Events at Physics |
Events During the Week of April 6th through April 13th, 2008
Monday, April 7th, 2008
- Plasma Physics (Physics/ECE/NE 922) Seminar
- Recent Experimental Results from the Helicity Injected Torus with Steady Inductive Helicity Injection (HIT-SI)
- Time: 12:05 pm
- Place: 2241 Chamberlin Hall
- Speaker: Aaron Redd, University of Wisconsin, Dept. of Engineering Physics
- Chaos & Complex Systems Seminar
- Title to be announced
- Time: 12:05 pm
- Place: 4274 Chamberlin Hall
- Speaker: Sean Cornelius, Physics
- Astronomy Colloquium
- Title to be announced
- Time: 3:45 pm
- Place: 6515 Sterling Hall (coffee at 3:30 pm)
- Speaker: Roger Chevalier, University of Virginia
- Medical Physics Seminar
- Title to be announced
- Time: 4:00 pm
- Place: 1345 Health Science Learning Center (refreshments will be served)
- Speaker: RTingliang Zhuang (student of Dr. Guang-Hong Chen) Assistant Professor, Department of Therapeutic Radiology
Tuesday, April 8th, 2008
- R. G. Herb Condensed Matter Seminar
- The Metal-Insulator Transition (MIT) in MgTi2O4 Spinel
- Time: 10:00 am
- Place: 5310 Chamberlin
- Speaker: Stefano Leoni, Max Planck - Dresden
- Abstract: The study of transition metal oxide spinels represents a considerable challenge for theory, due to the richness of their properties and to their natural tendency to form ordered states upon phase transitions. MgTi2O4 for example undergoes a transition from a metallic to a spin-singlet insulator near Tc=260 K, with the opening of an optical gap and no sign of Drude weight. The metallic phase is characterized by a pyrochlore lattice occupied by Ti atoms. The transition to a tetragonal structure is accompanied by a selective shortening of some bonds under formation of 'molecular dimers'. In this talk we address the problem of the origin of structural distortion in this material, by performing state-of-art correlated band structure calculations within the LDA+ Hubbard U scheme. Therein we identify an orbital selective MIT that is carried by orbital rehybridization assisted by correlation, without full orbital polarization. Additionally, we derive the structural distortion by expressing the forces in the LDA+U potential and by performing Parrinello-Rahman structure relaxation. Along this line we find a lock-in into a dimerized phase for a critical U value. Reference: S.Leoni, A. Yaresko, N. Perkins, H. Rosner, L. Craco, submitted to PRL.
- Host: Perkins
- Astronomy Colloquium
- The Death and Afterlife of Massive Stars
- Time: 3:45 pm - 4:45 pm
- Place: 6515 Sterling Hall (refreshments served in the library at 3:30 pm)
- Speaker: Roger Chevalier, University of Virginia
- Abstract: Observations of supernovae and gamma-ray bursts show that massive stars end their lives with a wide variety of properties. For supernovae, mass loss plays a crucial role in that variety. The circumstellar interaction and explosion characteristics provide diagnostics for the stellar evolution leading up to the explosion.
- Host: Eric Wilcots
Wednesday, April 9th, 2008
- Atomic Physics Seminar
- Reconstructing the Wigner function of a photonic Schrodinger cat in a cavity: a movie of decoherence
- Time: 11:00 am
- Place: 5310 Chamberlin
- Speaker: Serge Haroche, Laboratoire Kastler Brossel
- Abstract: The development of super-high finesse cavities made of pairs of mirrors which bounce microwave photons back and forth for times up to a few tenths of a second has opened a new domain in quantum optics. It is now possible to "look" at stored light without destroying it, to prepare by quantum non-demolition (QND) measurements well-defined photons number states and to observe the quantum jumps between them(1). One can also generate and study mesoscopic non-classical states containing several photons, which survive long enough to be measured and fully characterized. We will describe how a single atom passing between mirrors trapping a coherent field prepares a superposition of photonic states with opposite phases. This state is called a "Schroedinger cat" by reference to the feline that the founding father of quantum physics imagined to be suspended in a superposition of life and death. This highly non-classical state is represented in phase space by a quasi-probability distribution, called Wigner function, which exhibits Gaussian peaks corresponding to its classical components (the "dead" and "alive" parts of the cat) along with interference fringes describing its quantum coherence. By performing QND measurements of the photon number parity, we have reconstructed these Wigner function and recorded the progressive disappearance of their interference features. Movies of the decoherence process are realized in this way, which directly illustrate how a mesoscopic system evolves from quantum to classical.
