The David and Edith Harris Physics Colloquium Archives

FALL 2013 Schedule

IBM Almaden Research Center
Hosted by Graduate Women in Physics

"First-principles Studies of Atomic-Scale Engineered Spin Chains"

We have studied the unusual charge and spin properties of magnetic atoms (Mn, Co, Fe, Ti, Gd) on a complex surface as constructed by STM. This surface, a monolayer of N or MgO, was designed to be insulating in order to inhibit the Kondo effect. However, the magnetic adatom may be drawn down into the surface, or stay high above and attract surface atoms to it, with very different resulting properties. We show illustrations from our electronic structure calculations of these systems. The various magnetic atoms exhibit behavior ranging from spin chains to large-anisotropy atomic-scale molecular magnets to a Kondo effect for Co and Ti. Finally, when two magnetic atoms are close to one another, their magnetic spins can intevract, with complex and interesting results. We calculate the behavior of dimers, with an excellent match to experimental values of the exchange coupling. We moreover show that the coupling can be decomposed into three different pairing interactions and how to extract the values of each separately, with unexpected contrasts between two binding sites. I will conclude with some comments about the role of first-principles calculations for nanostructures.

SEPTEMBER 12, 2013
University of Wisconsin-Madison
Hosted by Markus Klute

"First Observation of High-Energy Neutrinos with IceCube"

Cosmic rays are relativistic charged particles continuously raining down on Earth’s atmosphere from outer space.  They have been detected with energies that range over more than 12 orders of magnitude and extend to tens of joules per particle, making them the highest energy particles ever observed.  More than a hundred years after their discovery, their origins remain a mystery. Neutrinos, as neutral tracers of hadronic acceleration, may offer a new and unique window into this problem.  IceCube, a cubic kilometer-size neutrino detector frozen into the glacial ice sheet at the geographic South Pole, has recently observed several extremely high-energy neutrinos with energies up to several PeV that cannot easily be explained by processes occurring in cosmic-ray showers in the Earth's atmosphere.  These events may represent the first evidence for a population of high-energy neutrinos of extraterrestrial origin.  In this talk, I will discuss these results and give a general overview of the IceCube detector and its physics goals.

SEPTEMBER 19, 2013
Hosted bt Wolfgang Ketterle

"One-photon transistor and very attractive slow photons"

Photons are boring; they all move at one speed and do not interact with one another. I will present an unusual optical medium that is nonlinear at the quantum scale. In this medium, photons travel slowly, acquire mass, and exhibit mutual attraction - so strong that two photons can even form a two-body bound state. I will also present an all-optical transistor where a single photon switches on and off another light beam containing hundreds of photons.

SEPTEMBER 26, 2013
Hosted by Robert Redwine

"First two years of the Alpha Magnetic Spectrometer on the International Space Station - The latest Results"

The Alpha Magnetic Spectrometer (AMS) is an MIT-led precision magnetic spectrometer on the International Space Station.  It takes 19 years and 60 institutes from 16 countries to construct this particle physics detector for space. AMS was installed on the Space Station in May 2011. It has collected 40 billion cosmic rays up to energy of TeV.  The construction of the detector and the latest results of AMS will be presented.

OCTOBER 3, 2013
McGill University
Hosted by Deepto Chakrabarty

"The Cosmic Gift of Neutron Stars"

Though thousands of light years away, neutron stars can act as precise cosmic beacons, a cosmic gift that sheds light on some of the most interesting problems in modern science. In this talk I will introduce these bizarre objects, and show how astronomers are using them to study topics which range from the origins of the Universe to the very nature of matter.

OCTOBER 10, 2013
University of Vienna and Austrian Academy of Sciences
Hosted by David Pritchard

"Recent Experiments with Entangled Photons"

Recent Experiments with photon entanglement discussed include long-distance teleportation and entanglement swapping between two Canary Islands, entanglement in the orbital angular momentum of photons with 300 ħ units of angular momentum, entanglement in a very high-dimension discrete Hilbert space, entanglement of GHZ states over large distances, and closing the fair sampling loophole in Bell inequality experiments. Recalling my time at MIT in the 1970s and 1980s I will reflect on how the early neutron interference experiments performed then led us to ideas in quantum entanglement which are used in quantum communication concepts.

OCTOBER 17, 2013
ETH Zurich
Hosted by Edward Farhi

"Validating Quantum Devices"

About a century after the development of quantum mechanics we have now reached an exciting time where non-trivial devices that make use of quantum effects can be built. While a universal quantum computer of non-trivial size is still out of reach there are a number commercial and experimental devices: quantum random number generators, quantum encryption systems, and analog quantum simulators. In this colloquium I will present some of these devices and validation tests we performed on them. Quantum random number generators use the inherent randomness in quantum measurements to produce true random numbers, unlike classical pseudorandom number generators which are inherently deterministic. Optical lattice emulators use ultracold atomic gases in optical lattices to mimic typical models of condensed matter physics. Finally, I will discuss the devices built by Canadian company D-Wave systems, which are special purpose quantum simulators for solving hard classical optimization problems.

