6,618 research outputs found
Iterative procedure for computing accessible information in quantum communication
We present an iterative algorithm that finds the optimal measurement for
extracting the accessible information in any quantum communication scenario.
The maximization is achieved by a steepest-ascent approach toward the extremal
point, following the gradient uphill in sufficiently small steps. We apply it
to a simple ad-hoc example, as well as to a problem with a bearing on the
security of a tomographic protocol for quantum key distribution.Comment: REVTeX, 4 pages, 1 figure, 1 tabl
Inelastic cotunneling in quantum dots and molecules with weakly broken degeneracies
We calculate the nonlinear cotunneling conductance through interacting
quantum dot systems in the deep Coulomb blockade regime using a rate equation
approach based on the T-matrix formalism, which shows in the concerned regions
very good agreement with a generalized master equation approach. Our focus is
on inelastic cotunneling in systems with weakly broken degeneracies, such as
complex quantum dots or molecules. We find for these systems a characteristic
gate dependence of the non-equilibrium cotunneling conductance. While on one
side of a Coulomb diamond the conductance decreases after the inelastic
cotunneling threshold towards its saturation value, on the other side it
increases monotonously even after the threshold. We show that this behavior
originates from an asymmetric gate voltage dependence of the effective
cotunneling amplitudes.Comment: 12 pages, 12 figures; revised published versio
Diagnosis of venous incompetence inerectile dysfunction
In 26 of 214 patients with erectile dysfunction and proved venous incompetence by cavernosography, an additional bidirectional Doppler ultrasound was performed also to demonstrate venous outflow disturbances. All except one leakage in the superficial and deep dorsal veins could be demonstrated as well as 4 of 6 cavernosum-glandular shunts. Bidirectional Doppler ultrasound visualized a continuous retrograde blood flow from the sulcus coronarius to the root of the penis in superficial and deep dorsal penile veins as well as in ectopic penile veins, an orthograde blood flow in the sulcus coronarius in cavernosum-glandular shunt
Minimal qubit tomography
We present, and analyze thoroughly, a highly symmetric and efficient scheme
for the determination of a single-qubit state, such as the polarization
properties of photons emitted by a single-photon source. In our scheme there
are only four measured probabilities, just enough for the determination of the
three parameters that specify the qubit state, whereas the standard procedure
would measure six probabilities.Comment: 14 pages, 10 figures; final versio
The multiferroic phase of DyFeO:an ab--initio study
By performing accurate ab-initio density functional theory calculations, we
study the role of electrons in stabilizing the magnetic-field-induced
ferroelectric state of DyFeO. We confirm that the ferroelectric
polarization is driven by an exchange-strictive mechanism, working between
adjacent spin-polarized Fe and Dy layers, as suggested by Y. Tokunaga [Phys.
Rev. Lett, \textbf{101}, 097205 (2008)]. A careful electronic structure
analysis suggests that coupling between Dy and Fe spin sublattices is mediated
by Dy- and O- hybridization. Our results are robust with respect to the
different computational schemes used for and localized states, such as
the DFT+ method, the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional and the
GW approach. Our findings indicate that the interaction between the and
sublattice might be used to tailor ferroelectric and magnetic properties of
multiferroic compounds.Comment: 6 pages, 4 figures-Revised versio
Secure communication with a publicly known key
We present a scheme for direct and confidential communication between Alice and Bob, where there is no need for establishing a shared secret key first, and where the key used by Alice even will become known publicly. The communication is based on the exchange of single photons and each and every photon transmits one bit of Alice's message without revealing any information to a potential eavesdropper
Applying many-body expansion aiming to make ab in initio molecular dynamics more efficient
In this present work we present a methodology that aims to apply the many-body expansion to decrease the computational cost of ab initio molecular dynamics, keeping acceptable accuracy on the results. We implemented this methodology in a program which we called ManBo. In the many-body expansion approach, we partitioned the total energy E of the system in contributions of one body, two bodies, three bodies, etc., until the contribution of the Nth body [1-3]: E = E1 + E2 + E3 + …EN. The E1 term is the sum of the internal energy of the molecules; the term E2 is the energy due to interaction between all pairs of molecules; E3 is the energy due to interaction between all trios of molecules; and so on. In Manbo we chose to truncate the expansion in the contribution of two or three bodies, both for the calculation of the energy and for the calculation of the atomic forces. In order to partially include the many-body interactions neglected when we truncate the expansion, we can include an electrostatic embedding in the electronic structure calculations, instead of considering the monomers, pairs and trios as isolated molecules in space. In simulations we made we chose to simulate water molecules, and use the Gaussian 09 as external program to calculate the atomic forces and energy of the system, as well as reference program for analyzing the accuracy of the results obtained with the ManBo. The results show that the use of the many-body expansion seems to be an interesting approach for reducing the still prohibitive computational cost of ab initio molecular dynamics. The errors introduced on atomic forces in applying such methodology are very small. The inclusion of an embedding electrostatic seems to be a good solution for improving the results with only a small increase in simulation time. As we increase the level of calculation, the simulation time of ManBo tends to largely decrease in relation to a conventional BOMD simulation of Gaussian, due to better scalability of the methodology presented.\ud
References\ud
[1] E. E. Dahlke and D. G. Truhlar; J. Chem. Theory Comput., 3, 46 (2007).\ud
[2] E. E. Dahlke and D. G. Truhlar; J. Chem. Theory Comput., 4, 1 (2008).\ud
[3] R. Rivelino, P. Chaudhuri and S. Canuto; J. Chem. Phys., 118, 10593 (2003)
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