Maciej Lewenstein

Modeling Social Change with Cellular Automata

In this paper we will first discuss computer simulations of social processes as models of qualitative understanding. In the second part of the paper we will present the cellular automata model of dynamic social impact (Nowak et al. 1990) and its applications in the areas of the formation of public opinion and social change as an example of a model of qualitative understanding.

Quantum-orbit analysis of high-order harmonic generation by resonant plasmon field enhancement

We perform a detailed analysis of high-order harmonic generation (HHG) in atoms within the strong field approximation (SFA) by considering spatially inhomogeneous monochromatic laser fields. We investigate how the individual pairs of quantum orbits contribute to the harmonic spectra. We show that in the case of inhomogeneous fields, the electron tunnels with two different canonical momenta. One of these momenta leads to a higher cutoff and the other one develops a lower cutoff. Furthermore, we demonstrate that the quantum orbits have a very different behavior in comparison to the homogeneous field. We also conclude that in the case of the inhomogeneous fields, both odd and even harmonics are present in the HHG spectra. Within our model, we show that the HHG cutoff extends far beyond the standard semiclassical cutoff in spatially homogeneous fields. Our findings are in good agreement both with quantum mechanical and classical models.

Optical Abelian lattice gauge theories

We discuss a general framework for the realization of a family of abelian lattice gauge theories, i.e., link models or gauge magnets, in optical lattices. We analyze the properties of these models that make them suitable to quantum simulations. Within this class, we study in detail the phases of a U(1)-invariant lattice gauge theory in 2+1 dimensions originally proposed by Orland. By using exact diagonalization, we extract the low-energy states for small lattices, up to 4x4. We confirm that the model has two phases, with the confined entangled one characterized by strings wrapping around the whole lattice. We explain how to study larger lattices by using either tensor network techniques or digital quantum simulations with Rydberg atoms loaded in optical lattices where we discuss in detail a protocol for the preparation of the ground state. We also comment on the relation between standard compact U(1) LGT and the model considered.

Dipolar Molecules in Optical Lattices

We study the extended Bose-Hubbard model describing an ultracold gas of dipolar molecules in an optical lattice, taking into account all on-site and nearest-neighbor interactions, including occupation-dependent tunneling and pair tunneling terms. Using exact diagonalization and the multiscale entanglement renormalization ansatz, we show that these terms can destroy insulating phases and lead to novel quantum phases. These considerable changes of the phase diagram have to be taken into account in upcoming experiments with dipolar molecules.

Evidence for a Bose-Einstein condensate of excitons

We report compelling evidence for a “gray” condensate of dipolar excitons, electrically polarised in a 25 nm wide GaAs quantum well. The condensate is composed by a macroscopic population of dark excitons coherently coupled to a lower population of bright excitons. To create the exciton condensate we use an all-optical approach in order to produce microscopic traps which confine a dense exciton gas that yet exhibits an anomalously weak photoemission at sub-kelvin temperatures. This is the first fingerprint for the “gray” condensate. It is then confirmed by the macroscopic spatial coherence and the linear polarization of the weak excitonic photoluminescence emitted from the trap, as theoretically predicted.

Social dilemmas exist in space

Proper representation of many kinds of social dilemmas requires the consideration of social space. Social interdependence is mainly limited to other people located in a local neighborhood. The concept of social space may be more fruitfull in some instances than the network description. Several geometries that may be usefull describing the social space are discussed: Euclidean, City Block, Fractal, One-dimensional, Railroad, Random, Probabilistic, and Multiple spaces metric. The outcomes of the conformity game for several types of the geometries of the social space are discussed.

Theory of fluorescence spectra induced by short laser pulses

We consider a single-photon bound–bound and bound–free transitions induced by short, coherent laser pulses. Fluorescence energy spectra are calculated. In the case of the fluorescence to a side level, a number of analytic results are presented. For the resonance-fluorescence case the compact Bloch-like equations are derived for the ionization to an autoionization resonance and numerically analyzed. The multipeak structure of the spectra previously found in the simplified models is shown to be a universal feature. The number of maxima of the spectrum increases with the area of the pulse.

Are squeezed states necessary? A case study of photon detection based on quantum interference

Abstract Squeezed states are the particular quantum states of light with the very attractive property of having quantum fluctuations of one of the quadratures reduced below the vacuum limit. The present paper suggests an alternative view on the phenomenon of reduction of quantum noise by showing that an entirely equivalent effect can be achieved by using the appropriate photon detection. For this purpose the specific scheme of a new photon counter that employs the mechanism of quantum interference is introduced. The detection of signal photons in such a device takes place by the coherent combination of the processes of photon absorption and stimulated emission. We start our analysis with a brief review of the properties of squeezed states, putting the emphasis on the notion of reduction of quantum noise. Then, the standard theory of photon counting is reconsidered and revised from the new perspective, and the model of the new detecting device is formulated and thoroughly analyzed. It is proven, using the theory of laser-induced continuum structures, that the photon counting statistics obtained by using this device for detection of ordinary coherent states is strictly equivalent to that of standard detection of the appropriate squeez ed states of light. The implicit degree of effective squeezing, moreover, can be manipulated by the appropriate tuning of the detector. The performance characteristics of the new detector in various circumstances (different detector parameters and various measured signals) are investigated in detail. Also, the various sources of incoherence that may degrade the operation of the detector (spontaneous emission, thermal noise) are carefully examined. Throughout, the question of feasibility of practical realization of this thought experiment is constantly addressed, and it is argued that, albeit not easy, it is in principle possible.

Quantum phase transition of ultracold bosons in the presence of a non-Abelian synthetic gauge field

We study the Mott phases and the superfluid-insulator transition of two-component ultracold bosons on a square optical lattice in the presence of a non-Abelian synthetic gauge field, which renders a SU(2)-hopping matrix for the bosons. Using a resummed hopping expansion, we calculate the excitation spectra in the Mott insulating phases and demonstrate that the superfluid-insulator phase boundary displays a nonmonotonic dependence on the gauge-field strength. We also compute the momentum distribution of the bosons in the presence of the non-Abelian field and show that they develop peaks at nonzero momenta as the superfluid-insulator transition point is approached from the Mott side. Finally, we study the superfluid phases near the transition and discuss the induced spatial pattern of the superfluid density due to the presence of the non-Abelian gauge potential.

Noise reduction in a Raman ring-laser driven by a chaotic pump

Abstract The theory of a single mode ring cavity Raman laser is investigated for a broad-band, chaotic pump. We identify the regimes of significant stabilization of the Stokes output.