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April 18, 2014

Arpil 24 Colloquium

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Jeff Lundeen, University of Ottawa

Seeing is Believing: Direct Observation of a General Quantum State

Host: Michael Raymer

Abstract: Central to quantum theory, the wavefunction is a complex distribution associated with a quantum system. Despite its fundamental role, it is typically introduced as an abstract element of the theory with no explicit definition. Rather, physicists come to a working understanding of it through its use to calculate measurement outcome probabilities through the Born Rule. Tomographic methods can reconstruct the wavefunction from measured probabilities. In contrast, I present a method to directly measure the wavefunction so that its real and imaginary components appear straight on our measurement apparatus. At the heart of the method is a joint measurement of position and momentum that is made possible by weak measurement (a concept that I will attempt to demystify). I will describe an experimental example of the method in which we directly measured the transverse spatial wavefunction of a single photon. New experimental work extending this to mixed states will be presented as well. Our direct measurement method gives the wavefunction a plain and general meaning in terms of a specific set of operations in the lab.

Related Papers:

Nature, 474, 188 (2011). http://arxiv.org/abs/1112.5471

Phys. Rev. Lett. 108, 070402 (2012). http://arxiv.org/abs/1112.3575

Physical Review Letters, 112, 070405 (2014). http://arxiv.org/abs/1309.1491

April 11, 2014

April 17 Colloquium

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Alex Small, Cal State Panoma

Host: Raghu Parthasarathy

Theoretical Limits to Superresolution Fluorescence Microscopy

Superresolution microscopy techniques enable fluorescence imaging of biological specimens with sub-wavelength resolution, down to as low as 10 nanometers in some cases.  Interestingly, in essentially all techniques considered thus far, the resolution scales as the wavelength of light divided by the square root of some measure of the number of photons used.  We have developed a proof that the relationship between image resolution and measures of photon count has a universal 1/sqrt(photon count) scaling for very wide classes of superresolution techniques.  Additionally, we have developed a theoretical framework for benchmarking the performance of the algorithms used to analyze the images, and have developed fast image analysis algorithms and mathematical approaches for optimizing image acquisition.

April 7, 2014

April 10 Colloquium

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Marc Janoscheck, Los Alamos National Laboratory

Investigating Strongly Correlated Electron Materials by Means of Neutron Scattering

Abstract: Materials governed by strong electronic correlations exhibit a broad set of interesting properties and ground states, such as heavy fermion behavior, complex magnetism, the Mott insulator, and unconventional superconductivity, and furthermore show potential for application spanning from thermoelectricity to spintroncis. Here it is generally believed that being able to manipulate and control these states that emerge through the cooperative behavior of in the order of 1023 electrons is critical to develop tomorrow’s devices and applications. Using two examples we will demonstrate that neutron scattering represents a powerful tool in shedding light on the properties of such complex materials.
First, we will report our work on the B20 helimagnet MnSi. The discovery of a new topological form of magnetic order in the family of cubic Dzyaloshinskii-Moriya helimagnets crystallizing in the B20 structure has recently attracted increasing scientific interest. This new magnetic state consists of a lattice of skyrmions, i.e., magnetic vortices characterized by a topological winding number [1], which is however only thermodynamically stable in a little pocket of the phase diagram at finite field close to the critical temperature Tc. In the skyrmion phase of MnSi spin transfer torques have been observed at record low current densities j = 106A m-2 making these materials promising for spintronic applications [2]. Here it was argued in Ref. [1] that the skyrmion lattice phase only exists due to strong renormalizations attributed to thermal fluctuations, which in turn motivated our detailed small angle neutron scattering (SANS) study of the magnetic fluctuations in MnSi. Our study demonstrates that the properties of MnSi are indeed governed by strongly-interacting critical fluctuations, resulting in a fluctuation-induced first-order transition at Tc. Notably, we show that within a self-consistent Hartree-Fock-Brazovskii theory the critical magnetic fluctuations determined by our SANS measurements quantitatively reproduce the observed specific heat and magnetic susceptibility data [3]. While our study contributes to our better understanding of the skyrmion-lattice compounds, we note that fluctuation-induced first-order transitions are of interest for a wide range of topics such liquid crystals, superconductors, cold atom systems or even phase transitions in the early universe (see references in Ref. [3]).

