Joint Atomic Physics Colloquium Series Jointly sponsored by Harvard Physics Department and ITAMP The colloquia are held Wednesdays in Jefferson Lab 356 at 4:30 p.m unless otherwise noted. Tea Is served at 4 p.m.
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Fall 2009 September, 2009 October, 2009 November, 2009 December, 2009 Spring 2009 February, 2009  March, 2009 April, 2009 May, 2009
Spring Semester 09
January
1/28/09	Tilman Pfau, University of Stuttgart Rydberg atoms in a Bose-Einstein Condensate Rydberg atoms provide a wide range of possibilities to tailor interactions in a quantum gas. Here we report on Rydberg excitation of Bose-Einstein condensed 87 Rb atoms [1]. We observe coherent collective behavior for ensembles of up to several thousand atoms [2]. Despite the strong interactions between Rydberg atoms the evolution can still be reversed by a simple phase shift in the excitation laser field. We experimentally proof the coherence of the excitation in the strong blockade regime by applying an optical ro- tary echo technique to a sample of magnetically trapped ultracold atoms, analogous to a method known from nuclear magnetic resonance. Using this echo technique we measured the dephasing time due to the interaction between the Rydberg atoms [3]. We also show that the ground state of the Pseudo-Spin Hamiltonian describing the driven system exhibits a second order quantum phase transition. We present the results of a critical theory for the quantum phase transition and show that it describes the properties of the driven Rydberg system in the saturated regime. We find that the suppression of Rydberg excitations known as blockade phenomena exhibits an algebraic scaling law with a universal exponent [4]. Very recently we successfully excited ultra–long–range Rydberg molecules (dimers and trimers) [5,6]. The underlying novel mechanism for the bond is low-energy electron scat- tering of Rydberg electrons from polarizable ground state atoms. We determine the low energy scattering length for electron– Rb(5S) scattering and the lifetimes and polarizabil- ities of these exotic molecules. References [1] R. Heidemann, U. Raitzsch, V. Bendkowsky, B. Butscher, R. L?ow, T. Pfau ”Rydberg excitation of Bose-Einstein condensates” Phys. Rev. Lett. 100 , 033601 (2008). [2] R. Heidemann, U. Raitzsch, V. Bendkowsky, B. Butscher, R. L?ow, L. Santos, T. Pfau ”Evidence for coherent collective Rydberg excitation in the strong blockade regime” Phys. Rev. Lett. 99, 163601 (2007). [3] U. Raitzsch, V. Bendkowsky, R. Heidemann, B. Butscher, R. L?ow, T. Pfau ”An echo experiment in a strongly interacting Rydberg gas” Phys. Rev. Lett. 100 , 013002 (2008). [4] H. Weimer, R. L?ow, T. Pfau, H.P. B?uchler ”Quantum critical behavior in strongly interacting Rydberg gases” Phys. Rev. Lett. 101 250601 (2008). [5] C. H. Greene, A. S. Dickinson, and H. R. Sadeghpour ”Creation of Polar and Nonpolar Ultra-Long-Range Rydberg Molecules ” Phys. Rev. Lett. 85, 2458 (2000). [6] V. Bendkowsky, B. Butscher, J. Nipper, J.P. Shaffer, R. L?ow, T. Pfau ”Observation of ultra-long range Rydberg molecules” accepted for publication in Nature.
February
March
3/4/09	John Bohn, JILA Which way is up? Or, How a BEC lives with dipolar interactions Ultracold, quantum degenerate gases consisting of dipolar particles bring a new richness to the phenomenon of Bose-Einstein condensation (BEC). The dipolar interaction cares about the direction the dipoles are pointing, which leads to novel structure and properties in such a gas. Thanks to the efforts of the Pfau group in Stuttgart in Bose-condensing chromium, dipolar BEC is now a reality, and its properties are steadily emerging in the lab as well as in computer simulations. In this talk I will report on our recent theoretical work on dipolar BEC. I will show how the dipolar interaction leads to novel ground state density profiles in the condensate, as well as to certain collective excitations that bear a qualitative resemblance to rotons in superfluid helium. These roton-like modes play a decisive role in the dynamical stability of the condensate, for both non-rotating and vortex states. We find that the stability diagram computed within mean field theory is in excellent agreement with the experiment of Pfau and collaborators.
