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Fall 2018 Colloquium Series

Date:  Thursday, November 29th, 2018
Speaker: Çağlar Girit, Collège de France

Title: Spectroscopy with Josephson Junctions

Abstract: Spectroscopy, by providing a direct measurement of the energy spectrum, is a powerful tool to probe matter. Conventional spectroscopy techniques suffer several drawbacks when applied to mesoscopic systems, or artificial quantum coherent atoms. I present an on-chip, Josephson-junction based spectrometer which surpasses state-of-the-art instruments, works between 2-200 GHz, and is ideally suited for probing elementary excitations in mesoscopic systems. I describe the operating principle and design of the spectrometer, show spectra of several superconducting quantum circuits, and outline experiments to investigate single quasiparticles in superconductors.

Host: Benjamín J. Alemán


Date:  Thursday, November 15th, 2018
Speaker: Arpita Upadhyaya, University of Maryland

Title: Forces and Mechanosensing in Immune Cells

Abstract: Cells need to sense and adaptively respond to their physical environment in diverse biological contexts such as development, cancer and the immune response. In addition to chemical signals and the genetic blueprint, cellular function and dynamics are modulated by the physical properties of their environment such as stiffness and topography. In order to probe and respond to these environmental attributes, cells exert forces on their surroundings and generate appropriate biochemical and genetic responses. These forces arise from the spatiotemporal organization and dynamics of the cell cytoskeleton, a network of entangled biopolymer filaments that is driven out of thermal equilibrium by enzymes that actively convert chemical energy to mechanical energy. Understanding how cells generate forces and sense the mechanical environment (mechanosensing) is an important challenge with implications for physics and biology. We have investigated the principles of cellular force generation, the statistical properties of these forces, and their role in stiffness and topography sensing by immune and cancer cells. During activation, immune cells interact with structures possessing a diverse range of physical properties and respond to physical cues such as stiffness, topography and ligand mobility. We have used traction force microscopy to measure the forces exerted by T cells during activation on elastic substrates. I will discuss the distinct roles of the actin and microtubule cytoskeleton in the exertion of mechanical stresses that support signaling activation, microcluster assembly and receptor movement in T cells. We found two spatially distinct regimes of force generation, potentially arising from different actin-based structures. Furthermore, T cells are mechanosensitive, as cytoskeletal dynamics, force generation and signaling are modulated by substrate stiffness. Our recent studies have also shown that actin dynamics and signaling in B cells is modulated by subcellular topography of the antigen-presenting surface. Our work highlights the importance of cytoskeletal forces in immune cell receptor activation.

Host: Raghu Parthasarathy


Date:  Thursday, November 8th, 2018
Speaker: Ronald Walsworth, Harvard University and Smithsonian Institution

Title: Quantum Diamond Sensors for the Life & Physical Sciences

Abstract: In recent years, optically probed nitrogen–vacancy (NV) quantum defects in diamond have become a leading modality for magnetic, electrical, and temperature sensing at short length scales (nanometers to millimeters) under ambient conditions.  This technology has wide-ranging application across the physical and life sciences — from NMR spectroscopy at the scale of individual cells to improved biomedical diagnostics to understanding the formation of the solar system to the search for dark matter.  I will provide an overview of quantum diamond sensors and their diverse applications.

Host: Michael Raymer


Date:  Thursday, November 1st, 2018
Speaker: John Crocker, University of Pennsylvania

Title:  Why is everything squishy?

Abstract: A broad variety of different biomaterials, foods and personal products are found to have remarkably similar mechanical properties when deformed–which might typically be called ‘squishy’.  Technically termed soft-glassy materials (SGMs), these materials include such examples as soap foams, mayonnaise, ketchup, toothpaste, as well as remarkably, the actin cytoskeleton and the chromatin in cells’ nuclei. When gently activated by internal energy sources, these SGMs display dynamic shear moduli that have a power-law frequency dependence, super-diffusive particle motion, and large cooperative particle rearrangements, or avalanches, all phenomena which are essentially unexplained. I will report recent micromechanical studies of the cytoskeleton of fibroblast that show such behavior, as well as three-dimensional tracking experiments that show similar behavior in a transparent dense emulsion–essentially clear mayonnaise. We recently constructed a minimal computational model for SGMs whose physics was determined solely by energy minimization on an energy landscape spanning a high-dimensional configuration space (Nature Materials, 15, 1031-1036, 2016). The model is essentially a wet soap foam consisting of compressible spherical bubbles, whose sizes slowly evolve due to ripening. Surprisingly, we find that the steepest-descent configuration space path is a self-similar fractal curve, resembling a river cascading down a tortuous mountain canyon. The previously unexplained SGM rheology and Lévy-like super-diffusive motion in our model stem directly from these paths’ fractal dimension and energy function. In the clear mayonnaise, we are able to show experimentally that the high-dimensional configuration space path is a fractal. The cell finding suggests that the cytoskeleton is an active network that robustly self-organizes into marginally stable mechanical state akin to that in an SGM or jammed solid.

