823,503 research outputs found
Low delta-V near-Earth asteroids: A survey of suitable targets for space missions
In the last decades Near-Earth Objects (NEOs) have become very important
targets to study, since they can give us clues to the formation, evolution and
composition of the Solar System. In addition, they may represent either a
threat to humankind, or a repository of extraterrestrial resources for suitable
space-borne missions. Within this framework, the choice of next-generation
mission targets and the characterisation of a potential threat to our planet
deserve special attention. To date, only a small part of the 11,000 discovered
NEOs have been physically characterised. From ground and space-based
observations one can determine some basic physical properties of these objects
using visible and infrared spectroscopy. We present data for 13 objects
observed with different telescopes around the world (NASA-IRTF, ESO-NTT, TNG)
in the 0.4 - 2.5 um spectral range, within the NEOSURFACE survey
(http://www.oa-roma.inaf.it/planet/NEOSurface.html). Objects are chosen from
among the more accessible for a rendez-vous mission. All of them are
characterised by a delta-V (the change in velocity needed for transferring a
spacecraft from low-Earth orbit to rendez-vous with NEOs) lower than 10.5 km/s,
well below the Solar System escape velocity (12.3 km/s). We taxonomically
classify 9 of these objects for the first time. 11 objects belong to the
S-complex taxonomy; the other 2 belong to the C-complex. We constrain the
surface composition of these objects by comparing their spectra with meteorites
from the RELAB database. We also compute olivine and pyroxene mineralogy for
asteroids with a clear evidence of pyroxene bands. Mineralogy confirms the
similarity with the already found H, L or LL ordinary chondrite analogues.Comment: 9 pages, 7 figures, to be published in A&A Minor changes by language
edito
Hierarchical Structure of Azbel-Hofstader Problem: Strings and loose ends of Bethe Ansatz
We present numerical evidence that solutions of the Bethe Ansatz equations
for a Bloch particle in an incommensurate magnetic field (Azbel-Hofstadter or
AH model), consist of complexes-"strings". String solutions are well-known from
integrable field theories. They become asymptotically exact in the
thermodynamic limit. The string solutions for the AH model are exact in the
incommensurate limit, where the flux through the unit cell is an irrational
number in units of the elementary flux quantum.
We introduce the notion of the integral spectral flow and conjecture a
hierarchical tree for the problem. The hierarchical tree describes the topology
of the singular continuous spectrum of the problem. We show that the string
content of a state is determined uniquely by the rate of the spectral flow
(Hall conductance) along the tree. We identify the Hall conductances with the
set of Takahashi-Suzuki numbers (the set of dimensions of the irreducible
representations of with definite parity).
In this paper we consider the approximation of noninteracting strings. It
provides the gap distribution function, the mean scaling dimension for the
bandwidths and gives a very good approximation for some wave functions which
even captures their multifractal properties. However, it misses the
multifractal character of the spectrum.Comment: revtex, 30 pages, 6 Figs, 8 postscript files are enclosed, important
references are adde
Spontaneous Generation of Photons in Transmission of Quantum Fields in PT Symmetric Optical Systems
We develop a rigorous mathematically consistent description of PT symmetric
optical systems by using second quantization. We demonstrate the possibility of
significant spontaneous generation of photons in PT symmetric systems. Further
we show the emergence of Hanbury-Brown Twiss (HBT) correlations in spontaneous
generation. We show that the spontaneous generation determines decisively the
nonclassical nature of fields in PT symmetric systems. Our work can be applied
to other systems like plasmonic structure where losses are compensated by gain
mechanisms.Comment: 4 pages, 5 figure
Nonlinear dynamics of sand banks and sand waves
Sand banks and sand waves are two types of sand structures that are commonly observed on an off-shore sea bed. We describe the formation of these features using the equations of the fluid motion coupled with the mass conservation law for the sediment transport. The bottom features are a result of an instability due to tide–bottom interactions. There are at least two mechanisms responsible for the growth of sand banks and sand waves. One is linear instability, and the other is nonlinear coupling between long sand banks and short sand waves. One novel feature of this work is the suggestion that the latter is more important for the generation of sand banks. We derive nonlinear amplitude equations governing the coupled dynamics of sand waves and sand banks. Based on these equations, we estimate characteristic features for sand banks and find that the estimates are consistent with measurements
Simple preparation of Bell and GHZ states using ultrastrong-coupling circuit QED
The ability to entangle quantum systems is crucial for many applications in
quantum technology, including quantum communication and quantum computing.
