2,337 research outputs found
Inferring Chemical Reaction Patterns Using Rule Composition in Graph Grammars
Modeling molecules as undirected graphs and chemical reactions as graph
rewriting operations is a natural and convenient approach tom odeling
chemistry. Graph grammar rules are most naturally employed to model elementary
reactions like merging, splitting, and isomerisation of molecules. It is often
convenient, in particular in the analysis of larger systems, to summarize
several subsequent reactions into a single composite chemical reaction. We use
a generic approach for composing graph grammar rules to define a chemically
useful rule compositions. We iteratively apply these rule compositions to
elementary transformations in order to automatically infer complex
transformation patterns. This is useful for instance to understand the net
effect of complex catalytic cycles such as the Formose reaction. The
automatically inferred graph grammar rule is a generic representative that also
covers the overall reaction pattern of the Formose cycle, namely two carbonyl
groups that can react with a bound glycolaldehyde to a second glycolaldehyde.
Rule composition also can be used to study polymerization reactions as well as
more complicated iterative reaction schemes. Terpenes and the polyketides, for
instance, form two naturally occurring classes of compounds of utmost
pharmaceutical interest that can be understood as "generalized polymers"
consisting of five-carbon (isoprene) and two-carbon units, respectively
Generic Strategies for Chemical Space Exploration
Computational approaches to exploring "chemical universes", i.e., very large
sets, potentially infinite sets of compounds that can be constructed by a
prescribed collection of reaction mechanisms, in practice suffer from a
combinatorial explosion. It quickly becomes impossible to test, for all pairs
of compounds in a rapidly growing network, whether they can react with each
other. More sophisticated and efficient strategies are therefore required to
construct very large chemical reaction networks.
Undirected labeled graphs and graph rewriting are natural models of chemical
compounds and chemical reactions. Borrowing the idea of partial evaluation from
functional programming, we introduce partial applications of rewrite rules.
Binding substrate to rules increases the number of rules but drastically prunes
the substrate sets to which it might match, resulting in dramatically reduced
resource requirements. At the same time, exploration strategies can be guided,
e.g. based on restrictions on the product molecules to avoid the explicit
enumeration of very unlikely compounds. To this end we introduce here a generic
framework for the specification of exploration strategies in graph-rewriting
systems. Using key examples of complex chemical networks from sugar chemistry
and the realm of metabolic networks we demonstrate the feasibility of a
high-level strategy framework.
The ideas presented here can not only be used for a strategy-based chemical
space exploration that has close correspondence of experimental results, but
are much more general. In particular, the framework can be used to emulate
higher-level transformation models such as illustrated in a small puzzle game
Observation of spatial quantum correlations induced by multiple scattering of non-classical light
We present the experimental realization of spatial quantum correlations of
photons that are induced by multiple scattering of squeezed light. The quantum
correlation relates photons propagating along two different light trajectories
through the random medium and is infinite in range. Both positive and negative
spatial quantum correlations are observed when varying the quantum state
incident to the multiple scattering medium, and the magnitude of the
correlations is controlled by the number of photons. The experimental results
are in excellent agreement with recent theoretical proposals by implementing
the full quantum model of multiple scattering
Hybrid quantum information processing
The development of quantum information processing has traditionally followed
two separate and not immediately connected lines of study. The main line has
focused on the implementation of quantum bit (qubit) based protocols whereas
the other line has been devoted to implementations based on high-dimensional
Gaussian states (such as coherent and squeezed states). The separation has been
driven by the experimental difficulty in interconnecting the standard
technologies of the two lines. However, in recent years, there has been a
significant experimental progress in refining and connecting the technologies
of the two fields which has resulted in the development and experimental
realization of numerous new hybrid protocols. In this Review, we summarize
these recent efforts on hybridizing the two types of schemes based on discrete
and continuous variables.Comment: 13 pages, 6 figure
Raman-induced limits to efficient squeezing in optical fibers
We report new experiments on polarization squeezing using ultrashort photonic
pulses in a single pass of a birefringent fiber. We measure what is to our
knowledge a record squeezing of -6.8 +/- 0.3 dB in optical fibers which when
corrected for linear losses is -10.4 +/- 0.8 dB. The measured polarization
squeezing as a function of optical pulse energy, which spans a wide range from
3.5-178.8 pJ, shows a very good agreement with the quantum simulations and for
the first time we see the experimental proof that Raman effects limit and
reduce squeezing at high pulse energy.Comment: 3 pages, 3 figure
Continuous-wave spatial quantum correlations of light induced by multiple scattering
We present theoretical and experimental results on spatial quantum
correlations induced by multiple scattering of nonclassical light. A continuous
mode quantum theory is derived that enables determining the spatial quantum
correlation function from the fluctuations of the total transmittance and
reflectance. Utilizing frequency-resolved quantum noise measurements, we
observe that the strength of the spatial quantum correlation function can be
controlled by changing the quantum state of an incident bright squeezed-light
source. Our results are found to be in excellent agreement with the developed
theory and form a basis for future research on, e.g., quantum interference of
multiple quantum states in a multiple scattering medium.Comment: 8 pages, 6 figure
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