponential decay of the conditional mutual information of the measured
states, and numerically confirmed they are generically true for certain random
MPDO.Comment: Added Github code for Propostion III.6; added few names in
acknowledgement after discussion with them about DPI for CM
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