- Host: Deniz Yavuz
Thursday, April 10th, 2008
- R. G. Herb Condensed Matter Seminar
- Quantum computation using tunable flux qubits
- Time: 10:00 am
- Place: 5310 Chamberlin
- Speaker: Matthias Steffen, IBM Yorktown Heights
- Abstract: The quest towards building a practical quantum computer using superconducting materials and Josephson junctions has made rapid progress over the past several years. Experimental demonstrations ranged from high fidelity single qubit gates to elementary two-qubit gates. One major step towards continued progress consists of understanding all major sources of decoherence that destroy the fragile quantum states. In this talk we provide an overview of our experimental efforts using superconducting qubits. Our qubit is a tunable flux qubit coupled to a high quality transmission line resonator. We show quantum information storage in the resonator with lifetimes of a few microseconds. The qubit itself is shown to have short coherence times, which are not well understood at present. One possible mechanism may be due to stray capacitance of the qubit to ground and bias leads. We present a simple model of decoherence due to the stray capacitances and speculate on future design strategies.
- Host: Robert McDermott
- NPAC (Nuclear/Particle/Astro/Cosmo) Forum
- The role of SUSY flat directions in reheating
- Time: 4:00 pm
- Place: 4274 Chamberlin Hall
- Speaker: Marco Peloso, University of Minnesota
- Abstract: Reheating after inflation describes all the particle physics processes occurring from the decay of the inflaton up to the establishment of thermal equilibrium. Due to the strong model dependence, it is one of the most unknown stages in the history of the universe. It has been argued that SUSY flat directions are naturally excited during inflation, and that they slow down the thermalization of the inflaton decay products by providing a large effective mass to the fields they are coupled to. We discuss a nonperturbative decay channel for the flat directions which has been generally overlooked in the literature. It typically leads to a quick decay of the flat directions, and, consequently, to a suppression of their delaying effect for thermalization.
Friday, April 11th, 2008
- Seminar
- Temporal resolution and bandwidth of human hearing
- Time: 8:30 am
- Place: MSC Room 281
- Speaker: Milind N. Kunchur, Department of Physics and Astronomy, University of South Carolina
- Abstract: Experiments were conducted to assess the bandwidth and temporal resolution of human hearing using special ultrahigh-fidelity equipment. The experiments demonstrated discernment at a ~5 microsecond timescale, which is much shorter than found previously and falls below the nominal 9 microseconds (=1/[2 pi 18kHz]) threshold expected for a linear system. Analysis of the stimuli suggests that the discernment does not arise solely from direct spectral-amplitude differences but may involve additional non-linear and/or temporal mechanisms. Possible mechanisms, which are quantitatively consistent with the findings, are discussed. The present work also shows that typical instrumentation used in psychoacoustic research has insufficient temporal speed and fidelity for properly assessing all aspects of human hearing and that digital sampling rates used in consumer audio are insufficient for fully preserving transparency. [This work was partially supported by the University of South Carolina Office of Research and Health Sciences Research Funding Program.]
- Host: Donata Oertel (Physiology)
- NPAC (Nuclear/Particle/Astro/Cosmo) Forum
- Joint NPAC and Pheno Seminar
- Dark-matter sterile neutrinos from decays of a gauge-singlet Higgs
- Time: 2:30 pm - 3:30 pm
- Place: 5280 Chamberlin
- Speaker: Kalliopi Petraki, UCLA
- Abstract: Sterile neutrinos are usually introduced to explain the masses of active neutrinos. If one of these fermions has mass of several keV, it can also account for the cosmological dark matter. The same particle can explain the observed velocities of pulsars, speed up the formation of the first stars and stir up supernova explosions. I will describe a mechanism for sterile neutrino production that involves a minimal extension of the Higgs sector by a gauge-singlet scalar. The relic abundance of sterile neutrinos is produced from decays of the singlet Higgs and does not depend on their mixing angle. The resulting dark matter is colder than the warm dark matter produced in neutrino oscillations. I will discuss the small-scale structure formation properties of these neutrinos and show that they comply with current observations. The presence of the gauge singlet in the Higgs sector has important implications for the electroweak phase transition, baryogenesis, and the upcoming experiments at the LHC.
- Physics Department Colloquium
- Trapping and Counting Photons without Destroying Them: A New Way to Look at Light
- Time: 4:00 pm
- Place: 2241 Chamberlin Hall (coffee at 3:30 pm)
- Speaker: Serge Haroche, Ecole Normale Superieure
- Abstract: An experiment realizing a quantum non-demolition measurement of a microwave field trapped in a cavity will be described. While usual photo-detection destroys light quanta, we have developed a new way to count photons without absorbing them, making it possible to measure the same field repeatedly. We use as detectors atoms prepared in a superposition of Rydberg states which cross the cavity one at a time and behave as microscopic clocks whose ticking rate is affected by light. By measuring the clocks' delay, information is extracted without energy absorption and the field progressively collapses into a well-defined photon number state. Quantum jumps between decreasing photon numbers are recorded as the cavity field subsequently relaxes towards vacuum. This new way to "look" at light also generates coherent superpositions of photonic states with different phases called "Schroedinger cats". By monitoring the evolution of these states, we directly observe the process of decohere once in experiments opening new avenues for the exploration of the boundary between the quantum and classical worlds.
- Host: Yavuz