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| ETH PHYSICS DEPARTMENT | ||
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COLLOQUIA WINTER SEMESTER 2004-2005 Wednesdays, 4:45pm, ROOM HPV-G4 Tea starts at 4:15pm ([AV]=Antrittsvorlesung) | ||
| Date | Title | Speaker |
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October 20 Host: R. Douglas | Biologically plausible synaptic models for reinforcement learning | S. Seung (MIT) |
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October 27 Host: D. Wyler | Physics in Search of Oil and Gas | M. Lueling (Schlumberger) |
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November 3 Host: M. Gaberdiel | Particles and Strings - Probing the Structure of Matter and Space-Time | J. Louis (Hamburg) |
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November 10 Host: J. Froelich | The Use of Geometry and Entropy in Analyzing Large Networks | J-P. Eckmann (Geneva) |
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November 17 Host: T. Esslinger | Cold Molecules | G. Meijer (FHI, Berlin) |
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November 24 Host: K. Ensslin | Quantum Dots for Quantum Computation | L. Kouwenhoven (Delft) |
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December 1 | NO COLLOQUIUM | European Physics Prize! |
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December 8 Host: A. Rubbia | Possibilities and Surprises of Vacuum Dark Energy | I. Dymnikowa (IPT, St. Petersburg) |
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December 15 Host: A. Imamoglu | Circuit Quantum Electrodynamics: Doing Quantum Optics with Superconductors | A. Wallraff (Yale) |
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December 22 Host: M. Sigrist | Surprises in Transport Theory | A. Rosch (Koeln) |
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January 12 Host: A. Imamoglu | Putting the Mechanics back into Quantum Mechanics | K. Schwab (Univ. Maryland) |
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January 19 Host: S. Lilly | Cosmological observations and fundamental physics | J-L. Puget (IAS, Paris) |
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January 26 Host: A. Imamoglu | Quantum memory for light: can the immeasurable be remembered? | E. Polzik (Aarhus) |
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February 2 | NO COLLOQUIUM | M. Rice's Abschiedsvorlesung! |
| Abstracts | ||
| October 20 | Biologically plausible synaptic models for reinforcement learning | S. Seung |
| Suppose that we find you practicing your serve on the tennis court, and ask what is happening in your brain as you learn. If you are a neuroscientist, you might reply that you are changing the synapses in your brain to improve your performance. But if we were able to single out a particular synapse in your brain, and ask whether strengthening it would improve your serve, you would be unable to answer. Nevertheless, knowledge of this sort must be available in your brain, if learning indeed proceeds by changing the strengths of synapses. I will discuss current theories of how the brain changes its synapses to optimize behavior, and the prospects for testing these theories experimentally. | ||
| October 27 | Physics in Search of Oil and Gas | M. Lueling |
| Oil and gas are natural resources that are hidden in various deposits in the earth crust. The physical properties of oil and gas influence the material properties of the entire rock. The search for oil and gas uses different physical measurements to determine the spatial distribution of these materials with high measurement accuracy and good spatial resolution. This presentation offers a historic review of the evolution of such measurement methods, followed by a comprehensive discussion of present measurement technologies and their impact on the oil and gas production. Directional drilling directly benefits from real-time measurements that permit to steer a horizontal well into target zones full of oil and gas up to 10 km away. Future developments foresee environmentally more responsible intervention, such as the sequestering and storage of carbon dioxide to reduce atmospheric warming. | ||
| November 3 | Particles and Strings - Probing the Structure of Matter and Space-Time | J. Louis |
| After a brief review of Particle Physics, General Relativity and Cosmology String Theory is introduced as a possible unifying concept. | ||
| November 10 | The Use of Geometry and Entropy in Analyzing Large Networks | J-P. Eckmann |
| In the last few years, large Networks have become an object of intense study, and statistical properties of many examples have been analyzed: The Web, biological networks,... In my talk, I will summarize work which goes in a more conceptual direction, done with Elisha Moses and Danilo Sergi. Our aim is to relate "context" and "geometrical properties" in such networks. We use local structures to capture these properties: On the static level, this is the "clustering coefficient" (a ratio of triangles and links) which can be used to extract relevant information from the local geometry of large networks. On the dynamic level, we use relative entropy (Kullback-Leibler) in the time-domain to capture the creation of a dynamic network in e-mail traffic. I will try to explain the geometric ideas behind these studies and will illustrate how they are being applied and tested in a variety of networks we have studied: World wide web, Protein Networks, Citations, e-mail. | ||
| November 17 | Cold Molecules | G. Meijer |
