Eric Sembrat's Test Bonanza

Tatiana Engel, Stanford University

Abstract:
Neural responses and behavior are influenced by internal brain states, such as arousal, vigilance, or task context. Ongoing variations of these internal states affect global patterns of neural activity, giving rise to apparent variability of neural responses to sensory stimuli, from trial-to-trial and across time within single trials. Demultiplexing these endogenously generated and externally driven signals proved difficult with traditional techniques based on trial-averaged responses of single neurons, which dismiss neural variability as noise. In this talk, I will describe my recent work leveraging multi-electrode neural activity recordings and computational models to uncover how internal brain states interact with goal-directed behavior. I will show that ensemble neural activity within single columns of the primate visual cortex spontaneously fluctuates between phases of vigorous (On) and faint (Off) spiking. These endogenous On-Off dynamics, reflecting global changes in arousal, are also modulated at a local scale during spatial attention and predict behavioral performance. I will also demonstrate that these On-Off dynamics provide a single unifying mechanism that explains general features of correlated variability classically observed in cortical responses (e.g., changes in neural correlations during attention). I will conclude by sketching out a roadmap for developing a general theory that will allow us to discover dynamic computations from large-scale neural recordings and to link these computations to behavior.

Turbulence, the complicated fluid behavior of nonlinear and statistical nature, arises in many physical systems across various disciplines, from tiny laboratory scales to geophysical and astrophysical ones. The notion of turbulence in the quantum world was conceived long ago by Onsager and Feynman, but the occurrence of turbulence in ultracold gases has been studied in the laboratory only very recently. Herein we review the general properties of quantum gases at ultralow temperatures paying particular attention to vortices, their dynamics and turbulent behavior.

We review the recent advances from experiment. We highlight recent measurements demonstrating the emergence of Turbulence, the scaling law for energy cascade and  quantification of disorder in such a fluid. The use of many recent tools to investigate turbulent clouds using correlation functions and extended entropy will be discussed.

The connection of turbulent cloud in expansions and speckle fields will be explained.

For a general reference see the review, Phys. Report, 2016, 622, 1-52. This work was performed in collaboration with: E. Henn, K. Magalhaes, G. Roati ( Italy), V. Yukalov ( Russia), G. Telles, , P. Castilho, P.E. Tavares M. C. Tsatsos, A. R. Fritsch, , F. E. A. dos Santos, M.  A. Caracanhas, P. E.S. Tavares, E. Gutieerez, A. Orozco.

Over the past decade, the physics of low dimensional electronic systems has been revolutionized by the discovery of materials with very unusual electronic properties where the behavior of the electrons is governed by the Dirac equation. Among these, graphene has taken center stage due to its ultrarelativistic-like electron dynamics and its potential applications in nanotechnology. Moreover, recent advances in the design and nanofabrication of heterostructures based on van der Waals materials have enabled a new generation of quantum electronic transport experiments in graphene.

In this talk, I will describe our recent experiments exploring electron-electron interaction driven quantum phenomena in ultra-high quality graphene-based van der Waals heterostructures. In particular, I will show two novel realizations of a symmetry-protected topological insulator state, specifically a quantum spin Hall state, characterized by an insulating bulk and conducting counterpropagating spin-polarized states at the system edges.

Our experiments establish graphene-based heterostructures as highly tunable systems to study topological properties of condensed matter systems in the regime of strong e-e interactions and I will end my talk with an outlook of some of the exciting directions in the field.

Individual superconducting qubits have seen impressive improvements in nearly all aspects over the past decade and now sit at the threshold of being able to perform quantum error correction. Scaling to larger numbers of qubits is challenging both in the fundamental aspects of understanding the behavior of large numbers of strongly coupled qubits and in the technical aspects of controlling such systems. We take a unique approach in which we use a superconducting qubit as a quantum FM radio manipulate individual photons in an array of harmonic oscillators. This allows control with a single qubit's worth of classical hardware and simplifies the process of designing and building large devices. Further, these techniques are compatible with current brute force scaling efforts and represent a promising avenue to reach hundreds of qubits in the next several years. In this talk, I will describe the architecture and show how parametric control can be used to realize universal quantum logic operations between arbitrary memory modes.

