1,963 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
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
Self-energy and critical temperature of weakly interacting bosons
Using the exact renormalization group we calculate the momentum-dependent
self-energy Sigma (k) at zero frequency of weakly interacting bosons at the
critical temperature T_c of Bose-Einstein condensation in dimensions 3 <= D <
4. We obtain the complete crossover function interpolating between the critical
regime k << k_c, where Sigma (k) propto k^{2 - eta}, and the short-wavelength
regime k >> k_c, where Sigma (k) propto k^{2 (D-3)} in D> 3 and Sigma (k)
\propto ln (k/k_c) in D=3. Our approach yields the crossover scale k_c on the
same footing with a reasonable estimate for the critical exponent eta in D=3.
From our Sigma (k) we find for the interaction-induced shift of T_c in three
dimensions Delta T_c / T_c approx 1.23 a n^{1/3}, where a is the s-wave
scattering length and n is the density.Comment: 4 pages,1 figur
Maximizing Output and Recognizing Autocatalysis in Chemical Reaction Networks is NP-Complete
Background: A classical problem in metabolic design is to maximize the
production of desired compound in a given chemical reaction network by
appropriately directing the mass flow through the network. Computationally,
this problem is addressed as a linear optimization problem over the "flux
cone". The prior construction of the flux cone is computationally expensive and
no polynomial-time algorithms are known. Results: Here we show that the output
maximization problem in chemical reaction networks is NP-complete. This
statement remains true even if all reactions are monomolecular or bimolecular
and if only a single molecular species is used as influx. As a corollary we
show, furthermore, that the detection of autocatalytic species, i.e., types
that can only be produced from the influx material when they are present in the
initial reaction mixture, is an NP-complete computational problem. Conclusions:
Hardness results on combinatorial problems and optimization problems are
important to guide the development of computational tools for the analysis of
metabolic networks in particular and chemical reaction networks in general. Our
results indicate that efficient heuristics and approximate algorithms need to
be employed for the analysis of large chemical networks since even conceptually
simple flow problems are provably intractable
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