Host: Eric Corwin


Date:  Thursday, October 25th, 2018
Speaker: Mark Kasevich, Stanford

Title: Quantum optimal measurements for clocks and microscopes

Abstract: When and how can quantum entanglement be exploited as a resource to improve measurement precision? This talk will discuss this question in the context of two sensing scenarios:  i) precision atomic clocks [1] and ii) phase contrast optical and electron microscopy [2].  In the first case, massively entangled atomic states have been exploited to realize nearly 20 dB metrological improvement in atomic clock precision.  In the second, quantum optimal performance is obtained without entanglement, but rather through the use of a mulit-pass measurement protocol.   In either case, quantum optimal measurements show promise of enabling a new generation of sensors with at least 10-fold improved performance.  Future applications, which include include low-damage imaging of nm-scale proteins and new tests of quantum mechanics and gravitation, will be described.

[1]  Hosten, O., et al.  “Measurement Noise 100 Times Lower than the Quantum-Projection Limit Using Entangled Atoms.” Nature 529 (Jan. 2016).

[2]  Juffmann, T., et al., “Multi-Pass Microscopy.” Nature Communications 7 (Sept. 2016).

Host:  Benjamin McMorran


Date:  Thursday, October 18th, 2018
Speaker: David P. Landau, Center for Simulational Physics, The University of Georgia

Title: Exploring Complex Free Energy Landscapes with Innovative Monte Carlo Simulations

Abstract: Complexity is everywhere in nature, and it often manifests itself in the existence of a rough free energy landscape that is extraordinarily difficult to investigate.  Computer simulations have become the method of choice for studying a wide variety of systems, but traditional algorithms fail when the free energy has multiple minima and maxima that may be widely separated in phase space.  We will introduce a generic, parallel Wang-Landau Monte Carlo sampling method[1] that is naturally suited for implementation on massively parallel, petaflop supercomputers. The approach introduces a replica-exchange framework involving densities of states that are determined iteratively for overlapping windows in energy space, each via traditional Wang-Landau sampling.  The framework is valid for models of soft and hard condensed matter, including systems of biological interest.  The significant scalability, performance advantages, and general applicability of the method are demonstrated using thousands of computing cores for several quite different models of interacting particles.  Systems studied include those possessing discrete as well as those with continuous degrees of freedom, including those with complex free energy landscapes and topological constraints.

[1] T. Vogel, Y. W. Li, T. Wüst, and D. P. Landau, Phys. Rev. Lett. 110, 210603 (2013); Phys. Rev. E 90, 023302 (2014).

Host:  Jayanth Banavar


Date:  Thursday, October 11th, 2018
Speaker: Valentin Taufour, UC Davis

Title: Quantum tricritical points, quantum wing critical points and more in the phase diagram of metallic quantum ferromagnets

Abstract: Studies of the temperature-pressure (T-p) phase diagram of metallic quantum ferromagnets have revealed that ferromagnetic quantum criticality is avoided in two ways [1]: either the transition becomes of the first-order at a tricritical point before being suppressed such as in UGe2 [2,3], or a transition to modulated magnetic phases appear such as in LaCrGe3 [4]. We have shown that the addition of a magnetic field (H) can restore quantum criticality at the end of “wings” in the T-p-H phase diagram in both UGe2 and LaCrGe3 [5]. Our careful study of the “wings” near the tricritical point reveal new rules that apply to the T-p-H phase diagram [6]. We discuss our experimental T-p-H phase diagrams of UGe2, LaCrGe3, and CeTiGe3 and how these compounds illustrate different strength of quantum fluctuations based on recent theoretical results [7].

[1] M. Brando et al. Rev. Mod. Phys., 88, 025006 (2016).

[2] V. Taufour et al. Phys. Rev. Lett. 105, 217201 (2010).

[3] H. Kotegawa et al. J. Phys. Soc. Jpn., 80, 8, 083703 (2011).

[4] V. Taufour et al. Phys. Rev. Lett. 117, 037207 (2016).

[5] U. S. Kaluarachchi et al. Nature Communications 8, 546 (2017).

[6] V. Taufour et al. Phys. Rev. B 94 060410 (2016).

[7] Belitz et al. Phys. Rev. Lett. 119, 267202 (2017)

Host:  Dietrich Belitz


Date:  Thursday, October 4th, 2018
Speaker: Alessandra Corsi, Texas Tech University

Title: Multi-messenger time-domain astronomy: GW170817 and the future

Abstract: On 2017 August 17, the field of gravitational-wave (GW) astronomy made the big leagues with a dazzling discovery. After several GW detections of black hole (BH)-BH mergers with no convincing electromagnetic (EM) counterparts, advanced LIGO and Virgo scored their first direct detection of GWs from a binary neutron star (NS) merger, an event dubbed GW170817. Soon after the GW discovery, GW170817 started gifting the astronomical community with an EM counterpart spanning all bands of the spectrum. In this talk, I will review what we have learned from GW170817 and its radio counterpart, what questions remain open, and what are the prospects for future radio plus GW studies of the transient sky.

Host:  Raymond Frey


Date:  Thursday, September 27, 2018
Speaker:   Richard Taylor, Department Head, UO Physics

Title: The state of the department, the state of the department head, and the state of human vision

Abstract: After three months with a new departmental head, I’m sure we are all curious to reflect on the current  ‘state’ of our department! In this talk, I will give you some qualitative and quantitative observations to help guide us in the year ahead. I’ve already written the first line: ”This is an exciting but complex time for our department.” If we have time, I will also talk about the state of human vision and the research done in this department to restore vision to those who have lost it.

Host:  Dietrich Belitz