Seminars:Spring 2009


Department of Physics and Astronomy

George Mason University, Fairfax, VA 22030

Location: Room 201, Research I (unless otherwise noted)

Day and Time: Friday, 11:30-12:30 (unless otherwise noted)


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Driving Directions to GMU

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Visitor parking decks (printable map)


Thursday, January 22 (note date change), 10:00 AM (note time change), 310 Science & Technology I (note location change):

Sophia E. Economou, Naval Research Lab

Optically controlled spins in quantum dots

Recent years have witnessed a lively pursuit of feasible quantum information processing schemes that are cross-disciplinary in nature. One such scheme holding considerable promise realizes the quantum bit as a single electron spin trapped in a quantum dot and controlled optically. These systems are not only relevant to quantum technologies but also offer a unique realm in which to study new physics stemming from the interplay of solid state, atomic, and optical physics. I will briefly give an overview of this field, along with the goals that need to be met for quantum information processing. In the main part of the talk, I will present theoretical and experimental results on the various aspects of optical manipulation, in particular spin initialization, spin rotation, and two-qubit conditional control. I will conclude with a discussion of some remaining challenges and future directions.


Friday, January 23, 11:00 AM, (note time change), 310 Science & Technology I (note location change):

Michael Peterson, University of Maryland

Reality of the topological non-Abelian Pfaffian description of the FQHE at filling factor 5/2

The fractional quantum Hall effect (FQHE) is one of the most important discoveries in physics in the past 25 years and is the quintessential example of a many-body strongly interacting system. In the FQHE, we see the emergence of fractional charged excitations, excitations with fractional statistics, and even the likely existence of non-Abelian quasiparticles--an implementation of a fault tolerant topological quantum computer using these non-Abelian quasiparticles has recently been proposed (Das Sarma, Nayak, Freedman, PRL 94, 166802(2005)). I discuss the actual physical reality of the proposed topological non-Abelian Moore-Read Pfaffian description of the FQHE in the half-filled second Landua level (filling factor 5/2), a FQHE state that is still mysterious nearly 20 years after its discovery. Our work (Peterson, Joliceour, Das Sarma, PRL 101,016807(2008) and PRB 78,155308(2008)), taken along with other theoretical and experimental work, provides compelling ev idence that the Pfaffian description of the FQHE at 5/2 is indeed correct. We further consider the interesting spontaneous particle-hole symmetry breaking that possibly occurs upon the formation of the Pfaffian state (Peterson, Park, Das Sarma, PRL 101,156803(2008)). Time permitting, I will also discuss other work, outside the FQHE context, in ultra-cold atom optical lattice systems towards the experimental realization of the so-called d-Mott state (Peterson, Zhang, Tewari, Das Sarma, PRL 101,150406(2008)). We acknowledge support from Microsoft Project Q and DARPA.


Friday, January 30, 10:00 AM (note time change), JC 3rd Floor Meeting Rm E (note location change):

Erhai Zhao, University of Pittsburgh

A Tale of Two Fluids at Ten Nano-Kelvin

Recent spectacular advances in laser trapping and cooling of alkaline atoms have taken us to the coldest temperature ever achieved by mankind in the laboratory. Ultracold atoms offer us the opportunity to 1) crack some of the toughest problems in condensed matter physics, and 2) engineer brand-new quantum phases of matter. I will focus on a simple example in category 2): two species of Li atoms confined in a highly elongated trap (one-dimensional "tube"). These atoms would like to pair up, but are frustrated to do so due to a population imbalance. Moreover, the conventional wisdom of spin-charge separation breaks down in this case. I describe our struggle to understand the system in terms of two fluids, and show that a weakly coupled array of such gas tubes can realize an exotic superfluid which also has crystalline order.


Friday, January 30: 11:30, 104 Innovation Hall (note location change):

Eugenie Mielczarek, George Mason University

At the Nexus of Physics and Biology

Genetics determines the organism but all its functions are determined by gravity, electromagnetism, thermodynamics and quantum mechanics. In 2008 the National Academies of Sciences published –‘Inspired by Biology: From Molecules to Materials to Machines’, examining how research at the intersection of physics and biology will lead to new materials and devices, with applications ranging from nanotechnology to medicine. Professor Mielczarek will describe several of these systems and the physics which governs their motion. From the mechanics of fueling cells and plants, to the automotive-like clutches of E. coli bacteria and the ability of geckos to walk on the ceiling, the physics of these organisms is remarkable. This interdisciplinary seminar will be at a level understandable to both undergraduate and graduate science students.


