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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,765 records · Page 98
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DOE OSTI2010

Noisy evolution of graph-state entanglement

A general method for the study of the entanglement evolution of graph states under the action of Pauli maps was recently proposed in Cavalcanti et al. [Phys. Rev. Lett. 103, 030502 (2009)]. This method is based on lower and upper bounds to the entanglement of the entire state as a function only of the state of a (typically) considerably smaller subsystem undergoing an effective noise process related to the original map. This provides a huge decrease in the size of the matrices involved in the calculation of entanglement in these systems. In the present paper we elaborate on this method in detail and generalize it to other natural situations not described by Pauli maps. Specifically, for Pauli maps we introduce an explicit formula for the characterization of the resulting effective noise. Beyond Pauli maps, we show that the same ideas can be applied to the case of thermal reservoirs at arbitrary temperature. In the latter case, we discuss how to optimize the bounds as a function of the noise strength. We illustrate our ideas with explicit exemplary results for several different graphs and particular decoherence processes. The limitations of the method are also discussed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2010

Exact reduced dynamics for a qubit in a precessing magnetic field and in contact with a heat bath

The two-level quantum system (qubit) in a precessing magnetic field and in contact with a heat bath is investigated. The exact reduced dynamics for the qubit in question is obtained. We apply the approach based on the block operator matrices theory, in contrast with the standard methods provided by the theory of the open quantum systems. We also present the solution of the Riccati operator equation associated with the Hamiltonian of the system. Next, we study the adiabatic approximation for the system under consideration using quantum fidelity as a way to measure validity of the adiabatic theory. We find that in the weak-coupling domain the standard condition that ensures adiabatic behavior of the spin in the magnetic field also guarantees its adiabatic evolution in the open-system variant of this model. Therefore, we provide the explicit example of the open quantum system that satisfies the adiabatic approximation first formulated for the closed quantum systems.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2010

Quantum-to-classical transition and entanglement sudden death in Gaussian states under local-heat-bath dynamics

Entanglement sudden death (ESD) in spatially separated two-mode Gaussian states coupled to local thermal and squeezed thermal baths is studied by mapping the problem to that of the quantum-to-classical transition. Using Simon's criterion concerning the characterization of classicality in Gaussian states, the time to ESD is calculated by analyzing the covariance matrices of the system. The results for the two-mode system at T=0 and T>0 for the two types of bath states are generalized to n modes, and are shown to be similar in nature to the results for the general discrete n-qubit system.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2010

Constructing optimal entanglement witnesses. II. Witnessing entanglement in 4Nx4N systems

We provide a class of optimal nondecomposable entanglement witnesses for 4Nx4N composite quantum systems or, equivalently, another construction of nondecomposable positive maps in the algebra of 4Nx4N complex matrices. This construction provides natural generalization of the Robertson map. It is shown that their structural physical approximations give rise to entanglement breaking channels.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2010

Laser-cooled RaF as a promising candidate to measure molecular parity violation

The parameter W{sub a}, which characterizes nuclear-spin-dependent parity violation (PV) in the molecular spin-rotational Hamiltonian, was computed with a quasirelativistic Hartree-Fock approach for radium fluoride (RaF) and found to be one of the largest absolute values predicted so far. The peculiar electronic structure of RaF leads to highly diagonal Franck-Condon matrices between the energetically lowest two electronic states, which qualifies RaF for direct laser cooling. A subset of diatomic molecules with a wide range of internal structures suitable for this cooling technique is also indicated. As trapped cold molecules offer superior coherence times, RaF can be considered promising for high-precision experiments aimed at molecular PV.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2010

Modal approach to Casimir forces in periodic structures

We present a modal approach to calculate finite-temperature Casimir interactions between two periodically modulated surfaces. The scattering formula is used and the reflection matrices of the patterned surfaces are calculated by decomposing the electromagnetic field into the natural modes of the structures. The Casimir force gradient from a deeply etched silicon grating is evaluated using the modal approach and compared to experiment for validation. The Casimir force involving a two-dimensional periodic structure is computed and deviations from the proximity force approximation are examined.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2010

Negative entanglement measure for bipartite separable mixed states

We define a negative entanglement measure for separable states which shows how much entanglement one should compensate the unentangled state, at the least, to change it into an entangled state. For two-qubit systems and some special classes of states in higher-dimensional systems, the explicit formula and the lower bounds for the negative entanglement measure (NEM) have been presented, and it always vanishes for bipartite separable pure states. The negative entanglement measure can be used as a useful quantity to describe the entanglement dynamics and the quantum phase transition. In the transverse Ising model, the first derivatives of negative entanglement measure diverge on approaching the critical value of the quantum phase transition, although these two-site reduced density matrices have no entanglement at all. In the one-dimensional (1D) Bose-Hubbard model, the NEM as a function of t/U changes from zero to negative on approaching the critical point of quantum phase transition.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Maximally discordant mixed states of two qubits

