Towards Realistic Time-Resolved Simulations of Quantum Devices

We report on our recent efforts to perform realistic simulations of large quantum devices in the time domain. In contrast to d.c. transport where the calculations are explicitly performed at the Fermi level, the presence of time-dependent terms in the Hamiltonian makes the system inelastic so that it is necessary to explicitly enforce the Pauli principle in the simulations. We illustrate our approach with calculations for a flying qubit interferometer, a nanoelectronic device that is currently under experimental investigation. Our calculations illustrate the fact that many degrees of freedom (16,700 tight-binding sites in the scattering region) and long simulation times (80,000 times the inverse Bandwidth of the tight-binding model) can be easily achieved on a local computer.Comment: 8 pages, 6 figure

Manipulating Andreev and Majorana Bound States with microwaves

We study the interplay between Andreev (Majorana) bound states that form at the boundary of a (topological) superconductor and a train of microwave pulses. We find that the extra dynamical phase coming from the pulses can shift the phase of the Andreev reflection, resulting in the appear- ance of dynamical Andreev states. As an application we study the presence of the zero bias peak in the differential conductance of a normal-topological superconductor junction - the simplest, yet somehow ambiguous, experimental signature for Majorana states. Adding microwave radiation to the measuring electrodes provides an unambiguous probe of the Andreev nature of the zero bias peak.Comment: 4 pages, 4 figure

Stopping electrons with radio-frequency pulses in the quantum Hall regime

Most functionalities of modern electronic circuits rely on the possibility to modify the path fol- lowed by the electrons using, e.g. field effect transistors. Here we discuss the interplay between the modification of this path and the quantum dynamics of the electronic flow. Specifically, we study the propagation of charge pulses through the edge states of a two-dimensional electron gas in the quantum Hall regime. By sending radio-frequency (RF) excitations on a top gate capacitively coupled to the electron gas, we manipulate these edge state dynamically. We find that a fast RF change of the gate voltage can stop the propagation of the charge pulse inside the sample. This effect is intimately linked to the vanishing velocity of bulk states in the quantum Hall regime and the peculiar connection between momentum and transverse confinement of Landau levels. Our findings suggest new possibilities for stopping, releasing and switching the trajectory of charge pulses in quantum Hall systems.Comment: 5 pages, 4 figure

Classical and quantum spreading of a charge pulse

With the technical progress of radio-frequency setups, high frequency quantum transport experiments have moved from theory to the lab. So far the standard theoretical approach used to treat such problems numerically--known as Keldysh or NEGF (Non Equilibrium Green's Functions) formalism--has not been very successful mainly because of a prohibitive computational cost. We propose a reformulation of the non-equilibrium Green's function technique in terms of the electronic wave functions of the system in an energy-time representation. The numerical algorithm we obtain scales now linearly with the simulated time and the volume of the system, and makes simulation of systems with 10^5 - 10^6 atoms/sites feasible. We illustrate our method with the propagation and spreading of a charge pulse in the quantum Hall regime. We identify a classical and a quantum regime for the spreading, depending on the number of particles contained in the pulse. This numerical experiment is the condensed matter analogue to the spreading of a Gaussian wavepacket discussed in quantum mechanics textbooks.Comment: 4 pages, 5 figures; to be published in IEEE Xplore, in Proceedings to IEEE 17th International Workshop on Computational Electronics 2014, June 3 - 6, 2014, Paris, France. Correction of typographic mistakes and update of ref. 1

Toward a functional characterization of the Acidic-domain of the chloroplast protein import receptor Toc159

