Screening and plasmons in pure and disordered single- and bilayer black phosphorus

We study collective plasmon excitations and screening of disordered single- and bilayer black phosphorus beyond the low energy continuum approximation. The dynamical polarizability of phosphorene is computed using a tight-binding model that properly accounts for the band structure in a wide energy range. Electron-electron interaction is considered within the Random Phase Approximation. Damping of the plasmon modes due to different kinds of disorder, such as resonant scatterers and long-range disorder potentials, is analyzed. We further show that an electric field applied perpendicular to bilayer phosphorene can be used to tune the dispersion of the plasmon modes. For sufficiently large electric field, the bilayer BP enters in a topological phase with a characteristic plasmon spectrum, which is gaped in the armchair direction.Comment: 9 pages, 9 figure

Relaxation, thermalization and Markovian dynamics of two spins coupled to a spin bath

It is shown that by fitting a Markovian quantum master equation to the numerical solution of the time-dependent Schr\"odinger equation of a system of two spin-1/2 particles interacting with a bath of up to 34 spin-1/2 particles, the former can describe the dynamics of the two-spin system rather well. The fitting procedure that yields this Markovian quantum master equation accounts for all non-Markovian effects in as much the general structure of this equation allows and yields a description that is incompatible with the Lindblad equation.Comment: arXiv admin note: text overlap with arXiv:1605.0660

Real-Time Dynamics of Typical and Untypical States in Non-Integrable Systems

For a class of typical states, the real-time and real-space dynamics of non-equilibrium density profiles has been recently studied for integrable models, i.e. the spin-1/2 XXZ chain [PRB 95, 035155 (2017)] and the Fermi-Hubbard chain [PRE 96, 020105 (2017)]. It has been found that the non-equilibrium dynamics agrees with linear response theory. Moreover, in the regime of strong interactions, clear signatures of diffusion have been observed. However, this diffusive behavior strongly depends on the choice of the initial state and disappears for untypical states without internal randomness. In the present work, we address the question whether or not the above findings persist for non-integrable models. As a first step, we study the spin-1/2 XXZ chain, where integrability can be broken due to an additional next-nearest neighbor interaction. Furthermore, we analyze the differences of typical and untypical initial states on the basis of their entanglement and their local density of states.Comment: 15 pages, 15 figure

Magnetization and energy dynamics in spin ladders: Evidence of diffusion in time, frequency, position, and momentum

The dynamics of magnetization and energy densities are studied in the two-leg spin-1/2 ladder. Using an efficient pure-state approach based on the concept of typicality, we calculate spatio-temporal correlation functions for large systems with up to 40 lattice sites. In addition, two subsequent Fourier transforms from real to momentum space as well as from time to frequency domain yield the respective dynamic structure factors. Summarizing our main results, we unveil the existence of genuine diffusion both for spin and energy. In particular, this finding is based on four distinct signatures which can all be equally well detected: (i) Gaussian density profiles, (ii) time-independent diffusion coefficients, (iii) exponentially decaying density modes, and (iv) Lorentzian line shapes of the dynamic structure factor. The combination of (i) - (iv) provides a comprehensive picture of high-temperature dynamics in thisarchetypal nonintegrable quantum model.Comment: 12 pages, 11 figure

Massively parallel quantum computer simulator, eleven years later

A revised version of the massively parallel simulator of a universal quantum computer, described in this journal eleven years ago, is used to benchmark various gate-based quantum algorithms on some of the most powerful supercomputers that exist today. Adaptive encoding of the wave function reduces the memory requirement by a factor of eight, making it possible to simulate universal quantum computers with up to 48 qubits on the Sunway TaihuLight and on the K computer. The simulator exhibits close-to-ideal weak-scaling behavior on the Sunway TaihuLight,on the K computer, on an IBM Blue Gene/Q, and on Intel Xeon based clusters, implying that the combination of parallelization and hardware can track the exponential scaling due to the increasing number of qubits. Results of executing simple quantum circuits and Shor's factorization algorithm on quantum computers containing up to 48 qubits are presented.Comment: Substantially rewritten + new data. Published in Computer Physics Communicatio

