Non-Hermitian description of a superconducting phase qubit measurement

We present an approach based on a non-Hermitian Hamiltonian to describe the process of measurement by tunneling of a phase qubit state. We derive simple analytical expressions which describe the dynamics of measurement, and compare our results with those experimentally available.Comment: 8 pages, 4 figure

Nonlinear Dirac equation solitary waves under a spinor force with different components

We consider the nonlinear Dirac (NLD) equation in 1+1 dimension with scalar-scalar selfinteraction in the presence of external forces as well as damping of the form γͦf(x-t) - ιμγͦψ, where both f, {fj = rieiKjx} and ψ are two-component spinors. We develop an approximate variational approach using collective coordinates (CC) for studying the time dependent response of the solitary waves to these external forces. In our previous paper we assumed Kj = K, j = 1,2 which allowed a transformation to a simplifying coordinate system, and we also assumed the "small" component of the external force was zero. Here we include the effects of the small component and also the case K1 ≠ K2 which dramatically modi es the behavior of the solitary wave in the presence of these external forces.United States Department of EnergySanta Fe InstituteNational Natural Science Foundation of China (Nos. 11471025 and 11421101)Alexander von Humboldt Foundation (Germany) through Research Fellowship for Experienced Researchers SPA 1146358 STPMinisterio de Economía y Competitividad (Spain) through FIS2014-54497-PJunta de Andalucía (Spain) under Projects No. FQM207Excellent Grant P11-FQM-7276Mathematical Institute of the University of Seville (IMUS)Theoretical Division and Center for Nonlinear Studies at Los Alamos National LaboratoryPlan Propio of the University of Sevill

Sparse matrix-vector multiplication on GPGPU clusters: A new storage format and a scalable implementation

Sparse matrix-vector multiplication (spMVM) is the dominant operation in many sparse solvers. We investigate performance properties of spMVM with matrices of various sparsity patterns on the nVidia "Fermi" class of GPGPUs. A new "padded jagged diagonals storage" (pJDS) format is proposed which may substantially reduce the memory overhead intrinsic to the widespread ELLPACK-R scheme. In our test scenarios the pJDS format cuts the overall spMVM memory footprint on the GPGPU by up to 70%, and achieves 95% to 130% of the ELLPACK-R performance. Using a suitable performance model we identify performance bottlenecks on the node level that invalidate some types of matrix structures for efficient multi-GPGPU parallelization. For appropriate sparsity patterns we extend previous work on distributed-memory parallel spMVM to demonstrate a scalable hybrid MPI-GPGPU code, achieving efficient overlap of communication and computation.Comment: 10 pages, 5 figures. Added reference to other recent sparse matrix format

Soliton stability criterion for generalized nonlinear Schrödinger equations

A stability criterion for solitons of the driven nonlinear Schrödinger equation (NLSE) has been conjectured. The criterion states that p′(v)0 is a necessary condition for stability; here, v is the soliton velocity and p=P/N, where P and N are the soliton momentum and norm, respectively. To date, the curve p(v) was calculated approximately by a collective coordinate theory, and the criterion was confirmed by simulations. The goal of this paper is to calculate p(v) exactly for several classes and cases of the generalized NLSE: a soliton moving in a real potential, in particular a time-dependent ramp potential, and a time-dependent confining quadratic potential, where the nonlinearity in the NLSE also has a time-dependent coefficient. Moreover, we investigate a logarithmic and a cubic NLSE with a time-independent quadratic potential well. In the latter case, there is a bisoliton solution that consists of two solitons with asymmetric shapes, forming a bound state in which the shapes and the separation distance oscillate. Finally, we consider a cubic NLSE with parametric driving. In all cases, the p(v) curve is calculated either analytically or numerically, and the stability criterion is confirmed.Grant No. 1146358 STP from the Alexander von Humboldt-Stiftung, Germany, through Research Fellowship for Experienced Researchers SPAMICINN (Spain) through FIS2011-24540Projects No. FQM207, No. P11-FQM7276, and No. P09-FQM-4643 by Junta de Andalucia (Spain

Generalized traveling-wave method, variational approach, and modified conserved quantities for the perturbed nonlinear Schrödinger equation.

The generalized traveling wave method (GTWM) is developed for the nonlinear Schrödinger equation (NLSE) with general perturbations in order to obtain the equations of motion for an arbitrary number of collective coordinates. Regardless of the particular ansatz that is used, it is shown that this alternative approach is equivalent to the Lagrangian formalism, but has the advantage that only the Hamiltonian of the unperturbed system is required, instead of the Lagrangian for the perturbed system. As an explicit example, we take 4 collective coordinates, namely the position, velocity, amplitude and phase of the soliton, and show that the GTWM yields the same equations of motion as the perturbation theory based on the Inverse Scattering Transform and as the time variation of the norm, first moment of the norm, momentum, and energy for the perturbed NLSE.MEC, Spain through Grant No. FIS2008- 02380/FIS, and by the Junta de Andalucía under Project Nos. FQM207, FQM-00481, P06-FQM-01735, and P09-FQM- 464

Nonlinear Schrödinger solitons oscillate under a constant external force

We investigate the dynamics of solitons of the cubic nonlinear Schrödinger equation with an external time-independent force of the form f(x)=rexp(−iKx). Here the solitons travel with an oscillating velocity and all other characteristics of the solitons (amplitude, width, momentum, and phase) also oscillate. This behavior was predicted by a collective variable theory and confirmed by simulations. However, the reason for these oscillations remains unclear. Moreover, the spectrum of the oscillations exhibits a second strong peak, in addition to the intrinsic soliton peak. We show that the additional frequency belongs to a certain extended linear mode (which we refer to as a phonon for short) close to the lower band edge of the phonon continuum. Initially the soliton is at rest. When it starts to move it is deformed, begins to oscillate, and excites the above phonon mode such that the total momentum in a certain moving frame is conserved. In this frame the phonon does not move. However, not only does the soliton move in the homogeneous, time-periodic field of the phonon, but it also oscillates.Junta de Andalucia IAC11-III-11965MICINN FIS2011-24540,Junta de Andalucia Projects No. FQM207, No. P11-FQM7276, and No. P09-FQM-464