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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Spring Semester 09
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January
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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.
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February
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March
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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.
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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.
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April
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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.
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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).
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4/29/09 |
Mark Raizen, UT-Austin
Towards Trapping and Cooling of Atomic Tritium
for Precision Measurement of Beta Decay
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May
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5/13/09 |
Andreas Buchleitner, Univ. of Freiburg
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June
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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
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Fall Semester 09
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September
2009
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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.
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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.
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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.
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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).
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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.
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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.
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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.
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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).
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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.
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12/16/09 |
Dr. Liang Jiang, CalTech
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