3/18/09	Andrei Derevianko, Univ of Nevada - Reno Improved test of the standard model of elementary particles with atomic parity violation Atomic parity violation places powerful constraints on new physic beyond the Standard Model of elementary particles. The measurements are interpreted in terms of the nuclear weak charge, quantifying the strength of the electroweak coupling between atomic electrons and quarks of the nucleus. We report the most accurate to-date determination of this coupling strength by combining previous measurements by the Boulder group with our high-precision calculations in cesium atom. Our result is in a perfect agreement with the prediction of the Standard Model. In combination with the results of high-energy collider experiments, our work confirms the predicted energy dependence (or ``running'') of the electroweak interaction over an energy range spanning four orders of magnitude (from ~10 MeV to ~100 GeV) and places new limits on the masses of extra Z bosons (Z'). Our raised bound on the Z' masses carves out a lower-energy part of the discovery reach of the Large Hadron Collider. At the same time, a major goal of the LHC is to find evidence for supersymmetry (SUSY), one of the basic, yet experimentally unproven, concepts of particle physics. Our result is consistent with the R-parity conserving SUSY with relatively light (sub-TeV) superpartners. This raises additional hopes of discovering SUSY at the LHC.
April
4/1/09	Paul Julienne, NIST Ultracold Polar Molecules: A Case Study with KRb It is now possible to make ultracold gases of ultracold polar molecules. This process can be made very efficient by first associating two ultracold atoms into a weakly bound "Feshbach molecule" and subsequently coherently transferring such a molecule to the vibrational ground state. Whether one considers the collision of two atoms or two molecules, an ultracold collision prepares the system in a very precise and narrow energy range from which one can probe the bound and quasi-bound states of the "collision complex" made from the colliding species. It is especially helpful to understand the properties of the bound and scattering states of the long range potential in order to understand near-threshold properties of the "collision complex." Coupled channels and approximate model calculations for K + Rb collisions and the bound states of the KRb molecule from threshold to the vibrational ground state will be used to illustrate key principles needed to understand the formation and applications of polar molecules.
4/22/09	Alexey Gorshkov, Harvard University Quantum Information and Quantum Simulation with Ultracold Alkaline-Earth Atoms in Optical Lattices We describe a method for quantum information processing and quantum simulation with ultracold alkaline-earth atoms in optical lattices. First, we propose and analyze a novel approach to quantum information processing, in which multiple qubits can be encoded and manipulated using electronic and nuclear degrees of freedom associated with individual alkaline-earth atoms trapped in an optical lattice [1]. We discuss potential applications of this approach to fault-tolerant quantum computation and precision measurements. Second, we propose to use alkaline-earth atoms in optical lattices to realize the two-orbital SU(N)-symmetric Hubbard Hamiltonian (with N as large as 10), which relies on the interplay between nuclear spin and electronic degrees of freedom. The unprecedented degree of symmetry and the spin-orbital physics associated with this readily accessible experimental system make it rich in its own right and capable of providing valuable insights into strongly correlated physics of transition metal oxides, heavy fermion materials, and spin liquid phases. [1] A.V.G. et al., Phys. Rev. Lett. 102, 110503 (2009).
