Fall 2016

Individual spins in semiconductors can retain their quantum phase coherence for times exceeding one second. Such long coherence times makes spins a versatile platform for exploring quantum information processing and condensed matter physics. I will discuss recent work exploiting the joint spin-state of two electrons in a GaAs double quantum dot as a spin qubit. This qubit is highly sensitive to its local magnetic environment. We leverage this sensitivity to precisely measure the statistically fluctuating nuclear polarization in the semiconductor crystal....

Molecular crystals have applications in nonlinear optics, organic electronics, and particularly in pharmaceuticals, as most drugs are marketed in the form of crystals of the pharmaceutically active ingredient. Molecular crystals are bound by dispersion (van der Waals) interactions, whose weak nature generates potential energy landscapes with many shallow minima that are close in energy. As a result, molecular crystals often exhibit polymorphism, the ability of the same molecule to crystallize in several structures. Crystal structure may profoundly influence the...

What is called "orthodox" quantum mechanics, as presented in standard foundational discussions, relies on two substantive assumptions --- the projection postulate and the eigenvalue-eigenvector link --- that do not in fact play any part in practical applications of quantum mechanics. I argue for this conclusion on a number of grounds, but primarily on the grounds that the projection postulate fails correctly to account for repeated, continuous and unsharp measurements (all of which are standard in contemporary physics) and that the eigenvalue-eigenvector link...

The field of spintronics, or magnetic electronics, is maturing and giving rise to new subfields. An important ingredient to the vitality of magnetism research in general is the large complexity due to competitions between interactions crossing many length scales and the interplay of magnetic degrees of freedom with charge (electric currents), phonon (heat), and photons (light). One perfect example, of the surprising new concepts being generated in magnetism research is the recent discovery of magnetic skyrmions. Magnetic skyrmions are topologically distinct...

Metal and semiconductor quantum dots (QDs) exhibit unique size and composition dependent optical properties. The key elements that determine their optical properties are localized surface plasmons in metal nanoparticles and excitons in quantum dots. When QDs are placed near Au@SiO2 core@shell nanoparticles, their exciton/multiexciton emission lifetimes and quantum yields are modified due to the exciton-plasmon interaction. At the single QD level, we found that the ratio between the biexciton and exciton quantum yields of the QDs...

First-principles computational approaches are making steady progress to quantitatively predict, for specific materials, the conceptual phenomena that are central to phase stability, energy capture, energy conversion, and transport. This talk outlines the vision behind and ongoing evolution of an efficient, accurate all-electron computational framework for such simulations, FHI-aims [1], begun from scratch over ten years ago and now a global development by a large group of scientists and engineers spread around the globe. The primary methods are density-...

Throughout the history of mankind, scientists and engineers have relied on the slow and serendipitous trial-and-error approach for materials discovery. In 1990s, the combinatorial approach was pioneered in the pharmaceutical industry in order to dramatically increase the rate at which new chemicals are identified. The high throughput concept is now widely implemented in a variety of fields in materials science. We have developed combinatorial thin film synthesis and characterization techniques in order to perform rapid survey of previously unexplored materials...

Quantum transport theory yields the celebrated Landauer formula for the conductance of a two-terminal device at zero bias in terms of T(EF,0), the transmission coefficient T(E,V) evaluated at the Fermi energy EF and V=0. For finite biases, one must use the nonequilibrium Green’s function (NEGF) method, which entails substantial difficulties. Instead of NEGF calculations, T(E,0) is often interpreted as representing transport at V=E/e. This practice is seriously flawed. In its stead, we employ quantum transport theory to derive a simple finite-bias analog of the...

Probability density function (PDF) methods have been useful in describing many physical aspects of turbulent mixing. In applications of these methods, modeled PDF transport equations are commonly simulated via classical Monte Carlo techniques, which provide estimates of moments of the PDF at arbitrary accuracy. In this talk, I will describe recently developed techniques in quantum computing and quantum enhanced measurements (quantum metrology) and use them to construct a quantum algorithm that accelerates the computation of such estimates. I will show that the...