Here, we propose a new, simple, and versatile setup for deterministically
creating Bell and Greenberger-Horne-Zeilinger (GHZ) states between photons of
different frequencies in a two-step protocol. The setup consists of a quantum
bit (qubit) coupled ultrastrongly to three photonic resonator modes. The only
operations needed in our protocol are to put the qubit in a superposition
state, and then tune its frequency in and out of resonance with sums of the
resonator-mode frequencies. By choosing which frequency we tune the qubit to,
we select which entangled state we create. We show that our protocol can be
implemented with high fidelity using feasible experimental parameters in
state-of-the-art circuit quantum electrodynamics. One possible application of
our setup is as a node distributing entanglement in a quantum network.Comment: 15 pages, 7 figure
Spatially and Temporally Explicit Energy System Modelling to Support the Transition to a Low Carbon Energy Infrastructure – Case Study for Wind Energy in the UK
Renewable energy sources and electricity demand vary with time and space and the energy system is constrained by the location of the current infrastructure in place. The transitioning to a low carbon energy society can be facilitated by combining long term planning of infrastructure with taking spatial and temporal characteristics of the energy system into account. There is a lack of studies addressing this systemic view. We soft-link two models in order to analyse long term investment decisions in generation, transmission and storage capacities and the effects of short-term fluctuation of renewable supply: The national energy system model UKTM (UK TIMES model) and a dispatch model. The modelling approach combines the benefits of two models: an energy system model to analyse decarbonisation pathways and a power dispatch model that can evaluate the technical feasibility of those pathways and the impact of intermittent renewable energy sources on the power market. Results give us the technical feasibility of the UKTM solution from 2010 until 2050. This allows us to determine lower bounds of flexible elements and feeding them back in an iterative process (e.g. storage, demand side control, balancing). We apply the methodology to study the long-term investments of wind infrastructure in the United Kingdom
Second harmonic generation: Goursat problem on the semi-strip and explicit solutions
A rigorous and complete solution of the initial-boundary-value (Goursat)
problem for second harmonic generation (and its matrix analog) on the
semi-strip is given in terms of the Weyl functions. A wide class of the
explicit solutions and their Weyl functions is obtained also.Comment: 20 page
Growth of Sobolev norms for the quintic NLS on
We study the quintic Non Linear Schr\"odinger equation on a two dimensional
torus and exhibit orbits whose Sobolev norms grow with time. The main point is
to reduce to a sufficiently simple toy model, similar in many ways to the one
used in the case of the cubic NLS. This requires an accurate combinatorial
analysis.Comment: 41 pages, 5 figures. arXiv admin note: text overlap with
arXiv:0808.1742 by other author
Designing High-Fidelity Single-Shot Three-Qubit Gates: A Machine Learning Approach
Three-qubit quantum gates are key ingredients for quantum error correction
and quantum information processing. We generate quantum-control procedures to
design three types of three-qubit gates, namely Toffoli, Controlled-Not-Not and
Fredkin gates. The design procedures are applicable to a system comprising
three nearest-neighbor-coupled superconducting artificial atoms. For each
three-qubit gate, the numerical simulation of the proposed scheme achieves
99.9% fidelity, which is an accepted threshold fidelity for fault-tolerant
quantum computing. We test our procedure in the presence of decoherence-induced
noise as well as show its robustness against random external noise generated by
the control electronics. The three-qubit gates are designed via the machine
learning algorithm called Subspace-Selective Self-Adaptive Differential
Evolution (SuSSADE).Comment: 18 pages, 13 figures. Accepted for publication in Phys. Rev. Applie
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