1,985 research outputs found
OH+ in Diffuse Molecular Clouds
Near ultraviolet observations of OH+ and OH in diffuse molecular clouds
reveal a preference for different environments. The dominant absorption feature
in OH+ arises from a main component seen in CH+ (that with the highest CH+/CH
column density ratio), while OH follows CN absorption. This distinction
provides new constraints on OH chemistry in these clouds. Since CH+ detections
favor low-density gas with small fractions of molecular hydrogen, this must be
true for OH+ as well, confirming OH+ and H2O+ observations with the Herschel
Space Telescope. Our observed correspondence indicates that the cosmic ray
ionization rate derived from these measurements pertains to mainly atomic gas.
The association of OH absorption with gas rich in CN is attributed to the need
for high enough density and molecular fraction before detectable amounts are
seen. Thus, while OH+ leads to OH production, chemical arguments suggest that
their abundances are controlled by different sets of conditions and that they
coexist with different sets of observed species. Of particular note is that
non-thermal chemistry appears to play a limited role in the synthesis of OH in
diffuse molecular clouds.Comment: 15 pages, 4 figures, to appear in ApJ Letter
Convolutional Goppa Codes
We define Convolutional Goppa Codes over algebraic curves and construct their
corresponding dual codes. Examples over the projective line and over elliptic
curves are described, obtaining in particular some Maximum-Distance Separable
(MDS) convolutional codes.Comment: 8 pages, submitted to IEEE Trans. Inform. Theor
Quantum processing photonic states in optical lattices
The mapping of photonic states to collective excitations of atomic ensembles
is a powerful tool which finds a useful application in the realization of
quantum memories and quantum repeaters. In this work we show that cold atoms in
optical lattices can be used to perform an entangling unitary operation on the
transferred atomic excitations. After the release of the quantum atomic state,
our protocol results in a deterministic two qubit gate for photons. The
proposed scheme is feasible with current experimental techniques and robust
against the dominant sources of noise.Comment: 4 pages, 4 figure
Mesoscopic mean-field theory for spin-boson chains in quantum optical systems
We present a theoretical description of a system of many spins strongly coupled to a bosonic chain. We rely on the use of a spin-wave theory describing the Gaussian fluctuations around the mean-field solution, and focus on spin-boson chains arising as a generalization of the Dicke Hamiltonian. Our model is motivated by experimental setups such as trapped ions, or atoms/qubits coupled to cavity arrays. This situation corresponds to the cooperative (E⊗β) Jahn-Teller distortion studied in solid-state physics. However, the ability to tune the parameters of the model in quantum optical setups opens up a variety of novel intriguing situations. The main focus of this paper is to review the spin-wave theoretical description of this problem as well as to test the validity of mean-field theory. Our main result is that deviations from mean-field effects are determined by the interplay between magnetic order and mesoscopic cooperativity effects, being the latter strongly size-dependent
Coherent Modulation of the YBa2Cu3O6+x Atomic Structure by Displacive Stimulated Ionic Raman Scattering
We discuss the mechanism of coherent phonon generation by Stimulated Ionic
Raman Scattering, a process different from conventional excitation with near
visible optical pulses. Ionic Raman scattering is driven by anharmonic coupling
between a directly excited infrared-active phonon mode and other Raman modes.
We experimentally study the response of YBa2Cu3O6+x to the resonant excitation
of apical oxygen motions at 20 THz by mid-infrared pulses, which has been shown
in the past to enhance the interlayer superconducting coupling. We find
coherent oscillations of four totally symmetric (Ag) Raman modes and make a
critical assessment of the role of these oscillatory motions in the enhancement
of superconductivity.Comment: 12 pages, 4 figure
Matter--wave emission in optical lattices: Single particle and collective effects
We introduce a simple set--up corresponding to the matter-wave analogue of
impurity atoms embedded in an infinite photonic crystal and interacting with
the radiation field. Atoms in a given internal level are trapped in an optical
lattice, and play the role of the impurities. Atoms in an untrapped level play
the role of the radiation field. The interaction is mediated by means of lasers
that couple those levels. By tuning the lasers parameters, it is possible to
drive the system through different regimes, and observe phenomena like matter
wave superradiance, non-Markovian atom emission, and the appearance of bound
atomic states.Comment: 5 pages, 3 figure
Renormalization algorithm for the calculation of spectra of interacting quantum systems
We present an algorithm for the calculation of eigenstates with definite
linear momentum in quantum lattices. Our method is related to the Density
Matrix Renormalization Group, and makes use of the distribution of multipartite
entanglement to build variational wave--functions with translational symmetry.
Its virtues are shown in the study of bilinear--biquadratic S=1 chains.Comment: Corrected version. We have added an appendix with an extended
explanation of the implementation of our algorith
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