| Getting full control over both the internal and external degrees of freedom of molecules has been an important goal in molecular physics during the last decades. This control is es-sential in the presently very active field of Cold Molecules. Trapped samples of neutral molecules have been created by means of buffer gas cooling in a magnetic trap, by using deceleration of a molecular beam in combination with an electrostatic trap, and by pairing cold atoms to form molecules in optical or magnetic traps. Recently, spectacular progress has been made with association of ultra-cold atoms assisted by magnetically induced Feshbach resonances, resulting in the first molecular Bose-Einstein condensates. In the field of Cold Molecules there is a particular interest in cold dipolar molecules which stems from the presence of the anisotropic, long-range dipole-dipole interaction in these samples, which is predicted to lead to interesting physics and novel applications. In this presentation I will give an overview of the various experiments that we have performed during the last few years to explore the possibilities of manipulating neutral polar molecules with electric fields (H.L. Bethlem and G. Meijer, Int. Rev. Phys. Chem. 22, 73, 2003). | ||
| November 24 | Quantum Dots for Quantum Computation | L. Kouwenhoven |
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Quantum dots are nano-scale field-effect transistor devices, which only
contain a small number of electrons. This number can be changed with a gate
voltage such that one can create a box containing exactly one, two, three,
etc. electrons. This artificial, human-fabricated system has many similarities
with atoms: the electron energy spectrum is discrete with a shell structure,
which is filled with electrons according to Hund's rules. We exploit the
ability to tune in-situ the quantum dot properties for a controlled study of
quantum mechanical interactions for a specific number of electrons. These
interactions lead to phenomena such as two-electron singlet and triplet states
and the Kondo effect. We will further discuss various quantum dot systems
(semiconductor, nanocrystals and carbon nanotubes) including some of the
fabrication procedures. Although these studies are presently pure scientific
we will speculate on electronic applications. One such speculation concerns
the possible development of quantum computers. We will outline the basic
principles of a quantum computer as well as the foreseen advantages. The
realization of a quantum computer contains many difficulties. We will discuss
these hurdles using qu-bits based on quantum dots as an example. We will
particularly focus on the quantum information contained in the spin degree of
freedom of individual electrons. Our experimental efforts focus on realizing
spin-qubit circuits. These little circuits have to include a double quantum
dot with controllable tunnel coupling between the dots; electron-spin
resonance loop for performing single spin rotations; and a non-invasive
read-out system. Our read-out is performed by a quantum point
contact detector. Parts of this little qubit circuit are now being tested,
some parts are already working [3]. [1] For a review on quantum dots, see: Few-electron quantum dots, L.P. Kouwenhoven, D.G. Austing and S. Tarucha, Rep. Prog. Phys. 64, 701-736 (2001). This review and other papers can be found at http://qt.tn.tudelft.nl/ [2] Double transport through double quantum dots. W. G. van der Wiel, S. De Franceschi, J. M. Elzerman, T. Fujisawa, S. Tarucha and L. P. Kouwenhoven, Reviews of Modern Physics 75, No.1, 1-22 (2003) [3] Single shot read-out of an individual electron spin in a quantum dot J.M. Elzerman, R. Hanson, L.H. Willems van Beveren, B. Witkamp, L.M.K. Vandersypen and L.P. Kouwenhoven, Nature 430, 431-435 (2004). |
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| December 8 | Possibilities and Surprises of Vacuum Dark Energy | I. Dymnikowa |
| Astronomical data provide convincing evidence that the Universe is dominated in 70\% of its density by a dark energy, responsible for its accelerated expansion due to negative pressure $p=w\rho;~w<-1/3$. Current observations constrain the parameter $w$ in the equation of state to $w<-0.7$ with the best fit $w=-1$ which corresponds to cosmological constant $\Lambda$ related to a vacuum density $\Lambda=8\pi G\rho_{vac}$. The main competitor of cosmological vacuum $\Lambda$ is the quintessence $Q$ - a fifth element with negative pressure introduced actually as a time-evolving non-vacuum alternative to $\Lambda$ which must be constant by the Einstein equations. Future observations are planned to study evolution of the dark energy with time to distinguish between $\Lambda$ and $Q$. All that suggests a need in a time-dependent space-inhomogeneous version of a cosmological vacuum energy. The Einstein cosmological term $\Lambda g_{\mu\nu}$ is associated with a vacuum stress-energy tensor of maximal symmetry, the full Lorentz group for stress-energy tensor, the 10-parametric de Sitter group for space-time. Our mathematical instrument is the variable cosmological term $\Lambda_{\mu\nu}=8\pi G T_{\mu\nu}^{vac}$ based on the Petrov classification scheme. It describes a cosmological vacuum defined by symmetry of its stress-energy tensor $T_{\mu\nu}^{vac}$ and evolves from $\Lambda g_{\mu\nu}$ to $\lambda g_{\mu\nu}$ with $\lambda < \Lambda$. The full symmetry remains only asymptotically, in between it is reduced to the Lorentz boosts in a certain space direction. The Einstein cosmological term is the particular case of $\Lambda_{\mu\nu}$ with the maximal symmetry. In