36,482 research outputs found
Fundamental limitations in the purifications of tensor networks
We show a fundamental limitation in the description of quantum many-body
mixed states with tensor networks in purification form. Namely, we show that
there exist mixed states which can be represented as a translationally
invariant (TI) matrix product density operator (MPDO) valid for all system
sizes, but for which there does not exist a TI purification valid for all
system sizes. The proof is based on an undecidable problem and on the
uniqueness of canonical forms of matrix product states. The result also holds
for classical states.Comment: v1: 11 pages, 1 figure. v2: very minor changes. About to appear in
Journal of Mathematical Physic
The complete LQG propagator: I. Difficulties with the Barrett-Crane vertex
Some components of the graviton two-point function have been recently
computed in the context of loop quantum gravity, using the spinfoam
Barrett-Crane vertex. We complete the calculation of the remaining components.
We find that, under our assumptions, the Barrett-Crane vertex does not yield
the correct long distance limit. We argue that the problem is general and can
be traced to the intertwiner-independence of the Barrett-Crane vertex, and
therefore to the well-known mismatch between the Barrett-Crane formalism and
the standard canonical spin networks. In a companion paper we illustrate the
asymptotic behavior of a vertex amplitude that can correct this difficulty.Comment: 31 page
Path Integral Approach to Residual Gauge Fixing
In this paper we study the question of residual gauge fixing in the path
integral approach for a general class of axial-type gauges including the
light-cone gauge. We show that the two cases -- axial-type gauges and the
light-cone gauge -- lead to very different structures for the explicit forms of
the propagator. In the case of the axial-type gauges, fixing the residual
symmetry determines the propagator of the theory completely. On the other hand,
in the light-cone gauge there is still a prescription dependence even after
fixing the residual gauge symmetry, which is related to the existence of an
underlying global symmetry.Comment: revtex 13pages, slightly expanded discussion, version to be published
in Physical Review
The Case for Optically-Thick High Velocity Broad Line Region Gas in Active Galactic Nuclei
A combined analysis of the profiles of the main broad quasar emission lines
in both Hubble Space Telescope and optical spectra shows that while the
profiles of the strong UV lines are quite similar, there is frequently a strong
increase in the Ly-alpha/H-alpha ratio in the high-velocity gas. We show that
the suggestion that the high velocity gas is optically-thin presents many
problems. We show that the relative strengths of the high velocity wings arise
naturally in an optically-thick BLR component. An optically-thick model
successfully explains the equivalent widths of the lines, the Ly-alpha/H-alpha
ratios and flatter Balmer decrements in the line wings, the strengths of CIII]
and the lambda 1400 blend, and the strong variability of high-velocity,
high-ionization lines (especially HeII and HeI).Comment: 34 pages in AASTeX, including 10 pages of figures. Submitted to
Astrophysical Journa
Three dimensional loop quantum gravity: physical scalar product and spin foam models
In this paper, we address the problem of the dynamics in three dimensional
loop quantum gravity with zero cosmological constant. We construct a rigorous
definition of Rovelli's generalized projection operator from the kinematical
Hilbert space--corresponding to the quantization of the infinite dimensional
kinematical configuration space of the theory--to the physical Hilbert space.
In particular, we provide the definition of the physical scalar product which
can be represented in terms of a sum over (finite) spin-foam amplitudes.
Therefore, we establish a clear-cut connection between the canonical
quantization of three dimensional gravity and spin-foam models. We emphasize
two main properties of the result: first that no cut-off in the kinematical
degrees of freedom of the theory is introduced (in contrast to standard
`lattice' methods), and second that no ill-defined sum over spins (`bubble'
divergences) are present in the spin foam representation.Comment: Typos corrected, version appearing in Class. Quant. Gra
Models and experimental results from the wide aperture Nb-Ti magnets for the LHC upgrade
MQXC is a Nb-Ti quadrupole designed to meet the accelerator quality
requirements needed for the phase-1 LHC upgrade, now superseded by the high
luminosity upgrade foreseen in 2021. The 2-m-long model magnet was tested at
room temperature and 1.9 K. The technology developed for this magnet is
relevant for other magnets currently under development for the high-luminosity
upgrade, namely D1 (at KEK) and the large aperture twin quadrupole Q4 (at CEA).
In this paper we present MQXC test results, some of the specialized heat
extraction features, spot heaters, temperature sensor mounting and voltage tap
development for the special open cable insulation. We look at some problem
solving with noisy signals, give an overview of electrical testing, look at how
we calculate the coil resistance during at quench and show that the heaters are
not working We describe the quench signals and its timing, the development of
the quench heaters and give an explanation of an Excel quench calculation and
its comparison including the good agreement with the MQXC test results. We
propose an improvement to the magnet circuit design to reduce voltage to ground
values by factor 2. The program is then used to predict quench Hot-Spot and
Voltages values for the D1 dipole and the Q4 quadrupole.Comment: 8 pages, Contribution to WAMSDO 2013: Workshop on Accelerator Magnet,
Superconductor, Design and Optimization; 15 - 16 Jan 2013, CERN, Geneva,
Switzerlan
Calorons, instantons and constituent monopoles in SU(3) lattice gauge theory
We analyze the zero-modes of the Dirac operator in quenched SU(3) gauge
configurations at non-zero temperature and compare periodic and anti-periodic
temporal boundary conditions for the fermions. It is demonstrated that for the
different boundary conditions often the modes are localized at different
space-time points and have different sizes. Our observations are consistent
with patterns expected for Kraan - van Baal solutions of the classical
Yang-Mills equations. These solutions consist of constituent monopoles and the
zero-modes are localized on different constituents for different boundary
conditions. Our findings indicate that the excitations of the QCD vacuum are
more structured than simple instanton-like lumps.Comment: Remarks added. To appear in Phys. Rev.
The loop-quantum-gravity vertex-amplitude
Spinfoam theories are hoped to provide the dynamics of non-perturbative loop
quantum gravity. But a number of their features remain elusive. The best
studied one -the euclidean Barrett-Crane model- does not have the boundary
state space needed for this, and there are recent indications that,
consequently, it may fail to yield the correct low-energy -point functions.
These difficulties can be traced to the SO(4) -> SU(2) gauge fixing and the way
certain second class constraints are imposed, arguably incorrectly, strongly.
We present an alternative model, that can be derived as a bona fide
quantization of a Regge discretization of euclidean general relativity, and
where the constraints are imposed weakly. Its state space is a natural subspace
of the SO(4) spin-network space and matches the SO(3) hamiltonian spin network
space. The model provides a long sought SO(4)-covariant vertex amplitude for
loop quantum gravity.Comment: 6page
Motion in Quantum Gravity
We tackle the question of motion in Quantum Gravity: what does motion mean at
the Planck scale? Although we are still far from a complete answer we consider
here a toy model in which the problem can be formulated and resolved precisely.
The setting of the toy model is three dimensional Euclidean gravity. Before
studying the model in detail, we argue that Loop Quantum Gravity may provide a
very useful approach when discussing the question of motion in Quantum Gravity.Comment: 30 pages, to appear in the book "Mass and Motion in General
Relativity", proceedings of the C.N.R.S. School in Orleans, France, eds. L.
Blanchet, A. Spallicci and B. Whitin
- …