4/29/09	Mark Raizen, UT-Austin Towards Trapping and Cooling of Atomic Tritium for Precision Measurement of Beta Decay
May
5/13/09	Andreas Buchleitner, Univ. of Freiburg
June
6/23/09 Special Event held in Pratt Conference Room.	Dmitry Fursa, Curtin University, Australia Relativistic Convergent Close-Coupling method for excitation and ionization processes in electron collisions with atoms and ions
Fall Semester 09
September 2009
9/9/09	Prof. Ami Vardi , Ben Gurion University Dephasing and noise in weakly coupled Bose-Einstein condensates Bose-Einstein condensates have been coherently split so as to prepare a coherent state with a well-defined relative phase between their constituents. Due to interparticle interactions, the coherence of this preparation is lost over time, a process known as 'phase diffusion'.  The dynamics of phase-diffusion in weakly-coupled condensates (as opposed to the more familiar case of fully-separated BECs) will be discussed, focusing on its sensitivity to the initial phase [1]. Next, we consider the interplay between phase-diffusion and decoherence, showing that dephasing may be slowed down by noise, in a Bose-enhanced quantum Zeno scenario [2,3]. Finally, the reverse process of dynamical phase-locking will be discussed [4]. In this case interactions  result in the emergence of a definite relative phase between initially incoherent BECs. The many realization fringe visibility is universal at 1/3 throughout the Josephson regime as evident from the semiclassical picture. Time permitting, I will also discuss the implications for the implementation of a sub-shot-noise SU(1,1) atom interferometer using the coherent dissociation of a molecular BEC [5]. [1] E. Boukobza, M. Chuchem, D. Cohen, and AV, Phys. Rev. Lett. 102, 180403 (2009). [2] Y. Khodorkovsky, G. Kurizki, and AV, Phys. Rev. Lett. 100, 220403 (2008). [3] Y. Khodorkovsky, G. Kurizki, and AV, Phys. Rev. A 80, 023609 (2009). [4] E. Boukobza, D. Cohen, and AV, eprint arXiv:0909.0195. [5] I. Tikhonenkov and AV, eprint arXiv:0904.2121.
9/23/09	Prof. Chris Greene, JILA and University of Colorado Four-body collisions, universality, and the Efimov effect Recent theoretical progress on the four-body problem has resulted in a study of the bound and quasi-bound states of four interacting bosonic atoms. New developments in this area will be described, including recent theory and experiments on the surprisingly rapid process of four-body recombination, including the relationship of these phenomena to Efimov physics.
October
10/7/09	Dr Charles W. Clark, Joint Quantum Institute, National Institute of Standards and Technology and University of Maryland "Relativity at a billionth of the speed of light" Abstract: Ultracold atoms enable us to explore exotic physical phenomena in conditions far removed from their original, inaccessible manifestations. In particular, we can design optical lattice configurations which produce strong "spin-orbit coupling" in the motion of cold atoms. I describe several related themes of current research: cold-atom analogues of Dirac-particle physics, which provide ways of observing the elusive "Zitterbewegung" phenomenon first noted by Schroedinger in 1931; the design and consturction of non-Abelian gauge potentials; atomic analogues of spintronic devices; and atomic analogues of the topological insulators.
10/21/09	Dr Sebastiaan van de Meerakker, Fritz-Haber-Institut der Max-Planck-Gesellschaft "Collision Experiments with Stark Decelerated Beams" Over the last years our group has been developing methods to get improved control over the velocity of molecules in a molecular beam [1]. With the Stark decelerator, a part of a molecular beam can be selected and transferred to any arbitrary velocity, producing bunches of state-selected molecules with a computer-controlled velocity and with longitudinal temperatures as low as a few mK. So far, this new molecular beam technology has been used mainly to decelerate packets of molecules to standstill, and to subsequently confine these molecules in a trap. We will report on various experiments that have been performed with these samples of trapped molecules. Stark decelerated molecular beams also hold great promise in molecular beam scattering experiments. In a crossed-beam configuration, these beams offer the revolutionary capability to study elastic or inelastic and reactive scattering as a function of the continuously variable collision energy with a high intrinsic energy resolution. We will report on the first scattering experiment using a Stark decelerated beam of OH radicals [2], and our progress on a new crossed beam machine containing two Stark decelerators under 90 degrees crossing angle. The prospects of a molecular synchrotron for scattering studies will be discussed. [1] S.Y.T. van de Meerakker, H.L. Bethlem and G. Meijer, Taming molecular beams, Nature Physics 4, 595 (2008). [2] J.J. Gilijamse, S. Hoekstra, S.Y.T. van de Meerakker, G.C. Groenenboom, and G. Meijer, Near-threshold inelastic collisions using molecular beams with a tunable velocity, Science 313, 1617 (2006).
