http://hdl.handle.net/2289/147 Search the Channel search http://dspace.rri.res.in:8080/jspui/simple-search http://hdl.handle.net/2289/4216 Title: Operator-sum representation for bosonic Gaussian channels<br/><br/>Authors: Ivan, J. Solomon; Sabapathy, Krishna Kumar; Simon, R.<br/><br/>Abstract: Operator-sum or Kraus representations for single-mode bosonic Gaussian channels are developed, and several of their consequences explored. The fact that the two-mode metaplectic operators acting as unitary purification of these channels do not, in their canonical form, mix the position and momentum variables is exploited to present a procedure which applies uniformly to all families in the Holevo classification. In this procedure the Kraus operators of every quantum-limited Gaussian channel can be simply read off from the matrix elements of a corresponding metaplectic operator. Kraus operators are employed to bring out, in the Fock basis, the manner in which the antilinear, unphysical matrix transposition map when accompanied by injection of a threshold classical noise becomes a physical channel, denoted D(κ) in the Holevo classification. The matrix transposition channels D(κ), D(κ‑1) turn out to be a dual pair in the sense that their Kraus operators are related by the adjoint operation. The amplifier channel with amplification factor κ and the beam-splitter channel with attenuation factor κ‑1 turn out to be mutually dual in the same sense. The action of the quantum-limited attenuator and amplifier channels as simply scaling maps on suitable quasiprobabilities in phase space is examined in the Kraus picture. Consideration of cumulants is used to examine the issue of fixed points. The semigroup property of the amplifier and attenuator families leads in both cases to a Zeno-like effect arising as a consequence of interrupted evolution. In the cases of entanglement-breaking channels a description in terms of rank 1 Kraus operators is shown to emerge quite simply. In contradistinction, it is shown that there is not even one finite rank operator in the entire linear span of Kraus operators of the quantum-limited amplifier or attenuator families, an assertion far stronger than the statement that these are not entanglement breaking channels. A characterization of extremality in terms of Kraus operators, originally due to Choi, is employed to show that all quantum-limited Gaussian channels are extremal. The fact that almost every noisy Gaussian channel can be realized as a product of a pair of quantum-limited channels is used to construct a discrete set of linearly independent Kraus operators for these noisy Gaussian channels, including the classical noise channel, and these Kraus operators have a particularly simple structure.<br/><br/>Description: Open Access http://hdl.handle.net/2289/4210 Title: Quantum walk of light in frequency space and its controlled dephasing<br/><br/>Authors: Pandey, Deepak; Satapathy, Nandan; Meena, M. S.; Ramachandran, Hema<br/><br/>Abstract: We implement, using a coherent source, a coined quantum walk of light in frequency space in a tabletop experiment utilizing a series of modified Michelson interferometers that incorporate polarization optics and acousto-optic modulators. Manipulating the phase of the radio frequency that governs the acousto-optic interaction, we achieve symmetric or asymmetric quantum walks. Introducing rapid random phase shifts electronically, while regulating the amplitude, we cause controlled dephasing and thus simulate a gradual transition from the quantum walk to the classical random walk.<br/><br/>Description: Open Access http://hdl.handle.net/2289/4203 Title: Optical nonlinearity and power limiting in organic molecules and nanocomposites<br/><br/>Authors: Philip, Reji; Thomas, Jayan<br/><br/>Abstract: Nonlinear optical transmission in materials has several applications including laser mode-locking, pulse shaping, opticalbistability, optical switching, and optical power limiting. Organic molecules suitable for functionalization have beenextensively investigated for their third order nonlinearities. We have measured the optical nonlinearity of different novelorganic and composite systems including nanocomposite polymer films of Au, Ag and Pt, Organic ionic crystals(pyridinium and quinolinium salts), Au-alkanethiol clusters, thiophene based polymers, and Schiff base complexes, usingthe z-scan and degenerate four wave mixing techniques, employing laser pulses of nanosecond and femtoseconddurations respectively. Most of these materials are found to be efficient optical power limiters under our excitationconditions, and their nonlinear extinction coefficients have been calculated. Enhancement in the optical nonlinearity wassometimes obtained even by mixing of two organic media. From degenerate four wave mixing experiments, the thirdorder nonlinear susceptibility χ(3) and figure of merit χ(3)/α values also have been determined for some of thesematerials. The above experiments conducted in a large number of organic and composite materials unravel the potentialof these media for diverse photonic applications.<br/><br/>Description: Restricted Access. http://hdl.handle.net/2289/4178 Title: Improved optical limiting in dispersible carbon nanotubes and their metal oxide hybrids<br/><br/>Authors: Anand, Benoy; Susana, Addo Ntim; Muthukumar, V Sai; Sai, S Siva Sankara; Philip, Reji; Mitra, Somenath<br/><br/>Abstract: The mechanism and performance of optical limiting in carbon nanotubes (CNTs) and their metal oxide hybrids are presented. The mechanism of nonlinear absorption (NLA) in dispersed CNTs was an "effective" three-photon absorption arising due to sequential transitions between the real excited states. The effect of nonlinear scattering was minimal, and it was found that the metal oxide immobilization on the CNT surface does not alter either the mechanism of NLA, or the optical limiting threshold. With limiting thresholds in the range of 0.37-0.46 J/cm(2), these highly dispersible MWCNTs and their hybrids are excellent optical liming nanocarbons. (C) 2011 Elsevier Ltd. All rights reserved.<br/><br/>Description: Restricted Access.