1,359 research outputs found
GRB observations by Fermi LAT revisited: new candidates found
We search the Fermi-LAT photon database for an extended gamma-ray emission
which could be associated with any of the 581 previously detected gamma-ray
bursts (GRBs) visible to the Fermi-LAT. For this purpose we compare the number
of photons with energies E > 100 MeV and E > 1 GeV which arrived in the first
1500 seconds after the burst from the same region, to the expected background.
We require that the expected number of false detections does not exceed 0.05
for the entire search and find the high-energy emission in 19 bursts, four of
which (GRB 081009, GRB 090720B, GRB 100911 and GRB 100728A) were previously
unreported. The first three are detected at energies above 100 MeV, while the
last one shows a statistically significant signal only above 1 GeV.Comment: Updated after referee comments, published in MNRAS Letters; 5 pages,
2 table
Analytical approximation for single-impurity Anderson model
We have applied the recently developed dual fermion technique to the spectral
properties of single-band Anderson impurity problem (SIAM). In our approach a
series expansion is constructed in vertices of the corresponding atomic
Hamiltonian problem. This expansion contains a small parameter in two limiting
cases: in the weak coupling case (), due to the smallness of the
irreducible vertices, and near the atomic limit (), when bare
propagators are small. Reasonable results are obtained also for the most
interesting case of strong correlations (). The atomic problem of
the Anderson impurity model has a degenerate ground state, so the application
of the perturbation theory is not straightforward. We construct a special
approach dealing with symmetry-broken ground state of the renormalized atomic
problem. Formulae for the first-order dual diagram correction are obtained
analytically in the real-time domain. Most of the Kondo-physics is reproduced:
logarithmic contributions to the self energy arise, Kondo-like peak at the
Fermi level appears, and the Friedel sum rule is fulfilled. Our approach
describes also renormalization of atomic resonances due to hybridization with a
conduction band. A generalization of the proposed scheme to a multi-orbital
case can be important for the realistic description of correlated solids.Comment: 6 pages, 5 figure
Classical modelling of a bosonic sampler with photon collisions
When the problem of boson sampling was first proposed, it was assumed that
little or no photon collisions occur. However, modern experimental realizations
rely on setups where collisions are quite common, i.e. the number of photons
injected into the circuit is close to the number of detectors . Here we
present a classical algorithm that simulates a bosonic sampler: it calculates
the probability of a given photon distribution at the interferometer outputs
for a given distribution at the inputs. This algorithm is most effective in
cases with multiple photon collisions, and in those cases it outperforms known
algorithms
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