In the second part of the talk, we turn to our recent inelastic neutron scattering of δ-Pu. Pu is arguably the most complex elemental metal known because its 5f electrons are tenuously poised at the edge between localized and itinerant configurations [4]. Despite its 60-year old history, our collective understanding of this complicated element is still vague. However, it is clear that Pu’s complex structure leads to emergent behavior—all a direct consequence of its 5f electrons—including six allotropic phases, large volumetric changes associated with these transitions of up to 25%, and mechanical properties ranging from brittle α-Pu to ductile δ-Pu [5]. Pu also exhibits a Pauli-like magnetic susceptibility, electrical resistivity and a Sommerfeld coefficient of the specific heat that are an order of magnitude larger than in any other elemental metal [6]. Finally, while experiments find no sign for static magnetism in Pu, most theories that use the correct volume predict a magnetically ordered state [4]. This discrepancy might be reconciled by recent DMFT calculations that suggest that Pu fluctuates between localized and itinerant valence configurations [7]. Our measurements demonstrate the existence of magnetic fluctuations centered around 84 meV, in agreement with these calculations putting the understanding of the “missing” magnetism and electronic structure of Pu within reach for the first time.
[1] S. Mühlbauer et al., Science 323, 915 (2009) [2] F. Jonietz et al., Science 330, 1648, (2010); T. Schulz et al. Nature Physics 2231 (2012). [3] M. Janoschek et al., Phys. Rev B 87, 134407 (2013). [4] A. M Boring and J. L. Smith, in Challenges in Plutonium Science Vol. I, Los Alamos Science, No. 26, p. 91. (2000). [5] J. L. Smith, and E. A. Kmetko, J. Less-Common Met. 90, 83 (1983). [6] J. C. Lashley et al., Phys. Rev. B 72, 054416 (2005). [7] J. H. Shim , K. Haule, G. Kotliar Nature 446, 513–516 (2007).

Host: Dietrich Belitz

March 27, 2014

April 3 Colloquium

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John Toner, University of Oregon

Swarming in the dirt: Flocking in the presence of quenched disorder

Abstract: The effect of quenched (frozen) orientational disorder on the collective motion of active particles is analyzed. We find that, as with annealed disorder (Langevin noise), active polar systems are far more robust against quenched disorder than their equilibrium counterparts. In particular, long ranged order (i.e., the existence of a non-zero average velocity <\vec{v}>) persists in the presence of quenched disorder even in spatial dimension d=3, while it is destroyed even by arbitrarily weak disorder in d \le 4 in equilibrium systems. Furthermore, in d=2, quasi-long-ranged order (i.e., spatial velocity correlations that decay as a power law with distance) occur when quenched disorder is present, in contrast to the short-ranged order that is all that can survive in equilibrium.  These predictions are born out by simulations in both 2 and 3 dimensions.

 

March 7, 2014

March 13 Colloquium

Kwonmoo Lee, Harvard Medical School

Integrating Protein Dynamics and Cell Mechanics: Mechanosensitivity of Dynamic Actin Remodeling During Cell Migration

Migrating cells are mechanochemical machines where biochemistry and mechanics integrate to control cellular motion. The polymerization of the filamentous protein called actin provides force generation for membrane protrusion, an initial step of cell migration. In turn, mechanical force sensed by cells leads to dynamic responses of the intracellular biochemical pathways. The dissection of systems with such mechanochemical pathways has remained a fundamental challenge in biological investigation.

To unravel the complex nature of interplay between biochemistry and mechanics in space and time, we developed a novel statistical approach based on local image sampling and registration to directly visualize the dynamics of molecular/cellular events at the subcellular level. This method revealed distinct dynamics of molecular factors and traction force involved in cell protrusion, allowing us to establish in situ their differential functions. This quantitative framework also allowed us to exploit natural heterogeneity of cell protrusion to extract statistical relationships between different molecular/cellular events. Using this method we suggest that linear actin filaments along with adhesion formation initiate new protrusions. Then, the linear filaments undergo a structural transition to branched networks, mediated by exponential actin polymerization. Furthermore, we showed that increasing membrane tension during protrusion promotes this structural tension which leads to stronger force generation to support edge advancement against the membrane tension. This suggests that migrating cells can reorganize their biochemical machinery in response to mechanical cues in a highly dynamic manner at a time scale of 10 seconds. It also highlights that mechanical processes are tightly integrated with biochemical pathways and modulate protein dynamics, thereby playing critical roles in cell physiology.

