447 research outputs found
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
A pedagogical introduction to continuously monitored quantum systems and measurement-based feedback
No full textIn this manuscript we present a pedagogical introduction to continuously monitored quantum systems. We
start by giving a simplified derivation of the Markovian master equation in Lindblad form, in the spirit
of collision models and input-output theory, which describes the unconditional dynamics of a continuously
monitored system. The same formalism is then exploited to derive stochastic master equations that describe the
conditional dynamics. We focus on the two most paradigmatic examples of continuous monitoring: continuous
photodetection, leading to a discontinuous dynamics with “quantum jumps”, and continuous homodyne
measurements, leading to a diffusive dynamics. We then present a derivation of feedback master equations that
describe the dynamics (either conditional or unconditional) when the continuous measurement photocurrents
are fed back to the system as a linear driving Hamiltonian, a paradigm known as linear Markovian feedback.
In the second part of the manuscript we focus on continuous-variable Gaussian systems: we first present the
equations for first and second moments describing the dynamics under continuous general-dyne measurements,
and we then discuss in more detail the conditional and unconditional dynamics under Markovian and state-based
feedback
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
Optimal quantum repeaters for qubits and qudits
No full textA class of optimal quantum repeaters for qubits is suggested. The schemes are minimal, i.e., they involve a single additional probe qubit, and optimal, i.e., they provide the maximum information adding the minimum amount of noise. Information gain and state disturbance are quantified by fidelities which, for our schemes, saturate the ultimate bound imposed by quantum mechanics for randomly distributed signals. Special classes of signals are also investigated, in order to improve the information-disturbance trade-off. Extension to higherdimensional signal
Optical Phase Estimation in the Presence of Phase Diffusion
No full textThe 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
Information-disturbance tradeoff in continuous variable Gaussian systems
No full textWe address the information–disturbance tradeoff for state measurements on continuous variable Gaussian systems and suggest minimal schemes for implementations. In our schemes, the symbols from a given alphabet are encoded in a set of Gaussian signals which are coupled to a probe excited in a known state. After the interaction the probe is measured, in order to infer the transmitted state, while the conditional state of the signal is left for the subsequent user. The schemes are minimal, i.e., involve a single additional probe, and allow for the nondemolitive transmission of a continuous real alphabet over a quantum channel. The tradeoff between information gain and state disturbance is quantified by fidelities and, after optimization with respect to the measurement, analyzed in terms of the energy carried by the signal and the probe. We found that transmission fidelity only depends on the energy of the signal and the probe, whereas estimation fidelity also depends on the alphabet size and the measurement gain. Increasing the probe energy does not necessarily lead to a better tradeoff, the most relevant parameter being the ratio between the alphabet size and the signal width, which in turn determine the allocation of the signal energy
Protein Dynamics and Drug Design: The Role of Molecular Simulations
No full textThe motions of proteins underlie all processes in cells, ranging from substrate transport to signal transmission, trafficking, formation of complexes and catalysis. Taking dynamics into account in molecular recognition may hold great promise in understanding the determinants of complex formation, in the identification of new binding sites and in the discovery of new drugs. Several groups have started tackling these problems with the use of simulation methods. The study of ligandinduced dynamic variations has also been exploited to review the concept of allosteric changes. The dynamics of proteins and complexes has also been used to develop pharmacophore models based on ensembles of protein conformations. These models, taking flexibility explicitly into account, are able to distinguish active inhibitors vs nonactive drug-like compounds, to define new molecular motifs and to preferentially identify specific ligands for a certain protein target. In this chapter, examples illustrating how simulations can be used to understand dynamics in relation to ligand binding and eventually to drug design will be presented. Finally, we will present two examples illustrating the utility of including dynamics in the design process of inhibitors against a well-defined protein receptor and against the formation of self-aggregated peptide oligomers
Towards the Extremely Large Telescopes era in exoplanetary science: simulation tools, instrumental optimization and design for high resolution spectroscopy. The cases of ESPRESSO and ELT-HIRES.
No full textIn this thesis I present my PhD activities concerning the development of simulation tools both for ground-based high resolution spectrograph design, ESPRESSO and ELT-HIRES, and for scientific investigations in the field of exoplanetary high-dispersion transmission spectroscopy. In the ESPRESSO (the Echelle SPectrograph for Rocky Exoplanets Super Stable Observations) project, the instrument modeling through ray-tracing bsoftware and ad-hoc developed sensivity analysis tool were fruitfully used for component optimization and alignment verification. In the framework of the ELT-HIRES (the HIgh REsolution Spectrograph for the ELT) project, an End-to-End simulator and a parametric paraxial model of the spectrograph were developed with the purpose to evaluate the different effects which can affect the final instrument performances, since they directly influence the scientific data observational performances. The synthetic echellogram (raw frame) produced and successfully processed by the CRIRES+ instrument data reduction pipeline are presented, thus probing the full chain feasibility and consistency of the system. Large telescopes allowing very high contrast, could also imply the possibility to detect the light emitted from background sources and/or from gravitationally bounded companion of exoplanetary system, which could act as signal contamination. A transmission spactra simulator, a tool aimed at generating synthetic spectra, was developed and used to show that the maximum contamination occurs for background stars of G-to-M type, even though for high resolution spectra with a 4m class telescope this contamination seems to not introduce obvious shifts or line broadening in the exoplanet atmospheric features
Non-Orthogonal Bases for Quantum Metrology
No full textMany quantum statistical models are most conveniently formulated in terms of non-orthonormal bases. This is the case, for example, when mixtures and superpositions of coherent states are involved. In these instances, we show that the analytical evaluation of the quantum Fisher information matrix may be greatly simplified by bypassing both the diagonalization of the density matrix and the orthogonalization of the basis. The key ingredient in our method is the Gramian matrix (i.e. the matrix of scalar products between basis elements), which may be interpreted as a metric tensor for index contraction. As an application, we derive novel analytical results for several estimation problems involving noisy Schrödinger cat states
Assessing Data Postprocessing for Quantum Estimation
No full textQuantum 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
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