We study the relative strength of classical and quantum correlations, as measured by discord, for two-qubit states. Quantum correlations appear only in the presence of classical correlations, while the reverse is not always true. We identify the family of states that maximize the discord for a given value of the classical correlations and show that the largest attainable discord for mixed states is greater than for pure states. The difference between discord and entanglement is emphasized by the remarkable fact that these states do not maximize entanglement and are, in some cases, even separable. Finally, by random generation of density matrices uniformly distributed over the whole Hilbert space, we quantify the frequency of the appearance of quantum and classical correlations for different ranks.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Dynamics of slow light and light storage in a Doppler-broadened electromagnetically-induced-transparency medium: A numerical approach

We present a numerical scheme to study the dynamics of slow light and light storage in an electromagnetically-induced-transparency (EIT) medium at finite temperatures. Allowing for the motional coupling, we derive a set of coupled Schroedinger equations describing a boosted closed three-level EIT system according to the principle of Galilean relativity. The dynamics of a uniformly moving EIT medium can thus be determined by numerically integrating the coupled Schroedinger equations for atoms plus one ancillary Maxwell-Schroedinger equation for the probe pulse. The central idea of this work rests on the assumption that the loss of ground-state coherence at finite temperatures can be ascribed to the incoherent superposition of density matrices representing the EIT systems with various velocities. Close agreements are demonstrated in comparing the numerical results with the experimental data for both slow light and light storage. In particular, the distinct characters featuring the decay of ground-state coherence can be well verified for slow light and light storage. This warrants that the current scheme can be applied to determine the decaying profile of the ground-state coherence as well as the temperature of the EIT medium.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Quantum walks on complex networks with connection instabilities and community structure

A continuous-time quantum walk is investigated on complex networks with the characteristic property of community structure, which is shared by most real-world networks. Motivated by the prospect of viable quantum networks, I focus on the effects of network instabilities in the form of broken links, and examine the response of the quantum walk to such failures. It is shown that the reconfiguration of the quantum walk is determined by the community structure of the network. In this context, quantum walks based on the adjacency and Laplacian matrices of the network are compared, and their responses to link failures is analyzed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Violation of the area law and long-range correlations in infinite-dimensional-matrix product states

We propose to construct a family of states of one-dimensional spin chains by replacing the finite-dimensional matrices in matrix product states by infinite-dimensional operators constructed from bosonic annihilation and creation operators. The resulting states are demonstrated to violate the area law and to exhibit long-range correlations. In addition, we propose an efficient way to prepare the states experimentally, in which the spins interact sequentially with an ancilla system.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Influence of conformational changes in complex molecules on photon statistics of single molecule fluorescence

A single complex molecule with conformational changes (conformations 0 and 2) is considered. When such a molecule is irradiated by cw-laser light it can randomly change the intensity or polarization of its fluorescence due to jumps from one conformation to another. In fact, the molecule manifests itself either like the 0-type or the 2-type emitter. An expression for the matrix s{sub {alpha}{beta}}(t) called the start-stop correlator (waiting time distribution) in which {alpha}=0,2 and {beta}=0,2 is derived. An expression for the matrix p{sub {alpha}{beta}}(t) called the full correlator is derived as well. It determines the density of the probability of finding an event of {alpha} type and an event of {beta} type separated by time interval t. A relation between matrices s{sub {alpha}{beta}}(t) and p{sub {alpha}{beta}}(t) is found. A mathematical expression for the distribution w{sub N}(T) of events measured in time interval T is derived. It is expressed solely via the matrix s{sub {alpha}{beta}}(t). Numerical calculations of the event distribution function for various rates of intra- and interconformational jumps are carried out with the help of the formula for w{sub N}(T) and by the Monte Carlo method. Both methods of the calculation yield identical distributions. Fluctuating fluorescence intensity I(t) for a bin time of 5 ms is calculated for slow and fast interconformational jumps. A relation is found between the autocorrelation function g{sup (2)}(t) of fluorescence measurable in experiments and the matrix p{sub {alpha}{beta}}(t) calculated theoretically.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2010

Schroedinger equations for the square root density of an eigenmixture and the square root of an eigendensity spin matrix

