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Winter 2016 Colloquium Series

Searching for Ultra-High Energy Cosmic Rays with Smartphones

Physics Colloquium, Thursday, March 10th, 2016

Speaker: Daniel Whiteson, University of California, Irvine


We propose a novel approach for observing cosmic rays at ultra-high energy (10^18 eV) by repurposing the existing network of smartphones as a ground detector array. Extensive air showers generated by cosmic rays produce muons and high-energy photons, which can be detected by the CMOS sensors of smartphone cameras. The small size and low efficiency of each sensor is compensated by the large number of active phones. We show that if user adoption targets are met, such a network will have significant observing power at the highest energies.

Host: Michael Raymer


Universality in Soft Active Matter

Physics Colloquium, Thursday, March 3rd, 2016

Speaker: Chiu Fan Lee, Imperial College, London


Biology systems operate in the far from equilibrium regime and one defining feature of living organisms is their motility. In the hydrodynamic limit, a system of motile organisms may be viewed as a form of active matter, which has been shown to exhibit behaviour analogous to that found in equilibrium systems, such as phase separation in the case of motility-induced aggregation, and critical phase transition in incompressible active fluids. In this talk, I will use the concept of universality to categorise some of the emergent behaviour observed in active matter. Specifically, I will show that i) the coarsening kinetics of motility-induced phase separation belongs to the Lifshitz-Slyozov-Wagner universality class [1]; ii) the order-disorder phase transition in incompressible polar active fluids (IPAF) constitutes a novel universality class [2], and iii)  the behaviour of IPAF in the ordered phase in 2D belongs to the Kardar-Parisi-Zhang universality class [3].


[1]          C. F. Lee, “Interface stability, interface fluctuations, and the Gibbs-Thomson relation in motility-induced phase separations,” arXiv: 1503.08674, 2015.

[2]          L. Chen, J. Toner, and C. F. Lee, “Critical phenomenon of the order-disorder transition in incompressible active fluids,” New Journal of Phyics, 17, 042002, 2015.

[3]          L. Chen, C. F. Lee, and J. Toner, “Birds, magnets, soap, and sandblasting: surprising connections to incompressible polar active fluids in 2D,” arXiv:1601.01924, 2016.

Host: Dietrich Belitz


Complexity of Multi-photon Interferometry

Physics Colloquium, Thursday, February 25th, 2016

Speaker: Hubert de Guise, Lakehead University


The simulation of many fully indistinguishable photons interfering at the output of a random interferometer with many more channels than photons supplies a link between computational complexity theory and linear optics.  This link is the basic ingredient of the BosonSampling problem, an idea originating with Aaronson and Arkhipov in 2011.

This colloquium will discuss some simple aspects of multi-photon interferometry, with emphasis on partial distinguishability of photons and its consequences on and connections with computationally complex functions of the scattering matrix.

Some work reported in this colloquium has been done in collaboration with colleagues from Calgary, Vienna and Singapore, and others.

Host: Michael Raymer


Shaking Up Statistical Physics in Interacting Quantum Systems

Physics Colloquium, Thursday, February 18th, 2016

Speaker: Anushya Chandran, Perimeter Institute


Statistical mechanics is a central pillar of modern science with applications ranging from sociology to economics. At its core is the idea of thermal equilibrium, which allows for a simple description of an interacting quantum system in terms of a few properties like temperature, without keeping track of the entire wavefunction. But what if a quantum system fails to equilibrate?

In this talk, I will discuss how we are discovering the answer to this question theoretically and experimentally. I’ll focus on two settings: disordered systems and periodically driven systems. In the former, many-body localization can prevent thermalization even at very high energy densities. The transition between the localized and the thermal phase is a fascinating dynamical quantum transition about which little is known. I will derive a rigorous constraint on this transition and apply it to current numerical studies and cold atomic experiments. Clean periodically driven systems, on the other hand, generically absorb heat indefinitely. I will present one physical setting of interacting bosons in which this expectation fails.

Host: Dietrich Belitz


Physics of Information Processing in Living Systems

Physics Colloquium, Thursday, February 11th, 2016

Speaker: Yuhai Tu, IBM T.J. Watson Research Center


Living organisms need to obtain and process information that are crucial for their survival. These information processes, ranging from signal transduction in a single cell to image processing in the human brain, are performed by biological circuits (networks). However, these biochemical or neural circuits are inherently noisy. Yet, certain accuracy is required to carry out proper biological functions. How do biological networks process information accurately and efficiently? What is the energy cost of biological computing? Is there a fundamental limit for its performance? In this talk, we will describe our recent work in trying to address these questions in the context of two basic cellular computing tasks: sensory adaptation for memory encoding [1,2]; biochemical oscillation for accurate timekeeping [3].

[1] “The energy-speed-accuracy trade-off in sensory adaptation”, G. Lan, P. Sartori, S. Neumann, V. Sourjik, and Yuhai Tu, Nature Physics 8, 422-428, 2012.
[2] “Free energy cost of reducing noise while maintaining a high sensitivity”, Pablo Sartori and Yuhai Tu, Phys. Rev. Lett. 2015. 115: 118102.
[3] “The free-energy cost of accurate biochemical oscillations”, Y. Cao, H. Wang, Q. Ouyang, and Yuhai Tu, Nature Physics 11, 772, 2015.