November
Tuesday 11/3/09	Special Talk Prof. Edward Gerjuoy, University of Pittsburgh Pratt Conference Room, ITAMP, 60 Garden Street, Cambridge MA 1-2pm "Recollections of Oppenheimer and Schwinger" The career of J. Robert Oppenheimer, who was born on April 22, 1904, was celebrated in a June, 2004 Los Alamos Symposium, wherein I recalled my experiences as a Ph.D. student of Oppenheimer's in Berkeley, California during the period August 1938 to January 1942. I shall recount some of these recollections, concentrating on conveying a portrait of Oppenheimer as creator and inspiration of probably the most important pre-war United States school of theoretical physics. During a portion of this period (the 1940 academic year) Julian Schwinger, who shared the 1965 Nobel Prize for the development of the modern formulation of quantum electrodynamics and deservedly has been termed a genius, was employed as what today would be termed Oppenheimer's post doc. Therefore, especially because Schwinger now seems almost forgotten, although he died only fifteen years ago (on July 16, 1994), I also will recall some of Schwinger's interactions with Oppenheimer and Oppenheimer's students including myself, in an attempt to convey some comprehension of Schwinger's astonishing theoretical physics talents.
11/4/09	Prof. Thierry Giamarchi, DPMC-MaNEP, University of Geneva "Disorder in cold atomic gases" For quantum systems of fermionic and bosonic particles, disorder has drastic effects. For free fermions, it leads to Anderson localization where quantum interference due to scattering on disorder transform a metal and an insulator. When interactions are present they can either compete or help the disorder to localize the particles. Such a problem has been one of the most challenging questions in condensed matter physics. Cold atoms offer the unique possibility to realize very controlled disordered systems and thus offer a unique laboratory to explore these issues. I will thus review some of the most important issues on the interplay between disorder and interactions, focusing on what happens in low dimensional systems. I will discuss the various consequences for Bosons in either a disordered or a biperiodic optical lattice potential, in the light of recent experiments in cold atomic gases, and in particular whether one can probe the existence of the Bose glass phase by expansion or by shaking of the optical lattice.
Thursday 11/12/09	Prof. Michael Fleischhauer, Fachbereich Physik, Uni Kaiserslautern "Single- and many-body physics with stationary-light polaritons" When light interacts with coherently driven three-level atoms composite particles, called dark-state polaritons (DSP) are formed which behave as massive objects. They are bosons and thus can undergo a Bose-Einstein condensation, which due to the small effective mass happens at a high critical temperature. When the characteristic length of the light pulse becomes small, the dynamics of DSPs obeys a two-component Dirac equation. Since the corresponding effective mass $m^*$ and ``speed of light'' $c^*$ are rather small, relativistic effects such as Klein-tunneling and Zitterbewegung can arise at laboratory length scales. Furthermore anusual localization phenomena associated with the dynamics of Dirac particles with random mass can be observed. Multi-component i.e. spinor polaritons can be created with multi-chromatic drive fields. When confined to 1D and in the presence of a strong elastic scattering heavy photons can fermionize. Finally using a rotating medium one can generate effective magnetic fields for DSPs with small magnetic length which provides a new approach to study the fractional quantum-Hall effect.
11/18/09	POSTPONED:   Prof. Hanns-Christoph Nägerl, Institut für Experimentalphysik Universität Innsbruck "Tunable quantum gases in optical lattices: Ground state molecules and strongly-interacting 1D systems" I will report on several of our recent experiments with tunable quantum gases in optical lattices. In the first experiment, we produce ultracold and dense samples of rovibrational ground state (RGS) molecules near quantum degeneracy. We first associate dimer molecules out of a lattice-based Mott-insulator state loaded from an atomic BEC and then coherently transfer the molecules to the RGS by a multiphoton STIRAP process. With an overall efficiency of 50%, we prepare a molecular quantum gas state in which every second site of an optical lattice is occupied with a RGS molecule [1]. We expect that, with further optimization of the transfer procedure, a BEC of RGS molecules is possible. In the second experiment, we prepare an exotic many-body, highly-correlated quantum state in 1D geometry know as the super-Tonks-Girardeau (sTG) gas. In contrast to the well-known case of the Tonks-Girardeau (TG) gas, interactions are strongly attractive for the sTG gas. The question thus arises whether this state exists at all and how it can be accessed, as attractive interactions give rise to a family of more deeply bound states. We observe a confinement-induced resonance that allows us to first enter deeply into the TG regime and from there to cross over into the sTG regime, which we find to be surprisingly stable. We analyze the crossover in terms of the collective mode frequencies of the 1D system and in terms of the energy and particle loss [2]. In the third experiment, we observe the superfluid-to-Mott-insulator (SF-MI) phase transition for a strongly-interacting 1D gas. For sufficiently strong interactions, the insulating state is induced by an arbitrarily week lattice, in striking contrast to the SF-MI transition observed for weakly-interating 3D gases. We map out the phase diagram and find that our measurements agree well with a quantum field description of the transition based on the well-known sine-Gordon model. [1] //Quantum gas of rovibronic ground-state molecules in an optical lattice//, J.G. Danzl et al., arXiv:0909.4700 (2009). [2] //Realization of an Excited, Strongly Correlated Quantum Gas Phase//, E. Haller et al., Science 325, 1224 (2009).