Benchmarking gate-based quantum computers

With the advent of public access to small gate-based quantum processors, it becomes necessary to develop a benchmarking methodology such that independent researchers can validate the operation of these processors. We explore the usefulness of a number of simple quantum circuits as benchmarks for gate-based quantum computing devices and show that circuits performing identity operations are very simple, scalable and sensitive to gate errors and are therefore very well suited for this task. We illustrate the procedure by presenting benchmark results for the IBM Quantum Experience, a cloud-based platform for gate-based quantum computing.Comment: Accepted for publication in Computer Physics Communication

First-principles modelling of magnetic excitations in Mn12

We have developed a fully microscopic theory of magnetic properties of the prototype molecular magnet Mn12. First, the intra-molecular magnetic properties have been studied by means of first-principles density functional-based methods, with local correlation effects being taken into account within the local density approximation plus U (LDA+U) approach. Using the magnetic force theorem, we have calculated the interatomic isotropic and anisotropic exchange interactions and full tensors of single-ion anisotropy for each Mn ion. Dzyaloshinskii-Moriya (DM) interaction parameters turned out to be unusually large, reflecting a low symmetry of magnetic pairs in molecules, in comparison with bulk crystals. Based on these results we predict a distortion of ferrimagnetic ordering due to DM interactions. Further, we use an exact diagonalization approach allowing to work with as large Hilbert space dimension as 10^8 without any particular symmetry (the case of the constructed magnetic model). Based on the computational results for the excitation spectrum, we propose a distinct interpretation of the experimental inelastic neutron scattering spectra.Comment: 8 pages, 2 figures. To appear in Physical Review

Decoherence and Thermalization at Finite Temperatures for Quantum Systems

We consider a quantum system $S$ with Hamiltonian ${\cal H}_S$ coupled via a Hamiltonian ${\cal H}_{SE}$ to a quantum environment $E$ with Hamiltonian ${\cal H}_E$. We assume the entirety $S+E$ is in a canonical-thermal state at an inverse temperature $\beta$. The entirety is a closed quantum system which evolves via the time-dependent Schr{\”o}dinger equation with Hamiltonian ${\cal H}={\cal H}_S+{\cal H}_E+\lambda{\cal H}_{SE}$ where $\lambda$ is the overall strength of the system-environment coupling. Using both large-scale simulations and perturbation theory calculations in $\lambda$, we have studied a measure $\sigma(t)$ for decoherence and $\delta(t)$ for thermalization of $S$. We performed large-scale parallel calculations on spin systems with up to $N=40$ spins in the entirety, with both real-time and imaginary-time quantum calculations. We performed perturbation theory calculations about $\lambda=0$ and fluctuations about the average for the canonical-thermal ensemble, for both $\sigma$ and $\delta$. We obtained closed form expressions for both $\sigma$ and $\delta$, in terms of the free energies of $S$ and $E$. Our perturbation theory calculations agree very well with our numerical calculations, at least as long as $\beta\lambda$ is small

Towards a corpuscular model of optical phenomena

This thesis presents a collection of event-by-event models that simulate fundamental optical experiments. The simulation approach is completely based on the experimental facts. Each component in the model corresponds to one kind of optical device, such as a beam splitter, a wave plate, a detector and so on. Networks of such components build computational experiments which are one-to-one copies of real experiments. As all components share the same mechanism (leaning machine) as in the previous work, our event-by-event simulation models are systematic and consistent with each other. As the model provides a description of interference and other wave phenomena on the level of individual event, it goes beyond the description of quantum theory. All the results presented in this thesis demonstrate that it is possible to simulate quantum phenomena by classical, non-Hamiltonian, local, causal and dynamical models.