706 research outputs found
Non-Gaussianity and purity in finite dimension
No full textWe address truncated states of continuous variable systems and analyze their statistical
properties numerically by generating random states in finite-dimensional Hilbert spaces.
In particular, we focus to the distribution of purity and non-Gaussianity for dimension
up to d = 21. We found that both quantities are distributed around typical values
with variances that decrease for increasing dimension. Approximate formulas for typical
purity and non-Gaussianity as a function of the dimension are derived
Optical Phase Estimation in the Presence of Phase Diffusion
Get PDFThe measurement problem for the optical phase has been traditionally attacked for noiseless schemes or
in the presence of amplitude or detection noise. Here we address the estimation of phase in the presence of
phase diffusion and evaluate the ultimate quantum limits to precision for phase-shifted Gaussian states.
We look for the optimal detection scheme and derive approximate scaling laws for the quantum Fisher
information and the optimal squeezing fraction in terms of the total energy and the amount of noise. We
also find that homodyne detection is a nearly optimal detection scheme in the limit of very small and large
nois
Introduction of a weighting scheme for the X-ray restrained wavefunction approach: advantages and drawbacks
No full textIn a quite recent study [Genoni et al. (2017). IUCrJ, 4, 136-146], it was observed that the X-ray restrained wavefunction (XRW) approach allows a more efficient and larger capture of electron correlation effects on the electron density if high-angle reflections are not considered in the calculations. This is due to the occurrence of two concomitant effects when one uses theoretical X-ray diffraction data corresponding to a single-molecule electron density in a large unit cell: (i) the high-angle reflections are generally much more numerous than the low- and medium-angle ones, and (ii) they are already very well described at unrestrained level. Nevertheless, since high-angle data also contain important information that should not be disregarded, it is not advisable to neglect them completely. For this reason, based on the results of the previous investigation, this work introduces a weighting scheme for XRW calculations to up-weight the contribution of low- and medium-angle reflections, and, at the same time, to reasonably down-weight the importance of the high-angle data. The proposed strategy was tested through XRW computations with both theoretical and experimental structure-factor amplitudes. The tests have shown that the new weighting scheme works optimally if it is applied with theoretically generated X-ray diffraction data, while it is not advantageous when traditional experimental X-ray diffraction data (even of very high resolution) are employed. This also led to the conclusion that the use of a specific external parameter λJ for each resolution range might not be a suitable strategy to adopt in XRW calculations exploiting experimental X-ray data as restraints
Assessing Data Postprocessing for Quantum Estimation
Get PDFQuantum sensors are among the most promising quantum technologies, allowing to attain the ultimate precision limit for parameter estimation. In order to achieve this, it is required to fully control and optimize what constitutes the hardware part of the sensors, i.e. the preparation of the probe states and the correct choice of the measurements to be performed. However careful considerations must be drawn also for the software components: a strategy must be employed to find a so-called optimal estimator. Here we review the most common approaches used to find the optimal estimator both with unlimited and limited resources. Furthermore, we present an attempt at a more complete characterization of the estimator by means of higher-order moments of the probability distribution, showing that most information is already conveyed by the standard bounds
DENPOL : A new program to determine electron densities of polypeptides using extremely localized molecular orbitals
No full textA new method to compute high-quality electron densities of polypeptides is proposed. The method is based on the transferability properties of extremely localized molecular orbitals, which can be used to describe with great accuracy the different functional groups of a molecule. It is therefore possible to generate a database of such orbitals, each of them associated with specific amino acids or with the peptide bond. A new program, DENPOL, has been written in order to build up the electron density of a generic polypeptide using this database. Due to both the large number of orbitals required to describe a polypeptide and the non-orthogonal nature of these orbitals, a Divide & Conquer strategy has been used to assemble the final electron density. The application of this approach is particularly efficient thanks to the extreme localization of the orbitals. The comparison with the corresponding electron densities generated by the Hartree–Fock method, shows the accuracy of the proposed approach and indicates that the electron densities generated by DENPOL are very close to those generated by an ab initio approach
New Catalytic Methods for Carbon Nitrogen Double Bond Transformations
No full textOver the last years, the use of trichlorosilane as a reducing agent has attracted much attention; the employment of a metal-free methodology could address the cost and waste remediation issues associated with main group hydrides, as well as avoid the expense and potentially toxic nature of metal catalysts. To promote the reaction, the trichlorosilane needs to be activated by coordination with a Lewis base: in particular, the use of chiral Lewis bases offers the potential to control the absolute stereochemistry of the process.1
Recently we decided to extend this methodology to the enantioselective reduction of fluorinated ketoimines, due to the great interest that organofluorine chemistry has received in many fields, such as material and pharmaceutical sciences.2 In spite of the great activity that fluorine attracted lately, it continues to challenge the organic chemistry community, since the presence of fluorine functional groups profoundly modifies the physicochemical and biological properties. In particular, the stereocontrol at carbon center featuring a fluorinated motif is an highly challenging task. The use of trichlorosilane combines the advantages of an environmentally friendly technique and the avoidance of the problems linked to the stereoselective insertion of a fluorinated group, while retaining high levels of enantioselectivity.
