Scattering theory of topological phases in discrete-time quantum walks

One-dimensional discrete-time quantum walks show a rich spectrum of topological phases that have so far been exclusively analysed in momentum space. In this work we introduce an alternative approach to topology which is based on the scattering matrix of a quantum walk, adapting concepts from time-independent systems. For gapped quantum walks, topological invariants at quasienergies 0 and {\pi} probe directly the existence of protected boundary states, while quantum walks with a non-trivial quasienergy winding have a discrete number of perfectly transmistting unidirectional modes. Our classification provides a unified framework that includes all known types of topology in one dimensional discrete-time quantum walks and is very well suited for the analysis of finite size and disorder effects. We provide a simple scheme to directly measure the topological invariants in an optical quantum walk experiment.Comment: 12 pages. v2: minor correction

Chiral symmetry and bulk--boundary correspondence in periodically driven one-dimensional systems

Over the past few years, topological insulators have taken center stage in solid state physics. The desire to tune the topological invariants of the bulk and thus control the number of edge states has steered theorists and experimentalists towards periodically driving parameters of these systems. In such periodically driven setups, by varying the drive sequence the effective (Floquet) Hamiltonian can be engineered to be topological: then, the principle of bulk--boundary correspondence guarantees the existence of robust edge states. It has also been realized, however, that periodically driven systems can host edge states not predicted by the Floquet Hamiltonian. The exploration of such edge states, and the corresponding topological phases unique to periodically driven systems, has only recently begun. We contribute to this goal by identifying the bulk topological invariants of periodically driven one-dimensional lattice Hamiltonians with chiral symmetry. We find simple closed expressions for these invariants, as winding numbers of blocks of the unitary operator corresponding to a part of the time evolution, and ways to tune these invariants using sublattice shifts. We illustrate our ideas on the periodically driven Su-Schrieffer-Heeger model, which we map to a discrete time quantum walk, allowing theoretical results about either of these systems to be applied to the other. Our work helps interpret the results of recent simulations where a large number of Floquet Majorana fermions in periodically driven superconductors have been found, and of recent experiments on discrete time quantum walks

Coherent states engineering with linear optics: Possible and impossible tasks

The general transformation of the product of coherent states $\prod_{i=1}^N|\alpha_i>$ to the output state $\prod_{i=1}^M|\beta_i>$ ($N=M$ or $N\neq M$), which is realizable with linear optical circuit, is characterized with a linear map from the vector $(\alpha^{\ast}_1,...,\alpha^{\ast}_N)$ to $(\beta^{\ast}_1,...,\beta^{\ast}_M)$. A correspondence between the transformations of a product of coherent states and those of a single photon state is established with such linear maps. It is convenient to apply this linear transformation method to design any linear optical scheme working with coherent states. The examples include message encoding and quantum database searching. The limitation of manipulating entangled coherent states with linear optics is also discussed.Comment: 6 pages, 2 figure

Self-organization of atoms in a cavity field: threshold, bistability and scaling laws

We present a detailed study of the spatial self-organization of laser-driven atoms in an optical cavity, an effect predicted on the basis of numerical simulations [P. Domokos and H. Ritsch, Phys. Rev. Lett. 89, 253003 (2002)] and observed experimentally [A. T. Black et al., Phys. Rev. Lett. 91, 203001 (2003)]. Above a threshold in the driving laser intensity, from a uniform distribution the atoms evolve into one of two stable patterns that produce superradiant scattering into the cavity. We derive analytic formulas for the threshold and critical exponent of this phase transition from a mean-field approach. Numerical simulations of the microscopic dynamics reveal that, on laboratory timescale, a hysteresis masks the mean-field behaviour. Simple physical arguments explain this phenomenon and provide analytical expressions for the observable threshold. Above a certain density of the atoms a limited number of ``defects'' appear in the organized phase, and influence the statistical properties of the system. The scaling of the cavity cooling mechanism and the phase space density with the atom number is also studied.Comment: submitted to PR

Creative Goal Modeling for Innovative Requirements

Context: When determining the functions and qualities (a.k.a. requirements) for a system, creativity is key to drive innovation and foster business success. However, creative requirements must be practically operationalized, grounded in concrete functions and system interactions. Requirements Engineering (RE) has produced a wealth of methods centered around goal modeling, in order to graphically explore the space of alternative requirements, linking functions to goals and dependencies. In parallel work, creativity theories from the social sciences have been applied to the design of creative requirements workshops, pushing stakeholders to develop innovative systems. Goal models tend to focus on what is known, while creativity workshops are expensive, require a specific skill set to facilitate, and produce mainly paper-based, unstructured outputs. Objective: Our aim in this work is to explore beneficial combinations of the two areas of work in order to overcome these and other limitations, facilitating creative requirements elicitation, supported by a simple extension of a well-known and structured requirements modeling technique. Method: We take a Design Science approach, iterating over exploratory studies, design, and summative validation studies. Results: The result is the Creative Leaf tool and method supporting creative goal modeling for RE. Conclusion: We support creative RE by making creativity techniques more accessible, producing structured digital outputs which better match to existing RE methods with associated analysis procedures and transformations

Total Quality Management (TQM) in Hungary

In up-to-date manufacturing, industrial quality has proven to be a key competitive advantage. An active market position and profits are associated with high quality products and services. Like the optical properties of a natural crystal, quality reflects the state of the whole industry, its marketing, research and development, design, production methods, human resources, logistics, costing and pricing, and capital background. All advancement in the productive and commercial capability of a company relates somehow to the quality of the delivered goods. The holistic approach to industrial quality appeals to industrial policymaking and company leadership; quality improvement converted into an authoritative management issue. Active commitment and program implantation by the people at the top usually decides the whole quality image of the company. The outstanding role that quality plays in industry, obliges IIASA to follow closely the developments in international strategies of quality promotion. We highly appreciate the enthusiastic response of Hungarian industry. It has been proved that an advanced knowledge of comprehensive quality management, accumulated in leading industrial countries, is clearly compatible with the socio-economic environment of the Hungarian Republic. Encouraging results achieved so far invite other countries, with at least comparable societal structure, to follow the Hungarian example. IIASA would be pleased to provide an intellectual background for such breakthroughs in the challenging area of total quality movement

HerMES: a search for high-redshift dusty galaxies in the HerMES Large Mode Survey – catalogue, number counts and early results

Selecting sources with rising flux densities towards longer wavelengths from Herschel/Spectral and Photometric Imaging Receiver (SPIRE) maps is an efficient way to produce a catalogue rich in high-redshift (z > 4) dusty star-forming galaxies. The effectiveness of this approach has already been confirmed by spectroscopic follow-up observations, but the previously available catalogues made this way are limited by small survey areas. Here we apply a map-based search method to 274 deg^2 of the Herschel Multi-tiered Extragalactic Survey (HerMES) Large Mode Survey and create a catalogue of 477 objects with SPIRE flux densities S_(500) > S_(350) > S_(250) and a 5σ cut-off S_(500) > 52 mJy. From this catalogue we determine that the total number of these ‘red’ sources is at least an order of magnitude higher than predicted by galaxy evolution models. These results are in agreement with previous findings in smaller HerMES fields; however, due to our significantly larger sample size we are also able to investigate the shape of the red source counts for the first time. We have obtained spectroscopic redshift measurements for two of our sources using the Atacama Large Millimeter/submillimeter Array. The redshifts z = 5.1 and 3.8 confirm that with our selection method we can indeed find high-redshift dusty star-forming galaxies