REFRESHMENTS:  3:40 p.m. in the Willamette Atrium COLLOQUIUM:   4:00 p.m. in Willamette 100

March 5, 2014

March 6 Colloquium

Wolfgang Altmannshofer, Perimeter Institute

The Flavor Puzzle

The known basic building blocks of matter, the quarks and leptons, come in three generations or flavors.
The masses and interactions of the different flavors show a very hierarchical structure and the origin of these hierarchies remains an unsolved mystery of particle physics.The same hierarchies lead to a very high sensitivity of flavor changing processes to new undiscovered particles even outside the reach of direct searches at particle colliders.In this colloquium I will discuss the status of our understanding of flavor and highlight the complementarity of flavor and collider physics in searching for new phenomena at the TeV scale and beyond.

 

REFRESHMENTS:  3:40 p.m. in the Willamette Atrium COLLOQUIUM:   4:00 p.m. in Willamette 100

February 28, 2014

March 4 Colloquium

Timothy Cohen, SLAC

What Light Can Teach Us About Dark Matter

Consistency between big bang cosmology and precision data requires that about 80% of the matter in our Universe consists of a new particle — the dark matter.  Uncovering the identity of this state is one of the driving motivations for exploring theories beyond the Standard Model of particle physics.  One compelling hypothesis is that the dark matter is a massive particle whose only interactions occur via gravity and the weak force.  This so-called WIMP paradigm leads to a wide variety of experimental signatures.  In this colloquium, I will argue that one compelling way to explore WIMP models is by analyzing measurements of cosmic ray photons.  I will provide a pair of case studies to demonstrate the kind of physics that can be extracted from gamma-ray data.  The first is an analysis of the Fermi line at 130 GeV, with an emphasis on the implications for WIMP models; the second demonstrates that data from H.E.S.S. can be used to constrain compelling models for multi-TeV WIMPs.  These examples will illuminate how much there is to learn about the nature of dark matter by studying light.

REFRESHMENTS:  3:40 p.m. in the Willamette Atrium COLLOQUIUM:   4:00 p.m. in Willamette 100

February 26, 2014

February 27 Colloquium

Hernan Garcia, Princeton University

How, When and Where in Pattern Formation: Spying on Embryonic Development One Molecule at a Time

An abiding mystery in the study of living matter is how a single cell develops into a multicellular organism. As this cell divides, its progeny read the program encoded on their DNA and adopt different fates becoming familiar cell types such as those found in muscle, liver and our brains. We now know that the decisions that cells make during development are not so much based on which genes to express, but rather on when, where and how to express them. Despite advances in determining the identities of the molecules that mediate these decisions we are still incapable of predicting how simple physical parameters such as the number, position and affinity of binding sites for these molecules on the DNA determine developmental fates. Using the fruit fly, one of the classic model systems for embryonic development, I will show how a combination of new technologies, quantitative experiments, and statistical mechanics is providing new insights about cellular decision making during development. In particular, I will describe how the specification of macroscopic body parts in an organism is linked to the non-equilibrium molecular-scale processes inside single cells. The goal of this interdisciplinary research is to produce a predictive understanding of developmental programs which will enable the rational control of biological size, shape and function.

  •  REFRESHMENTS:  3:40 p.m. in the Willamette Atrium
  • COLLOQUIUM:   4:00 p.m. in Willamette 100
February 22, 2014

February 25 Colloquium

Nathaniel Craig, Rutgers University

The Once and Future Higgs

The discovery of a Higgs boson at the LHC marks both the culmination of a decades-long quest for the final piece of the Standard Model and the dawn of a new era in the search for more fundamental physics. I’ll explore the ways in which the Higgs provides a powerful tool in the hunt for physics beyond the Standard Model through its production, propagation, and decays.

  •  REFRESHMENTS:  3:40 p.m. in the Willamette Atrium
  • COLLOQUIUM:   4:00 p.m. in Willamette 100
February 17, 2014

February 20 Colloquium

Takemichi Okui, Florida State University

Searching For Matter-Antimatter Asymmetry Through the Higgs Boson

Abstract: Despite its extraordinary success as a theory of the microscopic world, the standard model of particle physics fails to explain some crucial cosmological observations. An example of such is the fact that we only see matter but no antimatter in the universe, while the standard model is too symmetric between matter and antimatter. I will discuss how to probe a possible new source of matter-antimatter asymmetry in the properties of the newly-discovered Higgs boson in future LHC data.

  •  REFRESHMENTS:  3:40 p.m. in the Willamette Atrium
  • COLLOQUIUM:   4:00 p.m. in Willamette 100

 

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