Monday, February 2 (note date change), 10:00 AM (note time change), SUB II Room 1 & 2 (note location change):

Ryan Barnett, California Institute of Technology

New Physics in Multicomponent Cold Atomic Gases

Ultracold atomic gases have proven to be enormously rich systems to study from the many-particle perspective. In addition to being able to mimic hamiltonians used to study solid state systems, ultracold atomic systems can also exhibit novel quantum phases and dynamics with no counterpart in the solid state. After an overview of recent experimental work, I will discuss multicomponent gases which have been a focus of my recent work. The first example I will consider is a rotating condensate composed of two types of atoms with different masses. I will discuss the structure of the vortex configurations for such mixtures, and argue the existence of a counterintuitive phase where the two superfluids and external drive all rotate at different rates. The remainder of the talk will focus on the so-called spinor condensates. In optical traps the macroscopic spin configuration is determined by the spin-exchange interaction. I will describe a range of interesting effects arising from these extra degrees of freedom.


Friday, February 6, 10:00 AM, SUB II Room 4 (note location change):

Predrag Nikolić, Rice University

Strongly Correlated Physics With Ultra-Cold Atoms

The experimental realization of a Bose-Einstein condensate fourteen years ago marked the beginning of a new era in physics. Since then, strongly correlated many body systems are no longer bound to the field of traditional condensed matter, but have been studied also from the unique perspective of atomic physics. I will review some of the extraordinary progress in this field and discuss the current challenges it faces. My focus will be the unconventional phenomena of fermionic ultra-cold atoms with attractive interactions, which parallel much of the behavior of correlated electronic superconductors. I will discuss the novel strongly interacting regime experimentally found near the Feshbach resonance, and show how the powerful theory of quantum phase transitions can, even quantitatively, access the universal properties of this regime. Due to the simplicity of cold-atom systems, much can be learned from them about diverse phenomena such as: strongly correlated superfluidity, the evolution of lattice insulators with interaction strength, lattice supersolids, the Pauli limit and FFLO states, vortex lattices and correlated vortex liquids. I will present my theoretical results in these areas and make connections to the physics of high-temperature superconductors.


Tuesday, Feb 10, 11:00, ST1 310:

Daniel Braun, University Paul Sabatier, Toulouse, France

Decoherence-enhanced measurements

The idea of quantum-enhanced measurements (QEM) is to use quantum-engineered states, such as squeezed light, for precision measurements of classical system properties. These methods promise to beat the standard quantum limit (SQL), and to potentially achieve the Heisenberg limit of precision. However, QEMs are difficult to implement experimentally as the quantum states required are in general highly non-classical and very prone to decoherence. In this talk I will show that decoherence itself can be exploited to reach the Heisenberg limit, without the need to produce highly entangled states. We will discuss the underlying physical principles of this new type of precision measurements, and provide a detailed analysis for the example of the measurement of the length of an optical cavity, including experimentally relevant questions such as state preparation and read-out, sources and scaling of background noise, and the stability of the method under various perturbations.


Thursday, Feb 12, 11:00, 302, Research I:

Carlos A. R. Sa de Melo, Georgia Institute of Technology

The Evolution from BCS to Bose-Einstein Condensation: Superfluidity in Metals, Neutrons Stars, Nuclei, and Ultra-Cold Atoms

Superfluidity is a very interesting phenomenon that has been found in metals, neutron stars, nuclei and more recently in ultra-cold atoms. For a given metal, neutron star, or nuclei there is essentially “zero” tunability of the particle density or interaction strength, and thus superfluid properties can not be controlled at the turn of a knob. However, in ultra-cold Fermi atoms the interaction strength and the particle density can be tuned to change qualitatively and quantitatively superfluid properties. This tunability allows for the study of the evolution from BCS (weak coupling) superfluidity of large Cooper pairs to Bose-Einstein condensation (strong coupling) superfluidity of tightly bound molecules. I will discuss the BCS to BEC evolution in s-wave and p-wave angular momentum channels, and will conclude that this evolution is just a crossover phenomenon for s-wave, while a quantum phase transition takes place for the p-wave case.