OCTOBER 24, 2013
Reed College
Hosted by MIT Society of Physics Students

"Hidden Momentum"

Electromagnetic fields carry energy, momentum, and even angular momentum. The momentum density is ε0(E×B), and it accounts (among other things) for the pressure of light. But even static fields can harbor momentum, and this would appear to contradict a general theorem: if the center of energy of a closed system is at rest, then its total momentum must be zero. Evidently in such cases there lurks some other momentum, not electromagnetic in nature, which cancels the field momentum. But finding this “hidden momentum” can be surprisingly subtle. I’ll discuss a particularly nice example.

October 31, 2013
Hosted by Matthew Evans

"Internal Structure of the Moon from the Gravity Recovery and Interior Laboratory (GRAIL) Mission"

The Gravity Recovery and Interior Laboratory (GRAIL) is a twin-spacecraft lunar gravity mission that has two primary objectives: to determine the structure of the lunar interior, from crust to core; and to advance understanding of the thermal evolution of the Moon. GRAIL mapped the Moon from March through December 2012 at average altitudes from 55 km down to 11 km. The current global gravity field has a spatially resolves blocks of 5 km and observations have been improved in quality by as much as a factor of 106 over previous lunar gravity models. The internal structure of the Moon represents a significant uncertainty in models of the dynamics of the Earth-Moon system and improvements in the Moon’s gravitational moments and tidal parameters have reduced substantially the range of plausible models of the lunar interior. GRAIL has provided a precise assessment of the lunar impact record, enabling major improvements in understanding of the bombardment history of the Moon, and by extension, Earth.

NOVEMBER 7, 2013
Max Planck Institute of Melecular Cell Biology and Genetics
Hosted by Jeremy England

"The Generation of Form in Biology: Principles of Mechanochemical Patterning"

Morphogenesis refers to the generation of form in Biology. Much is known about molecular mechanisms of regulation, but little is known about the physical mechanisms by which an unpatterned blob of cells develops into a fully structured and formed organism. The actomyosin cortex is a thin layer underneath the cellular membrane that can self contract, which drives many of the large-scale morphogenetic rearrangements that are observed during development. How this cortex reshapes and deforms, and how such morphogenetic processes couple to regulatory biochemical pathways is largely unknown. I will discuss two emergent physical activities of the actomyosin cytoskeleton, an active contractile tension and an active torque, both of which can serve to drive flows and large-scale chiral rotations of the actomyosin cytoskeleton. Discussing two biological examples, polarization of the Caenorhabditis elegans zygote and epiboly during zebrafish gastrulation, I will illustrate how active tension drive flows, how molecular constituents of the cortex affect flows, and how morphogenetic patterns can be formed by coupling regulatory biochemistry to active cortical mechanics. A particular focus will be the investigation of how active chiral torques drive chiral flow, and the resulting functions of such chiral activities of the actomyosin cytoskeleton for left-right symmetry breaking in development.

NOVEMBER 14, 2013
Max-Planck Institute of Quantum Optics and Ludwig Maximilian University of Munich
Hosted by PGSC

"Controlling and Exploring Quantum Matter at the Single Atom Limit"

Over the past years, ultracold quantum gases in optical lattices have offered remarkable opportunities to investigate static and dynamic properties of strongly correlated bosonic or fermionic quantum many-body systems. In this talk, I will show how it has recently not only become possible to image such quantum gases with single atom sensitivity and single site resolution, but also how it is now possible to coherently control single atoms on individual lattice sites and to reveal the presence of individual quantum fluctuations of the many-body system. This unique control has recently allowed us to realize novel quantum crystals of matter using Rydberg atoms. I will also demonstrate how 'Higgs' type excitations occur at 24 orders of magnitude lower energy scales than in high energy experiments and how they can be detected in our experimental setting. Finally, I will show how the unique control over ultracold quantum gases has enabled the creation of negative temperature states of matter and thereby the realization of Bose-Einstein condensation at absolute negative temperatures.

NOVEMBER 21, 2013
Hosted by Peter Fisher

"What’s the Matter with the Universe?"

Dark matter makes up 85% of the mass of the Universe, yet we know very little about what it is. We hunt for dark matter in an old iron mine half a mile underground using detectors operating only thousands of a degree above absolute zero. I will present results from our Cryogenic Dark Matter Search (CDMS) experiment that show a small excess of events which could be interpreted as a possible hint of dark matter, although more data is required. Although the composition of the other 15% of the mass in the Universe is understood, plenty of questions about its origin and evolution remain. I will introduce the Micro-X sounding rocket, a high-spectral resolution x-ray telescope which will study supernova remnants, the fascinating relics of stars whose explosive deaths gave birth to most of the atoms that form our planet.

DECEMBER 5, 2013
University of Oxford
Hosted by Jeremy England

"Nature’s engines: Powering life"

Active systems, from cells and bacteria to flocks of birds, create their own energy that they use to move and to control the complex processes needed for life. A goal of biophysicists to is construct the new theory needed to understand these living systems which operate far from equilibrium. To this end we are asking questions like: How do biomolecules find their way across crowded cells? How do birds, bacteria and cells self-organize into similar patterns? Can tiny marine creatures stir the ocean?