We generalize a 'one-eigenstate' version of the density square root theorem to the case of densities coming from eigenmixture density operators. The generalization is of a special interest for the radial density functional theory (RDFT) for nuclei, a consequence of the rotational invariance of the nuclear Hamiltonian; when nuclear ground states have a finite spin, the RDFT uses eigenmixture density operators to simplify predictions of ground-state energies into one-dimensional, radial calculations. We also study Schroedinger equations governing spin eigendensity matrices.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2011

Theoretical study of the KKK system and dynamical generation of the K(1460) resonance

The KKK system is investigated with a coupled-channel approach based on solving the Faddeev equations considering the KKK, K{pi}{pi}, and K{pi}{eta} channels and using as input two-body t matrices that generate dynamically the f{sub 0}(980) and a{sub 0}(980) resonances. In the present calculation, a quasibound state around 1420 MeV with total isospin I=1/2 and spin-parity J{sup {pi}=}0{sup -} is found below the three-kaon threshold. This state can be identified with the K(1460) resonance listed by the Particle Data Group. We also study the KKK system in a single-channel three-body potential model with two-body effective KK and KK interactions, in which the KK interaction is adjusted to reproduce the properties of the f{sub 0}(980) and a{sub 0}(980) resonances as KK bound states, obtaining a very similar result to the one found in the Faddeev approach.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2011

Analyzing power in elastic scattering of {sup 6}He from a polarized proton target at 71 MeV/nucleon

The vector analyzing power has been measured for the elastic scattering of neutron-rich {sup 6}He from polarized protons at 71 MeV/nucleon making use of a newly constructed solid polarized proton target operated in a low magnetic field and at high temperature. Two approaches based on local one-body potentials were applied to investigate the spin-orbit interaction between a proton and a {sup 6}He nucleus. An optical model analysis revealed that the spin-orbit potential for {sup 6}He is characterized by a shallow and long-ranged shape compared with the global systematics of stable nuclei. A semimicroscopic analysis with an {alpha}+n+n cluster folding model suggests that the interaction between a proton and the {alpha} core is essentially important in describing the p+{sup 6}He elastic scattering. The data are also compared with fully microscopic analyses using nonlocal optical potentials based on nucleon-nucleon g matrices.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2010

Flavored quantum Boltzmann equations

We derive from first principles, using nonequilibrium field theory, the quantum Boltzmann equations that describe the dynamics of flavor oscillations, collisions, and a time-dependent mass matrix in the early universe. Working to leading nontrivial order in ratios of relevant time scales, we study in detail a toy model for weak-scale baryogenesis: two scalar species that mix through a slowly varying time-dependent and CP-violating mass matrix, and interact with a thermal bath. This model clearly illustrates how the CP asymmetry arises through coherent flavor oscillations in a nontrivial background. We solve the Boltzmann equations numerically for the density matrices, investigating the impact of collisions in various regimes.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2010

Next-to-simplest quantum field theories

We describe new on-shell recursion relations for tree amplitudes in N=1 and N=2 gauge theories and use these to show that the structure of the one-loop S-matrix in pure (i.e. without any matter) N=1 and N=2 gauge theories resembles that of pure Yang-Mills theory. We proceed to study gluon scattering in gauge theories coupled to matter in arbitrary representations. The contribution of matter to individual bubble and triangle coefficients can depend on the fourth- and sixth-order indices of the matter representation, respectively. So, the condition that one-loop amplitudes be free of bubbles and triangles can be written as a set of linear Diophantine equations involving these higher-order indices. These equations simplify for supersymmetric theories. We present new examples of supersymmetric theories that have only boxes (and no triangles or bubbles at one-loop) and nonsupersymmetric theories that are free of bubbles. These theories see simplifications in their S-matrices that cannot be deduced just from naive power-counting. In particular, our results indicate that one-loop scattering amplitudes in the N=2, SU(N) theory with a symmetric tensor hypermultiplet and an antisymmetric tensor hypermultiplet are simple like those in the N=4 theory.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2010

SU(2|2) for theories with sixteen supercharges at weak and strong coupling

We consider the dimensional reductions of N=4 supersymmetric Yang-Mills theory on RxS{sup 3} to the three-dimensional theory on RxS{sup 2}, the orbifolded theory on RxS{sup 3}/Z{sub k}, and the plane-wave matrix model. With explicit emphasis on the three-dimensional theory, we demonstrate the realization of the SU(2|3) algebra in a radial Hamiltonian framework. Using this structure we constrain the form of the spin chains, their S matrices, and the corresponding one- and two-loop Hamiltonians of the three-dimensional theory, and find putative signs of integrability up to the two-loop order. The string duals of these theories admit the IIA plane-wave geometry as their Penrose limit. Using known results for strings quantized on this background, we explicitly construct the strong-coupling dual extended SU(2|2) algebra and discuss its implications for the gauge theories.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