Friday, February 13, 11:30, 201 Research I

Jim F. Drake, University of Maryland

"Magnetic Reconnection: a mechanism for cosmic particle acceleration?"

Solar and stellar flares, substorms in the Earth's magnetosphere, and disruptions in laboratory experiments are driven by the explosive release of magnetic energy through the process of magnetic reconnection. In magnetic reconnection regions of oppositely directed magnetic field annihilate, converting magnetic free energy into energetic beams, high velocity flows and thermal energy. Significant progress is being made on fundamental questions such as how magnetic energy is released so quickly and why the release occurs as an explosion. Suprisingly large amounts of the released energy appears in the form of energetic electrons -- up to 50% or more during solar flares. The mechanism for energetic electron production during magnetic reconnection has remained a mystery for more than three decades. I will discuss the basic physics of reconnection and then talk about a new model in which electrons are efficiently accelerated through Fermi reflection in a bath of contracting magnetic islands. A link between the total energy content of energetic electrons and the magnetic energy released is established. Further, solutions for energy spectra suggest that magnetic reconnection may compete with high mach number shocks as an accelerator of cosmic rays. The talk will review key observational data and emphasize basic physical principles to introduce the topic to the non-specialist.


Friday, March 6, 3:00, Place: 3rd floor JC room E

Robert Ehrlich, George Mason University

"The MIT TEAL program for teaching introductory physics"

MIT began its TEAL program in 2001 as a new approach to teaching the introductory calculus-based physics course taken by all students. TEAL stands for "Technology Enhanced Active Learning." TEAL which recently received national attention features a studio format (combined lecture-lab), and it was modeled after programs at RPI and North Carolina State University. The program was initially strongly opposed by many studentsand some faculty. However, after some of the kinks were ironed out, the student opposition has softened, but still persists to some degree. This is a report of what I learned about TEAL during a visit to MIT after talking with a number of physics faculty, students, and TA's. I also had the opportunity to sit in on several TEAL classes, and examine a 6-month evaluation recently performed by the physics faculty. We will consider the pros & cons of this approach, and its suitability for the Mason setting.


March 27:

D.N. Baker, Laboratory for Atmospheric and Space Physics, Univ. of Colorado at Boulder, Boulder, CO 80303-7814.

Magnetospheric substorm expansion and recovery phase features in the near- and mid-tail regions

The Cluster constellation of spacecraft is well-suited for the study of plasma sheet thinning and expansion associated with magnetospheric substorm dynamics in the magnetotail. The Cluster orbital apogee of r=19 RE often places the spacecraft set in a very good location to be close to the substorm neutral line (reconnection) position. Starting with energetic particle data from the Cluster spacecraft, we have identified numerous clear substorm events in the 2001-2008 tail passage seasons. We have also examined corresponding information in POLAR, geostationary orbit, and solar wind spacecraft data sets. We find that thinning/expansion sequences are observable quite broadly over large radial and azimuthal distances in most events. However, details of particle burst phenomena differ markedly with radial distance. We use global MHD magnetospheric modeling with realistic solar wind driving conditions to study substorm expansion phase features. We also devote special attention to the study of the substorm recovery phase: This is an aspect that has been much neglected both observationally and theoretically. In this presentation we focus on the implications of these new observations and model results for substorm models.

April 10:

Judith Lean, Space Science Division, Naval Research Lab

The Sun-Earth System: Our Home in Space

A generator in space, 150 million km away, heats the Earth, structures its atmosphere and organizes the surrounding space environment. The Sun’s energy output changes continually, with myriad Earthly consequences. How much of recent surface warming and atmospheric ozone depletion are solar- rather than human-induced? Will a solar storm be deadly, for space instruments and astronauts alike? How might solar effects on navigation, communication and Earth-orbiting objects compromise security and commerce? Scientific curiosity and societal utility both call for a robust understanding of the Sun-Earth system - our home in space that extends well beyond the surface where we live.