Chloroplasts are members of a diverse class of organelles called plastids that differentiate plants from other eukaryotes, and are the site of a number of essential biochemical pathways including photosynthesis. Nuclear-encoded pre-proteins, which account for ~95% of chloroplast proteins, are post-translationally imported into plastids across the double envelope membrane. In the model plant Arabidopsis thaliana, the majority of pre-proteins are imported via the Toc (translocon at outer envelope membrane of chloroplasts) and Tic (translocon at inner envelope membrane of chloroplasts) complexes, the key components of which have been identified. The Toc159 homologues, atToc159 and atToc132/120, have been shown to form structurally and functionally distinct Toc complexes and have been proposed to serve as the primary pre-protein receptors, recognizing and interacting with the N-terminal extensions of pre-proteins, called transit peptides. The tripartite structure of atToc159 includes a membrane anchor (M-) domain, a GTPase (G-) domain, and an acidic (A-) domain that is currently functionally uncharacterized. The A-domain is a large (733 a.a.), intrinsically unstructured protein domain. The sequence identity among Toc159 homologue A-domains is considerably lower than the G- and M-domains. The A-domain has been shown to play a role in the pre-protein specificity exhibited by distinct Toc complexes. In the current study, the A-domain of atToc159 was shown to remain associated with the chloroplast envelope membrane when proteolytically cleaved by thrombin and to interact specifically with the G-domain of atToc159 in vitro. There was not a large change in secondary structure associated with the interaction, as observed by CD, but the interaction between the A- and G-domains of atToc159 was observed to inhibit the hydrolysis of GTP by the G-domain

Probing (topological) Floquet states through DC transport

We consider the differential conductance of a periodically driven system connected to infinite electrodes. We focus on the situation where the dissipation occurs predominantly in these electrodes. Using analytical arguments and a detailed numerical study we relate the differential conductances of such a system in two and three terminal geometries to the spectrum of quasi-energies of the Floquet operator. Moreover these differential conductances are found to provide an accurate probe of the existence of gaps in this quasi-energy spectrum, being quantized when topological edge states occur within these gaps. Our analysis opens the perspective to describe the intermediate time dynamics of driven mesoscopic conductors as topological Floquet filters.Comment: 8 pages, 6 figures, invited contribution to the special issue of Physica E on "Frontiers in quantum electronic transport" in memory of Markus Buttike

Increased human pathogenic potential of Escherichia coli from polymicrobial urinary tract infections in comparison to isolates from monomicrobial culture samples

The current diagnostic standard procedure outlined by the Health Protection Agency for urinary tract infections (UTIs) in clinical laboratories does not report bacteria isolated from samples containing three or more different bacterial species. As a result many UTIs go unreported and untreated, particularly in elderly patients, where polymicrobial UTI samples are especially prevalent. This study reports the presence of the major uropathogenic species in mixed culture urine samples from elderly patients, and of resistance to front-line antibiotics, with potentially increased levels of resistance to ciprofloxacin and trimethoprim. Most importantly, the study highlights that Escherichia coli present in polymicrobial UTI samples are statistically more invasive (P<0.001) in in vitro epithelial cell infection assays than those isolated from monomicrobial culture samples. In summary, the results of this study suggest that the current diagnostic standard procedure for polymicrobial UTI samples needs to be reassessed, and that E. coli present in polymicrobial UTI samples may pose an increased risk to human health

Numerical simulations of time resolved quantum electronics

This paper discusses the technical aspects - mathematical and numerical - associated with the numerical simulations of a mesoscopic system in the time domain (i.e. beyond the single frequency AC limit). After a short review of the state of the art, we develop a theoretical framework for the calculation of time resolved observables in a general multiterminal system subject to an arbitrary time dependent perturbation (oscillating electrostatic gates, voltage pulses, time-vaying magnetic fields) The approach is mathematically equivalent to (i) the time dependent scattering formalism, (ii) the time resolved Non Equilibrium Green Function (NEGF) formalism and (iii) the partition-free approach. The central object of our theory is a wave function that obeys a simple Schrodinger equation with an additional source term that accounts for the electrons injected from the electrodes. The time resolved observables (current, density. . .) and the (inelastic) scattering matrix are simply expressed in term of this wave function. We use our approach to develop a numerical technique for simulating time resolved quantum transport. We find that the use of this wave function is advantageous for numerical simulations resulting in a speed up of many orders of magnitude with respect to the direct integration of NEGF equations. Our technique allows one to simulate realistic situations beyond simple models, a subject that was until now beyond the simulation capabilities of available approaches.Comment: Typographic mistakes in appendix C were correcte