During our studies we’ve synthesized a set of fluorinated aromatic ketimines, both aromatic and aliphatic. Their trichlorosilane mediated reduction, after a proper tuning of reaction and workup conditions, allowed us to isolate the corresponding amines with high chemical yield and very good enantioselectivity, up to 90% e.e. Some variously substitued aromatic substrates were also examined, showing a good tolerance for electrowithdrawing and electrodonating substituents on the aromatic ring.
References:
1. a) Guizzetti S., Benaglia M. Eur. J. Org. Chem. 2010, 5529–5541, b) Jones S., Warner C. J. A. Org. Biomol. Chem. 2012,
10, 2189–2200
2. Nie J., Guo H., Cahard D, Ma J. Chem. Rev. 2011, 111, 455–52
Analysis of spin-squeezing generation in cavity-coupled atomic ensembles with continuous measurements
Get PDFWe analyze the generation of spin-squeezed states via coupling of three-level atoms to an optical cavity and continuous quantum measurement of the transmitted cavity field in order to monitor the evolution of the atomic ensemble. Using analytical treatment and microscopic simulations of the dynamics, we show that one can achieve significant spin squeezing, favorably scaling with the number of atoms N. However, contrary to some previous literature, we clarify that it is not possible to obtain Heisenberg scaling without the continuous feedback that is proposed in optimal approaches. In fact, in the adiabatic cavity removal approximation and large N limit, we find the scaling behavior N - 2 / 3 for spin squeezing and N - 1 / 3 for the corresponding protocol duration. These results can be obtained only by considering the curvature of the Bloch sphere, since linearizing the collective spin operators tangentially to its equator yields inaccurate predictions. With full simulations, we characterize how spin-squeezing generation depends on the system parameters and departs from the bad cavity regime, by gradually mixing with cavity-filling dynamics until metrological advantage is lost. Finally, we discuss the relevance of this spin-squeezing protocol to state-of-the-art optical clocks
Quantifying non-Gaussianity for quantum information
Get PDFWe address the quantification of non-Gaussianity (nG) of states and operations in continuous-variable systems
and its use in quantum information. We start by illustrating in detail the properties and the relationships of two recently proposed measures of nG based on the Hilbert-Schmidt distance and the quantum relative entropy (QRE) between the state under examination and a reference Gaussian state. We then evaluate the non-Gaussianities of several families of non-Gaussian quantum states and show that the two measures have the same basic properties and also share the same qualitative behavior in most of the examples taken into account. However, we also show
that they introduce a different relation of order; that is, they are not strictly monotone to each other. We exploit the nG measures for states in order to introduce a measure of nG for quantum operations, to assess Gaussification and de-Gaussification protocols, and to investigate in detail the role played by nG in entanglement distillation protocols. Besides, we exploit the QRE-based nG measure to provide different insight on the extremality of
Gaussian states for some entropic quantities such as conditional entropy, mutual information, and the Holevo bound. We also deal with parameter estimation and present a theorem connecting the QRE nG to the quantum
Fisher information. Finally, since evaluation of the QRE nG measure requires the knowledge of the full density matrix, we derive some experimentally friendly lower bounds to nG for some classes of states and by considering the possibility of performing on the states only certain efficient or inefficient measurements
Computational Study of the Resistance Shown by the Subtype B/HIV-1 Protease to Currently Known Inhibitors
No full textHuman immunodeficiency virus type 1 protease (HIV-1 PR) is an essential
enzyme in the HIV-1 life cycle. As such, this protein represents a major
drug target in AIDS therapy, but emerging resistance to antiretroviral
inhibitor cocktails, caused by high viral mutation rates, represents a
significant challenge in AIDS treatment. Many mutations are not located
within the active site or binding pocket, nor they do significantly
modify the three-dimensional structural organization of the enzyme;
hence, the mechanism(s) by which they alter inhibitor affinity for the
protease remains uncertain. In this article, we present an all-atom
computational analysis of the dynamic residue-residue coordination
between the active site residues and the rest of the protein and of the
energetic properties of different HIV-1 PR complexes. We analyze both
the wildtype form and mutated forms that induce drug resistance, In
particular, the results show differences between the wild type and the
mutants in their mechanism of dynamic coordination, in the signal
propagation between the active site residues and the rest of the
protein, and in the energy networks responsible for the stabilization of
the bound inhibitor conformation. Finally, we propose a dynamic and
energetic explanation for HIV-1 protease drug resistance, and, through
this model, we identify a possible new site that could be helpful in the
design of a new family of HIV-1 PR allosteric inhibitors
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