April 24:

Brian Wood, Naval Research Laboratory

Structure and Kinematics of Coronal Mass Ejections Observed by STEREO

The two spacecraft that constitute the STEREO mission were in 2006 launched into orbits around the Sun resembling that of Earth, but with one spacecraft (STEREO-A) orbiting well ahead of the Earth and one (STEREO-B) orbiting well behind. The substantial separation of the spacecraft from the Earth and from each other means that each has a significantly different view of dynamic solar atmospheric phenomena. Of particular interest for STEREO are coronal mass ejections(CMEs), which STEREO telescopes can actually track continuously from inception on the solar surface all the way to 1 AU. One of the STEREO's primary goals is for stereoscopic viewing of CMEs to allow for the accurate reconstruction of their 3D morphologies and trajectories. I will present some early results concerning measurements of CME structure and kinematics from STEREO data.

May 01:

Jack Gosling, Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder CO 8030 The Solar Wind as a Magnetic Reconnection Laboratory

Magnetic reconnection plays a central role in a wide variety of observed solar and space phenomena. In the solar wind magnetic reconnection commonly occurs in a quasi-stationary mode at extended X-lines. It produces Petschek-like exhausts of roughly Alfvenic jetting plasma bounded by back-to-back rotational discontinuities or slow mode waves that bifurcate a reconnecting current sheet. It occurs most frequently in low beta plasma and at thin current sheets associated with relatively small (less than 90 deg) magnetic field rotations. Reconnection exhausts are observed most frequently (40-80 events/month at 1 AU) in the low-speed wind and within interplanetary coronal mass ejections, and less frequently in the Alfvenic turbulence characteristic of the high-speed wind from coronal holes. Reconnection occurs relatively infrequently at the heliospheric current sheet (HCS), but observations of exhausts at the HCS are particularly revealing of the magnetic topology changes associated with the reconnection process. Reconnection in the solar wind is usually “fast”, but not explosive – the magnetic energy release occurs over a long time interval following reconnection as the Alfvenic disturbances initiated by the process propagate into the surrounding solar wind plasma. Interestingly, there is as yet no hard evidence that would indicate that reconnection in the solar wind ever produces substantial particle acceleration. This paper provides an overview of magnetic reconnection in the solar wind as revealed by observations of reconnection exhausts.

May 08:

David Hafemeister, Prof. of Physics (emeritus), Cal Poly University and Science Affiliate at Stanford’s Center for International Security and Cooperation Progress in CTBT Monitoring since its 1999 Senate Defeat*

Progress in monitoring the Comprehensive Nuclear Test Ban Treaty (CTBT) is examined, beginning with the 2002 National Academy of Sciences CTBT study, followed by recent findings on regional seismology, array–monitoring, correlation–detection, seismic modeling and non-seismic technologies. The NAS–CTBT study concluded that the fully–completed International Monitoring System (IMS) will reliably detect and identify underground nuclear explosions with a threshold of 0.1 kt in hard rock, if conducted anywhere in Europe, Asia, North Africa, and North America. In some locations the threshold is 0.01 kt or lower, using arrays or regional seismic stations. As an example, the 0.6–kiloton North Korean test of October 9, 2006 was promptly detected by seismometers in Australia, Europe, North America and Asia. The P/S ratio between 1–15 Hz clearly showed that the event was an explosion and not an earthquake. Radioactive venting, observed as far as Canada, proved that it was a nuclear explosion. Advances in seismic monitoring strengthen the conclusions of the NAS study. Interferometric synthetic aperture radar can, in some cases, identify and locate 1–kt tests at 500 m depth by measuring subsidence to 2–5 mm. InSAR can discriminate between earthquakes and explosions from the subsidence pattern and it can locate nuclear tests to within 100 meters.

Friday, May 22:

Evgenya Shkolnik, Extra-solar planets DTM Carnegie Institute Star-Planet Interactions: A Probe of Extrasolar Planetary Magnetic Fields

Much effort has been invested in recent years, both observationally and theoretically, to understand the interacting processes taking place in planetary systems consisting of a hot Jupiter orbiting its star within 10 stellar radii. Several independent studies have converged on the same scenario: that a short-period planet can induce activity on the photosphere and upper atmosphere of its host star. The growing body of evidence for such magnetic star-planet interactions includes a diverse array of photometric, spectroscopic and spectropolarimetric studies. The nature of which is modeled to be strongly affected by both the stellar and planetary magnetic fields, possibly influencing the magnetic activity of both bodies, as well as affecting irradiation and non-thermal and dynamical processes. Studying such star-planet interactions (SPI) aids our understanding of the formation, migration and evolution of hot Jupiters, and improves SPI's potential as a probe of extrasolar planetary magnetic fields.

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