Host: Dietrich Belitz


Do two-dimensional metals exist?

Physics Colloquium, Thursday, February 4th, 2016

Speaker: Michael Mulligan, Stanford University

Conventional wisdom teaches us that electrons confined to a two-dimensional quantum well will do one of three things as the temperature is lowered to zero: superconduct, insulate, or exhibit the so-called quantum Hall effect. (Here, I am concentrating on the types of order as revealed in electrical charge transport; finer distinctions can be made, e.g., magnetic ordering.) Nature, however, is stubborn and doesn’t always listen. In this talk, I will describe two experimental systems that surprisingly appear to violate the conventional wisdom and instead exhibit a metallic phase at zero temperature. I will argue that there is a deep analogy between the two systems that relates their behaviors and discuss how such novel metallic phases can explain other unconventional low-temperature quantum orders.

Host: Dietrich Belitz


LSST: a color movie of the Universe coming near you!

Physics Colloquium, Thursday, January 28th, 2016

Speaker: Željko Ivezić,, University of Washington


The Large Synoptic Survey Telescope (LSST) will carry out an imaging survey covering the sky that is visible from Cerro Pachon in Northern Chile, with first light in 2019. With close to 1000 observations in ugrizy bands over a 10-year period, this data will enable deep coadded maps across half the sky reaching hundred times fainter flux level than the Sloan Digital Sky Survey (SDSS). About 20 billion galaxies and a similar number of stars will be detected in these maps — for the first time in history, the number of cataloged celestial objects will exceed the number of living people. The time-resolved observations will open a movie-like window on objects that change brightness, or move, on timescales ranging from 10 seconds to 10 years. With a raw data rate of about 15 TB per night (about the same as one SDSS per night), LSST will collect over 100 PB of data over its lifetime, resulting in an incredibly rich and extensive public archive that will be a treasure trove for breakthroughs in many areas of astronomy and physics, ranging from the properties of near-Earth asteroids to characterizations of dark energy and dark matter. I will provide an overview of the main science drivers and a status report for the federally-funded construction project that started in 2014.

Host: Spencer Chang


Emerging Phases and Phase Transitions in Quantum Matter

Physics Colloquium, Thursday, January 21st, 2016

Speaker: Thomas Vojta
Missouri University of Science and Technology


Condensed matter physics deals with the complex behavior of
many-particle systems. Novel phases of matter can emerge as a
result of strong interactions between the constituent
particles. A natural place to look for these phenomena are
quantum phase transitions, the boundaries between different
quantum ground states of matter.

This talk first gives an introduction into quantum phase
transitions and then discusses several novel phases of matter
that have been discovered in their vicinity in solids and in
ultracold atomic gases. These include exotic superconductors
and magnets as well as Griffiths phases that are dominated
by strong disorder.

Host: Dietrich Belitz


Non-perturbative Results for Itinerant Ferromagnetism in Multi-orbital Systems

January 14th, 2016

Speaker: Yi Li
Princeton Center for Theoretical Science, Princeton University

Title: Non-perturbative Results for Itinerant Ferromagnetism in Multi-orbital Systems


Itinerant ferromagnetism (FM) is intrinsically a strongly correlated phenomenon, which remains a major challenge of condensed matter physics. Most FM materials are orbital-active with prominent Hund’s coupling. However, the local physics of Hund’s rule usually does not lead to the FM long-range order. Furthermore, the magnetic phase transitions of itinerant electrons are also long-standing problems difficult to handle by using perturbative methods. In this talk, I will present non-perturbative studies on itinerant FM. Exact theorems are established for a stable itinerant FM phase in a large region of electron densities in multi-orbital systems, which provide sufficient conditions for Hund’s rule to build up global FM coherence. In addition, thermodynamic properties and magnetic phase transitions of itinerant electrons are studied via sign-problem-free quantum Monte Carlo simulations at generic fillings. Without introducing local moments as a priori, the Curie-Weiss metal behavior is identified in a wide range of temperatures. These results will provide important guidance to the current experimental search for novel itinerant FM states in a large class of systems ranging from the transition-metal-oxide heterostructures (e.g. LaAlO3/SrTiO3) to the p-orbital bands in optical lattices filled with ultra-cold fermions.


Topological Design of Mechanical Metamaterials

January 7th2016

Speaker: Jayson Paulose, Leiden University, the Netherlands

Topological phenomena lie at the forefront of condensed matter physics. When a physical observable is linked to a topological index characterizing a system, it is unaffected by local changes and thus robust against a range of perturbations. This concept of topological protection, originally developed in electronic systems, has recently been applied to mechanical systems as well. In this talk, I’ll show how a recent mapping between spring lattices and electronic topological insulators can be exploited to design topologically protected mechanical response in artificial repetitive structures, or metamaterials. First, I’ll demonstrate localized flexible regions in otherwise rigid lattices that arise due to an interplay between crystal defects and a bulk topological index characterizing lattice vibrations. I’ll then show how topological stress states single out regions for buckling in the interior of a structure. Finally, I’ll extend the mapping from central-force spring networks to a broader class of structures with more complex couplings among elements, encompassing gear networks and frictional disc packings. The results will be demonstrated in real-world prototypes of topological mechanical metamaterials, and point towards new ways of designing robust mechanical phenomena across different scales.