Beating of exciton-dressed states in a single semiconductor InGaAs/GaAs quantum dot

We report picosecond control of excitonic dressed states in a single semiconductor quantum dot. A strong laser pulse couples the exciton and biexciton states, to form an Autler-Townes doublet of the neutral exciton transition. The Rabi-splitting, and hence the admixture of the dressed states follows the envelope of the picosecond control laser. We create a superposition of dressed states, and observe the resulting beat: a direct measurement of a Rabi oscillation in time delay rather than the usual power domain

Direct measurement of the hole-nuclear spin interaction in single quantum dots

We use photoluminescence spectroscopy of ''bright'' and ''dark'' exciton states in single InP/GaInP quantum dots to measure hyperfine interaction of the valence band hole with nuclear spins polarized along the sample growth axis. The ratio of the hyperfine constants for the hole (C) and electron (A) is found to be C/A~-0.11. In InP dots the contribution of spin 1/2 phosphorus nuclei to the hole-nuclear interaction is weak, which enables us to determine experimentally the value of C for spin 9/2 indium nuclei as C_In~-5 micro-eV. This high value of C is in good agreement with recent theoretical predictions and suggests that the hole-nuclear spin interaction has to be taken into account when considering spin qubits based on holes.Comment: to be submitted to Phys Rev Let

Voltage controlled nuclear polarization switching in a single InGaAs quantum dot

Sharp threshold-like transitions between two stable nuclear spin polarizations are observed in optically pumped individual InGaAs self-assembled quantum dots embedded in a Schottky diode when the bias applied to the diode is tuned. The abrupt transitions lead to the switching of the Overhauser field in the dot by up to 3 Tesla. The bias-dependent photoluminescence measurements reveal the importance of the electron-tunneling-assisted nuclear spin pumping. We also find evidence for the resonant LO-phonon-mediated electron co-tunneling, the effect controlled by the applied bias and leading to the reduction of the nuclear spin pumping rate.Comment: 5 pages, 2 figures, submitted to Phys Rev

Optimizing Replica Exchange Moves For Molecular Dynamics

In this short note we sketch the statistical physics framework of the replica exchange technique when applied to molecular dynamics simulations. In particular, we draw attention to generalized move sets that allow a variety of optimizations as well as new applications of the method.Comment: 4 pages, 3 figures; revised version (1 figure added), PRE in pres

Constructing Science

An examination of children's causal reasoning capacities and how those capacities serve as the foundation of their scientific thinking. Young children have remarkable capacities for causal reasoning, which are part of the foundation of their scientific thinking abilities. In Constructing Science, Deena Weisberg and David Sobel trace the ways that young children's sophisticated causal reasoning abilities combine with other cognitive, metacognitive, and social factors to develop into a more mature set of scientific thinking abilities. Conceptualizing scientific thinking as the suite of skills that allows people to generate hypotheses, solve problems, and explain aspects of the world, Weisberg and Sobel argue that understanding how this capacity develops can offer insights into how we can become a more scientifically literate society. Investigating the development of causal reasoning and how it sets the stage for scientific thinking in the elementary school years and beyond, Weisberg and Sobel outline a framework for understanding how children represent and learn causal knowledge and identify key variables that differ between causal reasoning and scientific thinking. They present empirical studies suggesting ways to bridge the gap between causal reasoning and scientific thinking, focusing on two factors: contextualization and metacognitive thinking abilities. Finally, they examine children's explicit understanding of such concepts as science, learning, play, and teaching

Improved Temperature Performance of 1.31-mu/m Quantum Dot Lasers by Optimized Ridge Waveguide Design

In this letter, we demonstrate the importance of the fabricated device structure for the external differential efficiency, threshold current density, and maximum operating temperature for ground state operation of a 1.31-mu/m quantum dot laser. The introduction of a shallow ridge etch design and selective electroplating of the gold bondpads is demonstrated to offer improved performance in comparison to a deep ridge etch design with thinner evaporated gold bondpads