296 research outputs found
Dark matter distributions around massive black holes: A general relativistic analysis
The cold dark matter at the center of a galaxy will be redistributed by the
presence of a massive black hole. The redistribution may be determined using an
approach pioneered by Gondolo and Silk: begin with a model distribution
function for the dark matter, and ``grow'' the black hole adiabatically,
holding the adiabatic invariants of the motion constant. Unlike the approach of
Gondolo and Silk, which adopted Newtonian theory together with ad hoc
correction factors to mimic general relativistic effects, we carry out the
calculation fully relativistically, using the exact Schwarzschild geometry of
the black hole. We find that the density of dark matter generically vanishes at
r=2R_S, not 4R_S as found by Gondolo and Silk, where R_S is the Schwarzschild
radius, and that the spike very close to the black hole reaches significantly
higher densities. We apply the relativistic adiabatic growth framework to
obtain the final dark matter density for both cored and cusped initial
distributions. Besides the implications of these results for indirect detection
estimates, we show that the gravitational effects of such a dark matter spike
are significantly smaller than the relativistic effects of the black hole,
including frame dragging and quadrupolar effects, for stars orbiting close to
the black hole that might be candidates for testing the black hole no-hair
theorems.Comment: 18 pages, 5 figures, submitted to Phys. Rev
Forgetfulness of continuous Markovian quantum channels
The notion of forgetfulness, used in discrete quantum memory channels, is
slightly weakened in order to be applied to the case of continuous channels.
This is done in the context of quantum memory channels with Markovian noise. As
a case study, we apply the notion of weak-forgetfulness to a bosonic memory
channel with additive noise. A suitable encoding and decoding unitary
transformation allows us to unravel the effects of the memory, hence the
channel capacities can be computed using known results from the memoryless
setting.Comment: 6 pages, 2 figures, comments are welcome. Minor corrections and
acknoledgment adde
Testing the black hole no-hair theorem at the galactic center: Perturbing effects of stars in the surrounding cluster
Observations of the precessing orbits of stars very near the massive black
hole in the galactic center could provide measurements of the spin and
quadrupole moment of the hole and thereby test the no-hair theorem of general
relativity. Since the galactic center is likely to be populated by a
distribution of stars and small black holes, their gravitational interactions
will perturb the orbit of any given star. We estimate the effects of such
perturbations using analytic orbital perturbation theory, and show that for a
range of possible stellar distributions, and for an observed star sufficiently
close to the black hole, the relativistic spin and quadrupole effects will be
larger than the effects of stellar cluster perturbations. Our results are
consistent those from recent numerical N-body simulations by Merritt et al.Comment: 17 pages, 2 figures, submitted to Classical and Quantum Gravit
New Phase Transitions in Optimal States for Memory Channels
We investigate the question of optimal input ensembles for memory channels
and construct a rather large class of Pauli channels with correlated noise
which can be studied analytically with regard to the entanglement of their
optimal input ensembles. In a more detailed study of a subclass of these
channels, the complete phase diagram of the two-qubit channel, which shows
three distinct phases is obtained. While increasing the correlation generally
changes the optimal state from separable to maximally entangled states, this is
done via an intermediate region where both separable and maximally entangled
states are optimal. A more concrete model, based on random rotations of the
error operators which mimic the behavior of this subclass of channels is also
presented.Comment: 13 pages, Late
Characterization of qutrit channels in terms of their covariance and symmetry properties
We characterize the completely positive trace-preserving maps on qutrits
(qutrit channels) according to their covariance and symmetry properties. Both
discrete and continuous groups are considered. It is shown how each symmetry
group restricts arbitrariness in the parameters of the channel to a very small
set. Although the explicit examples are related to qutrit channels, the
formalism is sufficiently general to be applied to qudit channels
Quantum information reclaiming after amplitude damping
We investigate the quantum information reclaim from the environment after
amplitude damping has occurred. In particular we address the question of
optimal measurement on the environment to perform the best possible correction
on two and three dimensional quantum systems. Depending on the dimension we
show that the entanglement fidelity (the measure quantifying the correction
performance) is or is not the same for all possible measurements and uncover
the optimal measurement leading to the maximum entanglement fidelity
Recovering quantum information through partial access to the environment
We investigate the possibility of correcting errors occurring on a
multipartite system through a feedback mechanism that acquires information from
partial access to the environment. A partial control scheme of this kind might
be useful when dealing with correlated errors. In fact, in such a case, it
could be enough to gather local information to decide what kind of global
recovery to perform. Then, we apply this scheme to the depolarizing and
correlated errors, and quantify its performance by means of the entanglement
fidelity
Solar light induced photocatalytic degradation of tetracycline in the presence of ZnO/NiFe2O4/Co3O4 as a new and highly efficient magnetically separable photocatalyst
In this study, a new solar light-driven magnetic heterogeneous photocatalyst, denoted as ZnO/NiFe2O4/Co3O4, is successfully prepared. FT-IR, XPS, XRD, VSM, DRS, FESEM, TEM, EDS, elemental mapping, and ICP analysis are accomplished for full characterization of this catalyst. FESEM and TEM analyses of the photocatalyt clearly affirm the formation of a hexagonal structure of ZnO (25–40 nm) and the cubic structure of NiFe2O4 and Co3O4 (10–25 nm). Furthermore, the HRTEM images of the photocatalyst verify some key lattice fringes related to the photocatalyt structure. These data are in very good agreement with XRD analysis results. According to the ICP analysis, the molar ratio of ZnO/NiFe2O4/Co3O4 composite is obtained to be 1:0.75:0.5. Moreover, magnetization measurements reveals that the ZnO/NiFe2O4/Co3O4 has a superparamagnetic behavior with saturation magnetization of 32.38 emu/g. UV-vis DRS analysis indicates that the photocatalyst has a boosted and strong light response. ZnO/NiFe2O4/Co3O4, with band gap energy of about 2.65 eV [estimated according to the Tauc plot of (αhν)2 vs. hν], exhibits strong potential towards the efficacious degradation of tetracycline (TC) by natural solar light. It is supposed that the synergistic optical effects between ZnO, NiFe2O4, and Co3O4 species is responsible for the increased photocatalytic performance of this photocatalyst under the optimal conditions (photocatalyst dosage = 0.02 g L−1, TC concentration = 30 mg L−1, pH = 9, irradiation time = 20 min, and TC degradation efficiency = 98%). The kinetic study of this degradation process is evaluated and it is well-matched with the pseudo-first-order kinetics. Based on the radical quenching tests, it can be perceived that •O2− species and holes are the major contributors in such a process, whereas the •OH radicals identify to have no major participation. The application of this methodology is implemented in a facile and low-cost photocatalytic approach to easily degrade TC by using a very low amount of the photocatalyst under natural sunlight source in an air atmosphere. The convenient magnetic isolation and reuse of the photocatalyst, and almost complete mineralization of TC (based on TOC analysis), are surveyed too, which further highlights the operational application of the current method. Notably, this method has the preferred performance among the very few methods reported for the photocatalytic degradation of TC under natural sunlight. It is assumed that the achievements of this photocatalytic method have opened an avenue for sustainable environmental remediation of a broad range of contaminants.The authors gratefully acknowledge the financial support of this study by University of Birjand Research Council. Thanks to the Spanish Ministerio de Economía, Industria y Competitividad, Agencia Estatal de Investigación (AEI) and Fondo Europeo de Desarrollo Regional (FEDER, EU) (project PID 2019-107268GB-I00) and to the University of Alicante
- …