Strongly interacting fermions govern the physics of e.g. high-temperature superconductors, nuclear matter and neutron stars. The interplay of the Pauli principle with strong interactions can give rise to exotic properties that we do not even understand at a qualitative level. In recent years, ultracold Fermi gases of atoms have emerged as a pristine platform for the creation and study of strongly interacting systems of fermions. Near Feshbach resonances, such gases display superfluidity at 17% of the Fermi temperature. Scaled to the density of electrons in solids, this corresponds to superfluidity far above room temperature. Confined in optical lattices, fermionic atoms realize the Fermi-Hubbard model, believed to capture the essence of cuprate high-temperature superconductors. In recent experiments on two-dimensional Fermi gases under a microscope, we observe metallic, Mott insulating and band insulating states with single-site, single-atom resolution. The microscope allows for the site-resolved detection of charge and spin correlations, revealing the famous Pauli and correlation hole for low and intermediate lattice fillings, and correlated doublon-hole pairs near half filling. These correlations should play an important role for transport in the Fermi-Hubbard model.

Soft Condensed Matter & Physics of Living Systems Seminar

Animal Aeroacoustics is the study of the acoustics and biology of the sounds animals make when they fly.  We begin by exploring an acoustic mechanism that, while catastrophic for aircraft, many birds use to communicate: aeroelastic flutter. The tail of hummingbirds is essentially a 'musical instrument': by evolving different shapes of tail-feathers, different species produce a range of species-specific sounds. Moreover, we demonstrate three different types of interactions between adjacent fluttering feathers that enhance the acoustic diversity of sound that is produced.  Next, we explore how hummingbirds use behavior to modulate the sounds they produce with their tail during a courtship dive, as heard by a recipient, a female.  We recorded dives using two 'acoustic cameras' (phased arrays of microphones that use beamforming to localize sound) to track the bird through 3D space. We demonstrate that male Costa's Hummingbird (Calypte costae) places the female in a part of the sound field in which the Doppler shift is minimized, while simultaneously employing strategies to maximize loudness. Finally, we discuss ongoing projects on the hum of hummingbirds, buzzing of bee and mosquito wings, as well as work on the silent flight of hunting owls.

Inquiring Minds Public Lecture

The twenty-first century is poised to see dramatic advances in medicine. The rapid progress in understanding the molecular causes of disease and the emergence of new treatment strategies are fueled by the development of physical instruments that can characterize biological processes at extreme resolution and provide the means to harness biological systems for technological uses. One common target of such investigations is DNA, which, after water and oxygen, is the most famous molecule of life known. This is not surprising, as the eye-catching double helix of DNA carries instructions to manufacture and assemble all the components of a living organism. The wealth of information encoded in DNA often overshadows its unusual physical properties, for example, the possibility of an effective attraction between same-charge DNA molecules regulated by their nucleotide sequence. Furthermore, the methods used to determine the informational content of DNA—its nucleotide sequence—until now relied on biological processes. In this lecture, I will describe our recent efforts to characterize the physical properties of DNA and determine their role in orchestrating the function of a biological cell. I will demonstrate how the physical properties of DNA can be used to build a physics-based reader of the DNA sequence. Finally, I will describe how recent advances in the field of DNA nanobiotechnology are paving the way to personalized medicine.  

School of Physics Hard Condensed Matter Seminar: Prof. Israel Klich, University of Virginia

When spins are regularly arranged in a triangular fashion, the spins may not satisfy simultaneously their antiferromagnetic interactions with their 
neighbors. This phenomenon, called frustration, may lead to a large set of ground states and to exotic states such as spin ice and spin liquid. 
However, when frustration is present simultaneously with disorder, a spin-glass is known to be a typical state of such a magnetic system. Is disorder necessary for a spin glass to form? This fundamental question has been frequently asked and several theoretical models of clean glassy states has been proposed, typically relying on long range interactions. Motivated by puzzling behavior observed in a well studied magnetic systems (SCGO), I describe a novel mechanism wherein quantum fluctuations cause a clean system governed by simple local interactions to freeze into a glass. At the heart of the effect is an unusual scaling of the number of local minima, with a scaling extensive in the boundary length rather than the volume. I will explain how these properties follow by a combination of tools and mappings, leading to a problem of counting tessellations. I will also present recent experimental evidence for the spin jam scenario.