3,607 research outputs found
Entanglement between an electron and a nuclear spin 1/2
We report on the preparation and detection of entangled states between an
electron spin 1/2 and a nuclear spin 1/2 in a molecular single crystal. These
were created by applying pulses at ESR (9.5 GHz) and NMR (28 MHz) frequencies.
Entanglement was detected by using a special entanglement detector sequence
based on a unitary back transformation including phase rotation.Comment: 4 pages, 3 figure
Amino acid substitution during functionally constrained divergent evolution of protein sequences
In aligning homologous protein sequences, it is generally assumed that amino acid substitutions subsequent in time occur independently of amino acid substitutions previous in time, i.e. that patterns of mulation are similar at low and high sequence divergence. This assumption is examined here and shown to be incorrect in an interesting way. Separate mutation matrices were constructed for aligned protein sequence pairs at divergences ranging from 5 to 100 PAM units (point accepted mutations per 100 aligned positions). From these, the corresponding log-odds (Day-hoff) matrices, normalized to 250 PAM units, were constructed. The matrices show that the genetic code influences accepted point mutations strongly at early stages of divergence, while the chemical properties of the side chains dominate at more advanced stage
Geometry of the 3-Qubit State, Entanglement and Division Algebras
We present a generalization to 3-qubits of the standard Bloch sphere
representation for a single qubit and of the 7-dimensional sphere
representation for 2 qubits presented in Mosseri {\it et
al.}\cite{Mosseri2001}. The Hilbert space of the 3-qubit system is the
15-dimensional sphere , which allows for a natural (last) Hopf
fibration with as base and as fiber. A striking feature is, as in
the case of 1 and 2 qubits, that the map is entanglement sensitive, and the two
distinct ways of un-entangling 3 qubits are naturally related to the Hopf map.
We define a quantity that measures the degree of entanglement of the 3-qubit
state. Conjectures on the possibility to generalize the construction for higher
qubit states are also discussed.Comment: 12 pages, 2 figures, final versio
Observing Long Cosmic Strings Through Gravitational Lensing
We consider the gravitational lensing produced by long cosmic strings formed
in a GUT scale phase transition. We derive a formula for the deflection of
photons which pass near the strings that reduces to an integral over the light
cone projection of the string configuration plus constant terms which are not
important for lensing. Our strings are produced by performing numerical
simulations of cosmic string networks in flat, Minkowski space ignoring the
effects of cosmological expansion. These strings have more small scale
structure than those from an expanding universe simulation - fractal dimension
1.3 for Minkowski versus 1.1 for expanding - but share the same qualitative
features. Lensing simulations show that for both point-like and extended
objects, strings produce patterns unlike more traditional lenses, and, in
particluar, the kinks in strings tend to generate demagnified images which
reside close to the string. Thus lensing acts as a probe of the small scale
structure of a string. Estimates of lensing probablity suggest that for string
energy densities consistant with string seeded structure formation, on the
order of tens of string lenses should be observed in the Sloan Digital Sky
Survey quasar catalog. We propose a search strategy in which string lenses
would be identified in the SDSS quasar survey, and the string nature of the
lens can be confirmed by the observation of nearby high redshift galaxies which
are also be lensed by the string.Comment: 24 pages revtex with 12 postscript firgure
Synthesis and evaluation of influenza A viral neuraminidase candidate inhibitors based on a bicyclo[3.1.0]hexane scaffold
We describe the synthesis of constrained oseltamivir analogues designed to mimic the proposed boat conformation of the enzymatic transition state
Gravitational Lensing Signature of Long Cosmic Strings
The gravitational lensing by long, wiggly cosmic strings is shown to produce
a large number of lensed images of a background source. In addition to pairs of
images on either side of the string, a number of small images outline the
string due to small-scale structure on the string. This image pattern could
provide a highly distinctive signature of cosmic strings. Since the optical
depth for multiple imaging of distant quasar sources by long strings may be
comparable to that by galaxies, these image patterns should be clearly
observable in the next generation of redshift surveys such as the Sloan Digital
Sky Survey.Comment: 4 pages, revtex with 3 postscript figures include
Entanglement of electrons in interacting molecules
Quantum entanglement is a concept commonly used with reference to the
existence of certain correlations in quantum systems that have no classical
interpretation. It is a useful resource to enhance the mutual information of
memory channels or to accelerate some quantum processes as, for example, the
factorization in Shor's Algorithm. Moreover, entanglement is a physical
observable directly measured by the von Neumann entropy of the system. We have
used this concept in order to give a physical meaning to the electron
correlation energy in systems of interacting electrons. The electronic
correlation is not directly observable, since it is defined as the difference
between the exact ground state energy of the many--electrons Schroedinger
equation and the Hartree--Fock energy. We have calculated the correlation
energy and compared with the entanglement, as functions of the nucleus--nucleus
separation using, for the hydrogen molecule, the Configuration Interaction
method. Then, in the same spirit, we have analyzed a dimer of ethylene, which
represents the simplest organic conjugate system, changing the relative
orientation and distance of the molecules, in order to obtain the configuration
corresponding to maximum entanglement.Comment: 15 pages, 7 figures, standard late
Observation of a Tricyclic[4.1.0.0 2,4]heptane During a Michael Addition-Ring Closure Reaction and a Computational Study on Its Mechanism of Formation
We describe the formation of a bis-cyclopropane product, a tricyclic[4.1.0.02,4]heptane, that is formed during a Johnson-Corey-Chaykovsky reaction on a cyclopentenone. Two (of four possible) bicyclic products are selectively formed by addition of a COOEt-stabilized sulfur ylide onto the Michael acceptor. The tricyclic product is formed subsequently via a retro Michael elimination of a hindered ether followed by addition of a further cyclopropyl moiety, affecting only one of the two bicyclic products initially formed. The experimental reaction outcome was rationalized using density functional theory (DFT), investigating the different Michael-addition approaches of the sulfur ylide, the transition state (TS) energies for the formation of possible zwitterionic intermediates and subsequent reactions that give rise to cyclopropanation. Selective formation of only two of the four possible products occurs due to the epimerization of unreactive intermediates from the other two pathways, as revealed by energy barrier calculations. The formation of the tricyclic product was rationalized by evaluation of energy barriers for proton abstraction required to form the intermediate undergoing the second cyclopropanation. The selectivity-guiding factors discussed for the single and double cyclopropanation of this functionalized Michael-acceptor will be useful guidelines for the synthesis of future singly and doubly cyclopropanated compounds
Entanglement and the SU(2) phase states in atomic systems
We show that a system of 2n identical two-level atoms interacting with n
cavity photons manifests entanglement and that the set of entangled states
coincides with the so-called SU(2) phase states. In particular, violation of
classical realism in terms of the GHZ and GHSH conditions is proved. We discuss
a new property of entanglement expressed in terms of local measurements. We
also show that generation of entangled states in the atom-photon systems under
consideration strongly depends on the choice of initial conditions and that the
parasitic influence of cavity detuning can be compensated through the use of
Kerr medium.Comment: 10 pages, 1 figur
Separable approximation for mixed states of composite quantum systems
We describe a purely algebraic method for finding the best separable
approximation to a mixed state of a composite 2x2 quantum system, consisting of
a decomposition of the state into a linear combination of a mixed separable
part and a pure entangled one. We prove that, in a generic case, the weight of
the pure part in the decomposition equals the concurrence of the state.Comment: 13 pages, no figures; minor changes; accepted for publication in PR
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