Eric Sembrat's Test Bonanza

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While we would like to avoid frustration in our daily lives, frustration in condensed matter produces novel phenomena with important consequences. Materials that are geometrically frustrated cannot minimize the energy of every particle-particle interaction, even in their ground state; materials that are kinetically frustrated are trapped out of equilibrium, i.e., they cannot reach their ground state. In this talk I will review a series of experiments I have performed investigating the role of frustration in the assembly and phase behavior of colloidal systems. By tuning particle size, shape, and confinement, I systematically varied the degree of frustration in each experiment. This controlled approach enabled me to create geometrically frustrated colloidal "antiferromagnets," to observe the competition between crystallization and vitrification, and to learn how glasses age. These concepts also appeared in other non-equilibrium soft condensed matter experiments, from materials assembled on the surface of drying drops, to the directed assembly of protein-based structures.

Diffusion is an important physical process in many systems, from gases to biological cells. In the simplest case, a diffusing particle undergoes a random walk and its mean squared displacement increases linearly with time. In many cases, however, the diffusion is not ideal: The motion of the diffusing species can be restricted in a variety of ways, leading to  “sub-diffusion” in which the mean squared displacement increases more slowly. I will discuss experiments on the restricted diffusion of tracer particles in soft-matter systems close to a gel transition. Our experimental measurements provide information about the microstructure of the system and its evolution as the system gels on the bulk scale. Computer simulations help to clarify the interpretation of the data. I will also briefly discuss other systems in which restricted diffusion plays an important role.

Many years ago Crick and Klug suggested that sharp bends or kinks have to facilitate strong bending of the double-helix, and accurate theoretical analysis confirms this suggestion. It remains, however, to be determined what is the critical curvature of DNA that prompts the appearance of the kinks. Different experimental approaches to the problem will be briefly reviewed. Attention will be paid to influence of the torsional stress in the double helix on the kink formation. The most reliable data suggest that the stable kinks appear only in torsionally relaxed DNA circles smaller than 70 bp. It is possible that the kinks represent openings of isolated base pairs. Although the probability of these openings in long unstressed DNA molecules is close to 10^-5, it increases sharply in small DNA circles reaching 1 open bp per circle of 70 bp.

The control of quantum systems is limited by unwanted interactions with the environment and uncertainties in the applied control fields.  For ions these uncertainties include unwanted fluctuations in the intensity and the frequency of the electromagnetic field used to control the qubit. For unknown but static errors on the time scale of the experiment, compensating composite pulses sequences can be used to minimize the effect of these errors. In this talk, I will describe the general method of compensating composite pulse sequences for single qubit and multi-qubit systems. I will then discuss two experiments performed in collaboration with GTRI using composite pulse sequences. The first experiment uses known pulse sequences to effectively reduce the spatial variation in a microwave field. The second experiment tests a family of narrowband composite pulse sequences that we have developed. Narrowband pulse sequences can improve ion addressing in a chain by minimizing the effective rotation on neighboring ions.  The new pulse sequences are an improvement in both sequence time and cross talk minimization.
 

Most organisms live in aqueous environments and propel themselves by swimming. A large subclass is micro-organisms that have slender rod-like shapes, e.g. sperm. These organisms propel themselves using undulations that follow certain waveforms depending on the type of desired motion. At these small lengths scales and slow velocities, water behaves as a viscous fluid: the Reynolds number is small and Resistive Force Theory is a good approximation. Recently RFT has been extended to non-traditional types of fluids, such as dense granular matter, in order to model sand-swimming of undulating animals and robots.

The optimal planar undulatory strategy for a swimming filament in a viscous fluid is the sawtooth waveform, which was identified by Lighthill. Although this result was intended for infinite-length filaments, it also is applicable to finite-length filaments where the number of undulations is large, $U \gtrsim 10$. However the sawtooth's sharp kinks limits the applicability of Lighthill's result in nature and engineering applications, and thus we consider planar waveforms which have constrained curvatures, $| {\mathcal C} | \le {\mathcal C}_\mathrm{max}$. This naturally leads to the dimensionless number, $N = {\mathcal C}_\mathrm{max} S/(2\pi)$, which we call the winding number. 

We find that a piece-wise constant curvature function is optimal, which we determine for a range of winding numbers. These results for viscous fluids also transfer to sand-swimming, albeit the optimal choice for parameters is different.

The mechanisms governing the transfer of pathogens between infected and non-infected members of a population  are critical in  shaping the outcome of an  epidemic.  This is true whether one considers human,  animal or plant populations.  Despite major efforts aimed at the mathematical modeling and mitigation of infectious diseases, the fundamental mechanisms of pathogen spreading for most infectious diseases remain poorly understood.  I present here the results of  combined theoretical and experimental studies of the role of fluid dynamics and fragmentation in disease transmission.

The ATLAS Experiment at the Large Hadron Collider with its sister experiment CMS reported a discovery last summer of a new boson which is consistent with the Standard Model Higgs boson.  The Higgs particle has been searched for decades. It is the final jewel in the Standard Model of particle physics, a crowning achievement of 20th century science that gives a powerful understanding of fundamental particles and their interactions. In the Standard Model, the Higgs is the quantum of a field that accounts for the masses of those particles.  We will describe the apparatus, the data and other searches.