On the Nature of Charge Transport in Quantum-Cascade Lasers

The first global quantum simulation of semiconductor-based quantum-cascade lasers is presented. Our three-dimensional approach allows to study in a purely microscopic way the current-voltage characteristics of state-of-the-art unipolar nanostructures, and therefore to answer the long-standing controversial question: is charge transport in quantum-cascade lasers mainly coherent or incoherent? Our analysis shows that: (i) Quantum corrections to the semiclassical scenario are minor; (ii) Inclusion of carrier-phonon and carrier-carrier scattering gives excellent agreement with experimental results.Comment: 4 pages, 7 Postscript figures. Phys. Rev. Lett. (in press

Polynomial growth of discrete quantum groups, topological dimension of the dual and *-regularity of the Fourier algebra

Banica and Vergnioux have shown that the dual discrete quantum group of a compact simply connected Lie group has polynomial growth of order the real manifold dimension. We extend this result to a general compact group and its topological dimension, by connecting it with the Gelfand-Kirillov dimension of an algebra. Furthermore, we show that polynomial growth for a compact quantum group G of Kac type implies *-regularity of the Fourier algebra A(G), that is every closed ideal of C(G) has a dense intersection with A(G). In particular, A(G) has a unique C*-norm.Comment: to appear in Annales de l'Institut Fourie

Wigner-function formalism applied to semiconductor quantum devices: Need for nonlocal scattering models

In designing and optimizing new-generation nanomaterials and related quantum devices, dissipation versus decoherence phenomena are often accounted for via local scattering models, such as relaxation-time and Boltzmann-like schemes. Here we show that the use of such local scattering approaches within the Wigner-function formalism may lead to unphysical results, namely anomalous suppression of intersubband relaxation, incorrect thermalization dynamics, and violation of probability-density positivity. Furthermore, we propose a quantum-mechanical generalization of relaxation-time and Boltzmann-like models, resulting in nonlocal scattering superoperators that enable one to overcome such limitations.Comment: 12 pages, 7 figure

Why are we not flooded by involuntary autobiographical memories? Few cues are more effective than many

Recent research on involuntary autobiographical memories (IAMs) has shown that these memories can be elicited and studied in the laboratory under controlled conditions. Employing a modified version of a vigilance task developed by Schlagman and Kvavilashvili (Mem Cogn 36:920–932, 2008) to elicit IAMs, we investigated the effects of varying the frequency of external cues on the number of IAMs reported. During the vigilance task, participants had to detect an occasional target stimulus (vertical lines) in a constant stream of non-target stimuli (horizontal lines). Participants had to interrupt the task whenever they became aware of any task-unrelated mental contents and to report them. In addition to line patterns, participants were exposed to verbal cues and their frequency was experimentally manipulated in three conditions (frequent cues vs. infrequent cues vs. infrequent cues plus arithmetic operations). We found that, compared to infrequent cues, both conditions with frequent cues and infrequent cues plus arithmetic operations decreased the number of IAMs reported. The comparison between the three experimental conditions suggests that this reduction was due to the greater cognitive load in conditions of frequent cues and infrequent cue plus arithmetic operations. Possible mechanisms involved in this effect and their implications for research on IAMs are discussed

Connected components of compact matrix quantum groups and finiteness conditions

We introduce the notion of identity component of a compact quantum group and that of total disconnectedness. As a drawback of the generalized Burnside problem, we note that totally disconnected compact matrix quantum groups may fail to be profinite. We consider the problem of approximating the identity component as well as the maximal normal (in the sense of Wang) connected subgroup by introducing canonical, but possibly transfinite, sequences of subgroups. These sequences have a trivial behaviour in the classical case. We give examples, arising as free products, where the identity component is not normal and the associated sequence has length 1. We give necessary and sufficient conditions for normality of the identity component and finiteness or profiniteness of the quantum component group. Among them, we introduce an ascending chain condition on the representation ring, called Lie property, which characterizes Lie groups in the commutative case and reduces to group Noetherianity of the dual in the cocommutative case. It is weaker than ring Noetherianity but ensures existence of a generating representation. The Lie property and ring Noetherianity are inherited by quotient quantum groups. We show that A_u(F) is not of Lie type. We discuss an example arising from the compact real form of U_q(sl_2) for q<0.Comment: 43 pages. Changes in the introduction. The relation between our and Wang's notions of central subgroup has been clarifie

Photoexcitation of electron wave packets in quantum spin Hall edge states: effects of chiral anomaly from a localised electric pulse

We show that, when a spatially localised electric pulse is applied at the edge of a quantum spin Hall system, electron wavepackets of the helical states can be photoexcited by purely intra-branch electrical transitions, without invoking the bulk states or the magnetic Zeeman coupling. In particular, as long as the electric pulse remains applied, the photoexcited densities lose their character of right- and left-movers, whereas after the ending of the pulse they propagate in opposite directions without dispersion, i.e. maintaining their space profile unaltered. Notably we find that, while the momentum distribution of the photoexcited wave packets depends on the temperature $T$ and the chemical potential $\mu$ of the initial equilibrium state and displays a non-linear behavior on the amplitude of the applied pulse, in the mesoscopic regime the space profile of the wave packets is independent of $T$ and $\mu$. Instead, it depends purely on the applied electric pulse, in a linear manner, as a signature of the chiral anomaly characterising massless Dirac electrons. We also discuss how the photoexcited wave packets can be tailored with the electric pulse parameters, for both low and finite frequencies.Comment: 15 pages, 5 figure

Wigner-function formalism applied to semiconductor quantum devices: Failure of the conventional boundary-condition scheme

The Wigner-function formalism is a well known approach to model charge transport in semiconductor nanodevices. Primary goal of the present article is to point out and explain intrinsic limitations of the conventional quantum-device modeling based on such Wigner-function paradigm, providing a definite answer to open questions related to the application of the conventional spatial boundary-condition scheme to the Wigner transport equation. Our analysis shows that (i) in the absence of energy dissipation (coherent limit) the solution of the Wigner equation equipped with given boundary conditions is not unique, and (ii) when decoherence/dissipation phenomena are taken into account via a relaxation-time approximation the solution, although unique, is not necessarily a physical Wigner function.Comment: 18 pages, 8 figures, accepted by Phys. Rev.

Continuous-time quantum walks on dynamical percolation graphs

We address continuous-time quantum walks on graphs in the presence of time- and space-dependent noise. Noise is modeled as generalized dynamical percolation, i.e. classical time-dependent fluctuations affecting the tunneling amplitudes of the walker. In order to illustrate the general features of the model, we review recent results on two paradigmatic examples: the dynamics of quantum walks on the line and the effects of noise on the performances of quantum spatial search on the complete and the star graph. We also discuss future perspectives, including extension to many-particle quantum walk, to noise model for on-site energies and to the analysis of different noise spectra. Finally, we address the use of quantum walks as a quantum probe to characterize defects and perturbations occurring in complex, classical and quantum, networks.Comment: 7 pages, 4 figures. Accepted for publication in EPL Perspective