Eric Sembrat's Test Bonanza

Many different types of biological cells have the capability of sensing and generating mechanical forces.  These biophysical properties of cells are utilized for many different aspects of cell physiology, including cell migration and division as well as building multi-cellular assemblies.  To a large degree, the active mechanical behavior of cells is regulated by the filamentous actin (F-actin) cytoskeleton.  F-actin is a semi-flexible biopolymer that forms the basis of larger length scale structures in the cell through the action of other proteins that regulate assembly, cross-linking and force generation. 

Nearly all of these processes are driven far from thermal equilibrium by processes that rely on the consumption of chemical energy to regulate the spatial and temporal organization of network mechanics and force generation. To elucidate the physical properties of the actin cytoskeleton, we have studied the dynamics and biophysical properties of actin networks formed with myosin motors both in live cells and reconstituted networks of purified proteins.  A common feature among both actin/myosin and adhesive structures is that their stability and mechanics is highly tuned based on the amount of external tension.  This property enables rapid remodeling under low tension, but stabilizes the structures as forces are increases.   Thus, cellular materials provide insight into design principles that are utilized by highly adaptive matter.

The viruses that infect bacteria have a hallowed position in the development of modern biology, and once inspired Max Delbruck refer to them as "the atom of biology".  Recently, these viruses have become the subject of intensive physical investigation.  Using single-molecule techniques, it is actually possible to watch these viruses in the act of packing  and ejecting their DNA.    This talk will begin with a general introduction to viruses and their life cycles and will then focus on simple physical arguments about the forces that attend viral DNA packaging and ejection, predictions about the ejection process and single-molecule measurements of ejection itself.

The advent of x-rays sources with unprecedented intensity will enable the study of nonlinear physics in the high frequency regime. In 2009, a physicist dream became reality with the commissioning of the world’s first x-ray free-electron laser, the LCLS, at SLAC. In contrast to low frequency strong-field physics where valence electrons react to the optical field, at high frequency the atom will be ionized from the inside out. The question remains as to whether the atomic response to x-rays will be adequately described by low-order perturbation theory or necessitate a non-perturbative description which is more commonly used at low-frequency. In this talk, these issues will be raised along with the basics of x-ray free-electron laser operation and initial experiments performed at the LCLS.

As a student of numerical relativity who planned to work the rest of his life in academia, I had never envisioned the possibility of finding myself working in the commercial sector. The perception exists that opportunities to do novel research or to direct one's own career path are limited in industry, but I have found that this is largely not true. Rather, different constraints on your time and resources are imposed, with different challenges and rewards. In this talk, I will describe how I came to find myself leading the development of a cloud-scale information extraction and retrieval application for a customer within the Intelligence Community, share my experiences working with multiple small businesses on research and development projects, identify what skills and training carry over from an education in physics, and try to offer a frank look at what life in industry is really like.

At the small scale of a cell swimming in water, inertial effects are unimportant. Therefore, the motion of the fluid is governed by Stokes equations, which are linear. Nevertheless, there are many situations in which nonlinear effects are important. In this talk I will describe two such situations. The first is swimming in a viscoelastic material, which is motivated by the fact that many microorganisms move in non-Newtonian media such as mucus. I will present a simple model that shows how fading memory affects swimming speed. We will also present experimental results for a helix swimming in a viscoelastic fluid. The second situation I will consider is the synchronization of rotors via hydrodynamic interactions. This problem is motivated by the
observed coordination of nearby beating cilia.

Cavity optomechanics is rapidly developing into a major area of research.  This is a result of the developments from two converging perspectives on the physical world. From the top down perspective, ultra-sensitive micromechanical and nanomechanical detectors have become available utilizing the advanced materials and processing techniques of the semiconductor industry and nanoscience. These devices are capable of probing extremely tiny forces, often with spatial resolution at atomic scales.  From the bottom-up, we have gained a deep understanding from quantum optics and atomic physics of the mechanical effects of light, in particular in the context of laser cooling, and of how quantum mechanics limits the ultimate sensitivity of measurement through quantum back-action. Cavity optomechanics is a mix of these two approaches, with ultra-sensitive mechanical elements from the top-down, and the deep understanding of the mechanical effects of optical fields from the bottom-up.

The talk will review key recent developments of this field and speculate on promising directions in which it is likely to evolve.

This colloquium will discuss recent theoretical predictions and experimental confirmation of two unusual classes of ultra-long-range molecules. One class involves a highly-excited Rydberg electron that manages to bind a distant ground state atom at thousands of Bohr radii. The other class involves the bizarre Efimov effect for three or four ground state atoms with resonant interactions. Implications for the behavior of ultracold quantum gases will be addressed.

Photons do not interact strongly in nature, and have thus been relegated to a role as a tool rather than an object of study in condensed matter physics.
However, in cavity quantum electrodynamics, the strong interaction of light with a single atom can lead to strong atom-mediated photon-photon
interactions, even when the light and atomic transitions are not resonant. Recent theoretical proposals have predicted phase transitions in arrays of
these cavities, demonstrating that complex matter-like phenomena can emerge from a sea of interacting photons.

I will present our recent measurements demonstrating strong photon-photon interactions in superconducting cavity QED.  Here, we observe a photon
blockade effect, where the presence of one photon blocks further transmission through the cavity when probing the system with a method in direct analogy to electron transport measurements in quantum dots.  I will also present preliminary measurements of cavity arrays, and discuss prospects for observing phase transitions and effects of broken time reversal symmetry in these arrays.  I will conclude by briefly presenting work on a new superconducting qubit which allows for a tunable vacuum Rabi coupling, and thus in situ tunability of photon-photon interactions.

Twistor theory is now over 45 years old. In December 1963, I proposed the initial ideas of this scheme, based on complex-number geometry, which presents an alternative perspective to that of standard 4-dimensional space-time, for the basic arena in which (quantum) physics takes place. Over the succeeding years, there were numerous intriguing developments. But many of these were primarily mathematical, and there was little interest expressed by the physics community. Things changed rather dramatically, in December 2003, when E.Witten produced a 99-page article initiating the subject of “twistor-string theory” this providing a novel approach to high-energy scattering processes. In this talk, I shall provide an account of the original geometrical and physical ideas, and also outline various recent developments, some of which may help our understandings of the seeming paradoxes of quantum mechanics.