December
12/02/09	Prof. V. Sandoghdar, Laboratory of Physical Chemistry, ETH Zurich "Cavity-free coherent coupling of photons and emitters" In this seminar I shall discuss several experimental and theoretical investigations of coupling strength between propagating light beams and single emitters. We show that a single dipolar radiator can strongly attenuate a laser beam and shift its phase [1-5]. We find that such an efficient light-matter coupling allows several nonlinear processes to be achieved at low photon levels and present experimental results on the observation of the Mollow triplet [2], Rabi oscillations [5], and stimulated emission [6] using a single organic molecule. Furthermore, I present our recent results on the demonstration of quantum interference between single photons emitted by remote molecules [7]. In closing, I discuss our progress on the coupling of single molecules to optical microcavities and on the coupling and entanglement of two molecules at large distances. [1] G. Zumofen, N.M. Mojarad, V. Sandoghdar, and M. Agio, Phys. Rev. Lett. 101, 180404 (2008). [2] G. Wrigge, I. Gerhardt, J. Hwang, G. Zumofen, and V. Sandoghdar, Nature Phys. 4, 60-66 (2008). [3] I. Gerhardt, G. Wrigge, P. Bushev, G. Zumofen, M. Agio, R. Pfab, and V. Sandoghdar, Phys. Rev. Lett. 98, 033601 (2007). [4] M. Celebrano, R. Lettow, P. Kukura, M. Agio, A. Renn, S. Götzinger, V. Sandoghdar, submitted. [5] I. Gerhardt, G. Wrigge, G. Zumofen, J. Hwang, A. Renn, V. Sandoghdar, Phys. Rev. A 79, 011402(R) (2009). [6] J. Hwang, M. Pototschnig, G. Zumofen, R. Lettow, S. Götzinger, V. Sandoghdar, Nature 460, 76 (2008). [7] R. Lettow, Y. Rezus, G. Zumofen, A. Renn, E. Ikonen, S. Götzinger, V. Sandoghdar, submitted.
12/16/09	Dr. Liang Jiang, CalTech "Repetitive Readout of a Single Electronic Spin via Quantum Logic with Nuclear Spin Ancillae" Robust measurement of single quantum bits plays a key role in the realization of quantum computation and communication as well as in quantum metrology and sensing. We have implemented a method for the improved readout of single electronic spin qubits in solid-state systems. The method makes use of quantum logic operations on a system consisting of a single electronic spin and several proximal nuclear spin ancillae in order to repetitively readout the state of the electronic spin. Using coherent manipulation of a single nitrogen vacancy center in room-temperature diamond, full quantum control of an electronic-nuclear system consisting of up to three spins was achieved. We took advantage of a single nuclear-spin memory in order to obtain a 10-fold enhancement in the signal amplitude of the electronic spin readout. We also present a two-level, concatenated procedure to improve the readout by use of a pair of nuclear spin ancillae, an important step toward the realization of robust quantum information processors using electronic- and nuclear-spin qubits. Our technique can be used to improve the sensitivity and speed of spin-based nanoscale diamond magnetometers.