the spherically symmetric case $T_{\mu\nu}^{vac}$ generates regular spherically symmetric space-time with the de Sitter center. Dependently on parameters and choice of a coordinate frame, geometry describes cosmological models with variable vacuum density and pressures, and localized objects with de Sitter vacuum core: nonsingular black holes and self-gravitating particle-like structures. Existence of such geometries follows from imposing requirements of regularity of density, finiteness of the ADM mass, and certain energy conditions on a stress-energy tensor. Mass of the objects described by geometry with the de Sitter center, is related to both smooth breaking of space-time symmetry and de Sitter vacuum trapped in the origin. This has been tested by evaluating the gravito-electroweak unification scale from the measured mass-squared differences for solar and atmospheric neutrinos. Predicted unification scale gets within (6-16) TeV. | ||
| December 15 | Circuit Quantum Electrodynamics: Doing Quantum Optics with Superconductors | A. Wallraff |
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I will describe recent experiments in which the strong coupling
limit of cavity quantum electrodynamics has been realized for the
first time using superconducting circuits [1]. In our approach, we
use a Cooper-pair box as an artificial atom, which is coupled to a
one-dimensional cavity formed by a transmission line resonator. In
the case when the Cooper-pair box qubit is tuned into resonance
with the cavity, we observe the vacuum Rabi splitting of the
cavity mode, indicating that the strong coupling regime is
attained, and coherent superpositions between the qubit and a
single photon are generated. When the qubit is detuned from the
cavity resonance frequency, we perform high-fidelity dispersive
quantum non-demolition readout of the qubit state. Using this
readout technique, we have characterized the qubit properties
spectroscopically, performed Rabi oscillations of the qubit, and
attained coherence times greater than 500 ns, indicating that this
architecture is extremely attractive for quantum computing and
control [2]. [1] A. Wallraff, D. I. Schuster, A. Blais, L. Frunzio, R.-S. Huang, J. Majer, S. Kumar, S. M. Girvin and R. J. Schoelkopf Nature (London) 431, 162 (2004) [2] A. Blais, R.-S. Huang, A. Wallraff, S. M. Girvin and R. J. Schoelkopf Phys. Rev. A 69, 062320 (2004) |
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| December 22 | Surprises in Transport Theory | A. Rosch |
| Even in systems where thermodynamic properties are well understood, transport can be full of surprises. For example tiny amounts of disorder can completely change the resistivity in a metal close to a quantum phase transitions. In one-dimensional and quasi one-dimensional systems the heat and charge transport is often governed by (approximate) symmetries such that typically not the strongest but the second strongest scattering process determines heat and charge transport. We discuss how these effects can explain the huge heat conductivities observed in spin-chain and spin-ladder compounds. | ||
| January 12 | Putting the Mechanics back into Quantum Mechanic | K. Schwab |
| I will discuss our recent experiments where we have made the closest approach to the quantum limit for continuous position detection of a mechanical structure, a factor of ~5 from the uncertainty principle limit. We have developed a nano-electro-mechanical device with an integrated nanomechanical resonator and ultra-sensitive single electron transistor. The success of these experiments paves the way to the realization of truly quantum states of a mechanical device: squeezed states, number states, and most exciting, the formation of mechanical entangled states. I will also discuss the applications of this technology from advanced force microscopes to readout for quantum information devices. | ||
| January 19 | Cosmological observations and fundamental physics | J-L. Puget |
| I will recall how historically cosmological observations and fundamental physics have been linked from 1917 to our days, then talk about CMB observations and present some elements on the critical role of new technologies in these. | ||
| January 26 | Quantum memory for light: can the immeasurable be remembered? | E. Polzik |
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A weak pulse of light, the carrier of today's classical communications, is a
quantum object. Its amplitude (photon number) and color (phase) cannot be
measured with high accuracy due to the Heisenberg uncertainty principle.
This complementarity of amplitude and phase of light is widely used in the
emerging area of quantum information and quantum communication. Both
state-of-the-art classical, and quantum communication networks require
memory for light, which in this case has to be a quantum memory. The quantum
memory must be capable of storing both an amplitude and a phase of the light
pulse, i.e., two immeasurable, complementary variables. In the recent
experiment [1] we have demonstrated such a memory. Its principles and
directions for future development will be discussed in the talk.
[1] B. Julsgaard, J. Sherson, J. I. Cirac, J. Fiurasek, and E. S. Polzik. Nature, November 25 (2004), preprint quant-ph/04100 |
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| Organized by: Marcella Carollo, Astronomy Institute | ||
The WS
2003-2004 ETH PHYSICS COLLOQUIA
The WS
2002-2003 ETH PHYSICS COLLOQUIA
| Last Update: 7 November 2004 | marcella.carollo@phys.ethz.ch |
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