Self heating and nonlinear current-voltage characteristics in bilayer graphene

We demonstrate by experiments and numerical simulations that the low-temperature current-voltage characteristics in diffusive bilayer graphene (BLG) exhibit a strong superlinearity at finite bias voltages. The superlinearity is weakly dependent on doping and on the length of the graphene sample. This effect can be understood as a result of Joule heating. It is stronger in BLG than in monolayer graphene (MLG), since the conductivity of BLG is more sensitive to temperature due to the higher density of electronic states at the Dirac point.Comment: 9 pages, 7 figures, REVTeX 4.

Shot noise and conductivity at high bias in bilayer graphene: Signatures of electron-optical phonon coupling

We have studied electronic conductivity and shot noise of bilayer graphene (BLG) sheets at high bias voltages and low bath temperature $T_0=4.2$ K. As a function of bias, we find initially an increase of the differential conductivity, which we attribute to self-heating. At higher bias, the conductivity saturates and even decreases due to backscattering from optical phonons. The electron-phonon interactions are also responsible for the decay of the Fano factor at bias voltages $V>0.1$ V. The high bias electronic temperature has been calculated from shot noise measurements, and it goes up to $\sim1200$ K at $V=0.75$ V. Using the theoretical temperature dependence of BLG conductivity, we extract an effective electron-optical phonon scattering time $\tau_{e-op}$. In a 230 nm long BLG sample of mobility $\mu=3600$ cm$^2$V$^{-1}$s$^{-1}$, we find that $\tau_{e-op}$ decreases with increasing voltage and is close to the charged impurity scattering time $\tau_{imp}=60$ fs at $V=0.6$ V.Comment: 7 pages, 7 figures. Extended version of the high bias part of version 1. The low bias part is discussed in arXiv:1102.065

Increasing complexity with quantum physics

We argue that complex systems science and the rules of quantum physics are intricately related. We discuss a range of quantum phenomena, such as cryptography, computation and quantum phases, and the rules responsible for their complexity. We identify correlations as a central concept connecting quantum information and complex systems science. We present two examples for the power of correlations: using quantum resources to simulate the correlations of a stochastic process and to implement a classically impossible computational task.Comment: 22 pages, 4 figure

Efficient single-photon emission from electrically driven InP quantum dots epitaxially grown on Si(001)

The heteroepitaxy of III-V semiconductors on silicon is a promising approach for making silicon a photonic platform for on-chip optical interconnects and quantum optical applications. Monolithic integration of both material systems is a long-time challenge, since different material properties lead to high defect densities in the epitaxial layers. In recent years, nanostructures however have shown to be suitable for successfully realising light emitters on silicon, taking advantage of their geometry. Facet edges and sidewalls can minimise or eliminate the formation of dislocations, and due to the reduced contact area, nanostructures are little affected by dislocation networks. Here we demonstrate the potential of indium phosphide quantum dots as efficient light emitters on CMOS-compatible silicon substrates, with luminescence characteristics comparable to mature devices realised on III-V substrates. For the first time, electrically driven single-photon emission on silicon is presented, meeting the wavelength range of silicon avalanche photo diodes' highest detection efficiency

HIV-1 co-infection does not reduce exposure to rifampicin, isoniazid, and pyrazinamide in South African tuberculosis outpatients

There are contrasting data in the literature about antituberculosis plasma drug concentrations in HIV-1-coinfected patients. We report the pharmacokinetics of rifampin, isoniazid, and pyrazinamide in a cohort of patients being treated for active tuberculosis, the majority of whom were coinfected with HIV-1 and had commenced antiretroviral therapy within 2 months of starting antituberculosis treatment. We also examined the association between antituberculosis drug concentrations and reported drug side effects at the 2-month clinical review. One hundred patients with pulmonary tuberculosis (65% coinfected with HIV-1) were intensively sampled to determine rifampin, isoniazid, and pyrazinamide plasma concentrations after 7 to 8 weeks of a daily quadruple-therapy regimen dosed according to World Health Organization (WHO) weight bands. Pharmacokinetic parameters were determined for each patient by using nonlinear mixed-effects models. HIV-1-coinfected patients had lower clearance rates for rifampin (21% decrease) and isoniazid (23% decrease) than HIV-1-uninfected patients, with resulting higher areas under the concentration-time curve from 0 to 24 h (AUC0–24) and maximum concentrations of drug in serum (Cmax). Antiretroviral therapy (ART) that included double-standard-dose lopinavir/ritonavir further lowered rifampin clearance, by 46%, and increased the AUC0–24. The current uniform dosing (per kilogram of body weight) across WHO weight bands was associated with a trend of decreased pharmacokinetic exposures for the lowest weight band. Use of fat-free mass as opposed to total body weight for allometric scaling of clearance significantly improved the model. Ambulant HIV-1-coinfected patients, the majority of whom were coprescribed ART, did not have reduced antituberculosis drug concentrations compared to HIV-1-uninfected patients

Quantum Cryptography without Switching

We propose a new coherent state quantum key distribution protocol that eliminates the need to randomly switch between measurement bases. This protocol provides significantly higher secret key rates with increased bandwidths than previous schemes that only make single quadrature measurements. It also offers the further advantage of simplicity compared to all previous protocols which, to date, have relied on switching.Comment: 4 pages, 4 figures, Submitte

Many Roads to Synchrony: Natural Time Scales and Their Algorithms

We consider two important time scales---the Markov and cryptic orders---that monitor how an observer synchronizes to a finitary stochastic process. We show how to compute these orders exactly and that they are most efficiently calculated from the epsilon-machine, a process's minimal unifilar model. Surprisingly, though the Markov order is a basic concept from stochastic process theory, it is not a probabilistic property of a process. Rather, it is a topological property and, moreover, it is not computable from any finite-state model other than the epsilon-machine. Via an exhaustive survey, we close by demonstrating that infinite Markov and infinite cryptic orders are a dominant feature in the space of finite-memory processes. We draw out the roles played in statistical mechanical spin systems by these two complementary length scales.Comment: 17 pages, 16 figures: http://cse.ucdavis.edu/~cmg/compmech/pubs/kro.htm. Santa Fe Institute Working Paper 10-11-02

Quantifying nonorthogonality

An exploratory approach to the possibility of analyzing nonorthogonality as a quantifiable property is presented. Three different measures for the nonorthogonality of pure states are introduced, and one of these measures is extended to single-particle density matrices using methods that are similar to recently introduced techniques for quantifying entanglement. Several interesting special cases are considered. It is pointed out that a measure of nonorthogonality can meaningfully be associated with a single mixed quantum state. It is then shown how nonorthogonality can be unlocked with classical information; this analysis reveals interesting inequalities and points to a number of connections between nonorthogonality and entanglement.Comment: Accepted for publication in Phys. Rev.