7,873 research outputs found
Divergence of the axial current and fermion density in Gross-Neveu models
The divergence of the axial current is used to relate the spatial derivative
of the fermion density to the bare fermion mass and scalar/pseudoscalar
condensates in 1+1 dimensional Gross-Neveu models. This serves as a novel test
of known results, to explain simple features of the continuous chiral model and
to resolve a conflict concerning the assignment of baryon number to certain
multi-fermion bound states.Comment: 7 pages, no figure; v2: minor changes, reference adde
On the anomalously large extension of the Pulsar Wind Nebula HESS J1825-137
The very high energy (VHE) gamma-ray emission reported from a number of
pulsar wind nebulae (PWNe) is naturally explained by the inverse Compton
scattering of multi-TeV electrons. However, the physical dimensions of some
gamma-ray-emitting PWNe significantly exceed the scales anticipated by the
standard hydrodynamical paradigm of PWN formation. The most "disturbing" case
in this regard is HESS J1825-137, which extends to distances
from the central pulsar PSR J1826-1334. If the gamma-ray
emission is indeed produced inside the PWN, but not by electrons that escaped
the nebula and diffuse in the interstellar medium (ISM), the formation of such
an anomalously extended plerion could be realized, in a diluted environment
with the hydrogen number density . In this
paper, we explore an alternative scenario assuming that the pulsar responsible
for the formation of the nebula initially had a very short rotation period. In
this case, the sizes of both the PWN and the surrounding supernova remnant
depend on the initial pulsar period, the braking index, and the ISM density. To
check the feasibility of this scenario, we study the parameter space that would
reproduce the size of HESS J1825-137. We show that this demand can be achieved
if the braking index is small, and the pulsar birth period is short,
. This scenario can reproduce the wind termination
position, which is expected at , only in a dense
environment with . The requirement of the dense
surrounding gas is supported by the presence of molecular clouds found in the
source vicinity.Comment: 15 pages, 6 figures, ApJ accepte
Transverse Entanglement of Biphotons
We measure the transverse entanglement of photon pairs on their propagation
from the near to the far field of spontaneous parametric downconversion (SPDC).
The Fedorov ratio, depending on the widths of conditional and unconditional
intensity measurements, is shown to be only able to characterize entanglement
in the near and far field zones of the source. Therefore we also follow a
different approach. By evaluating the first-order coherence of a subsystem of
the state we can quantify its entanglement. Unlike previous measurements, which
determine the Fedorov ratio via intensity correlations, our setup is sensitive
to both phase and modulus of the biphoton state and thus always grants
experimental access to the full transverse entanglement of the SPDC state. It
is shown theoretically that this scheme represents a direct measurement of the
Schmidt number.Comment: 11 pages, 6 figure
Gamma-ray flares from red giant/jet interactions in AGN
Non-blazar AGN have been recently established as a class of gamma-ray
sources. M87, a nearby representative of this class, show fast TeV variability
on timescales of a few days. We suggest a scenario of flare gamma-ray emission
in non-blazar AGN based on a red giant interacting with the jet at the base. We
solve the hydrodynamical equations that describe the evolution of the envelope
of a red giant blown by the impact of the jet. If the red giant is at least
slightly tidally disrupted by the supermassive black hole, enough stellar
material will be blown by the jet, expanding quickly until a significant part
of the jet is shocked. This process can render suitable conditions for energy
dissipation and proton acceleration, which could explain the detected day-scale
TeV flares from M87 via proton-proton collisions. Since the produced radiation
would be unbeamed, such an events should be mostly detected from non-blazar
AGN. They may be frequent phenomena, detectable in the GeV-TeV range even up to
distances of Gpc for the most powerful jets. The counterparts at lower
energies are expected to be not too bright.} {M87, and nearby non-blazar AGN in
general, can be fast variable sources of gamma-rays through red giant/jet
interactions.Comment: 8 pages, 4 figure
Integrating out the Dirac sea: Effective field theory approach to exactly solvable four-fermion models
We use 1+1 dimensional large N Gross-Neveu models as a laboratory to derive
microscopically effective Lagrangians for positive energy fermions only. When
applied to baryons, the Euler-Lagrange equation for these effective theories
assumes the form of a non-linear Dirac equation. Its solution reproduces the
full semi-classical results including the Dirac sea to any desired accuracy.
Dynamical effects from the Dirac sea are encoded in higher order derivative
terms and multi-fermion interactions with perturbatively calculable, finite
coefficients. Characteristic differences between models with discrete and
continuous chiral symmetry are observed and clarified.Comment: 13 pages, 11 figures; v2: typos corrected (Eqs. 4 and 44
Autonomous absolute calibration of an ICCD camera in single-photon detection regime
Intensified charge coupled device (ICCD) cameras are widely used in various
applications such as microscopy, astronomy, spectroscopy. Often they are used
as single-photon detectors, with thresholding being an essential part of the
readout. In this paper, we measure the quantum efficiency of an ICCD camera in
the single-photon detection mode using the Klyshko absolute calibration
technique. The quantum efficiency is obtained as a function of the threshold
value and of the wavelength of the detected light. In addition, we study the
homogeneity of the photon sensitivity over the camera chip area. The experiment
is performed in the autonomous regime, without using any additional detectors.
We therefore demonstrate the self-calibration of an ICCD camera.Comment: 8 pages, 3 figure
Dephasing and leakage dynamics of noisy Majorana-based qubits: Topological versus Andreev
Topological quantum computation encodes quantum information nonlocally by nucleating non-Abelian anyons separated by distances L, typically spanning the qubit device size. This nonlocality renders topological qubits exponentially immune to dephasing from all sources of classical noise with operator support local on the scale of L. We perform detailed analytical and numerical analyses of a time-domain Ramsey-type protocol for noisy Majorana-based qubits that is designed to validate this coveted topological protection in near-term devices such as the so-called “tetron” design. By assessing dependence of dephasing times on tunable parameters, e.g., magnetic field, our proposed protocol can clearly distinguish a bona fide Majorana qubit from one constructed from semilocal Andreev bound states, which can otherwise closely mimic the true topological scenario in local probes. In addition, we analyze leakage of the qubit out of its low-energy manifold due to classical-noise-induced generation of quasiparticle excitations; leakage limits the qubit lifetime when the bulk gap collapses, and hence our protocol further reveals the onset of a topological phase transition. This experiment requires measurement of two nearby Majorana modes for both initialization and readout—achievable, for example, by tunnel coupling to a nearby quantum dot—but no further Majorana manipulations, and thus constitutes an enticing prebraiding experiment. Along the way, we address conceptual subtleties encountered when discussing dephasing and leakage in the context of Majorana qubits
- …