7,393 research outputs found
Time-resolved Observation and Control of Superexchange Interactions with Ultracold Atoms in Optical Lattices
Quantum mechanical superexchange interactions form the basis of quantum
magnetism in strongly correlated electronic media. We report on the direct
measurement of superexchange interactions with ultracold atoms in optical
lattices. After preparing a spin-mixture of ultracold atoms in an
antiferromagnetically ordered state, we measure a coherent
superexchange-mediated spin dynamics with coupling energies from 5 Hz up to 1
kHz. By dynamically modifying the potential bias between neighboring lattice
sites, the magnitude and sign of the superexchange interaction can be
controlled, thus allowing the system to be switched between antiferromagnetic
or ferromagnetic spin interactions. We compare our findings to predictions of a
two-site Bose-Hubbard model and find very good agreement, but are also able to
identify corrections which can be explained by the inclusion of direct
nearest-neighbor interactions.Comment: 24 pages, 7 figure
Nuclear quantum optics with x-ray laser pulses
The direct interaction of nuclei with super-intense laser fields is studied.
We show that present and upcoming high-frequency laser facilities, especially
together with a moderate acceleration of the target nuclei, do allow for
resonant laser-nucleus interaction. These direct interactions may be utilized
for the optical measurement of nuclear properties such as the transition
frequency and the dipole moment, thus opening the field of nuclear quantum
optics. As ultimate goal, one may hope that direct laser-nucleus interactions
could become a versatile tool to enhance preparation, control and detection in
nuclear physics.Comment: 5 pages, 3 eps figures, revised versio
Efficient Immunization Strategies for Computer Networks and Populations
We present an effective immunization strategy for computer networks and
populations with broad and, in particular, scale-free degree distributions. The
proposed strategy, acquaintance immunization, calls for the immunization of
random acquaintances of random nodes (individuals). The strategy requires no
knowledge of the node degrees or any other global knowledge, as do targeted
immunization strategies. We study analytically the critical threshold for
complete immunization. We also study the strategy with respect to the
susceptible-infected-removed epidemiological model. We show that the
immunization threshold is dramatically reduced with the suggested strategy, for
all studied cases.Comment: Revtex, 5 pages, 4 ps fig
Gravity a la Born-Infeld
A simple technique for the construction of gravity theories in Born-Infeld
style is presented, and the properties of some of these novel theories are
investigated. They regularize the positive energy Schwarzschild singularity,
and a large class of models allows for the cancellation of ghosts. The possible
correspondence to low energy string theory is discussed. By including curvature
corrections to all orders in alpha', the new theories nicely illustrate a
mechanism that string theory might use to regularize gravitational
singularities.Comment: 21 pages, 2 figures, new appendix B with corrigendum: Class. Quantum
Grav. 21 (2004) 529
Counting atoms using interaction blockade in an optical superlattice
We report on the observation of an interaction blockade effect for ultracold
atoms in optical lattices, analogous to Coulomb blockade observed in mesoscopic
solid state systems. When the lattice sites are converted into biased double
wells, we detect a discrete set of steps in the well population for increasing
bias potentials. These correspond to tunneling resonances where the atom number
on each side of the barrier changes one by one. This allows us to count and
control the number of atoms within a given well. By evaluating the amplitude of
the different plateaus, we can fully determine the number distribution of the
atoms in the lattice, which we demonstrate for the case of a superfluid and
Mott insulating regime of 87Rb.Comment: 4 pages, 4 figure
Calibration of second-order correlation functions for non-stationary sources with a multi-start multi-stop time-to-digital converter
A novel high-throughput second-order-correlation measurement system is
developed which records and makes use of all the arrival times of photons
detected at both start and stop detectors. This system is suitable particularly
for a light source having a high photon flux and a long coherence time since it
is more efficient than conventional methods by an amount equal to the product
of the count rate and the correlation time of the light source. We have used
this system in carefully investigating the dead time effects of detectors and
photon counters on the second-order correlation function in the two-detector
configuration. For a non-stationary light source, distortion of original signal
was observed at high photon flux. A systematic way of calibrating the
second-order correlation function has been devised by introducing a concept of
an effective dead time of the entire measurement system.Comment: 7 pages, 6 figure
Loudly sing cuckoo : More-than-human seasonalities in Britain
This research was funded by a grant from the Arts and Humanities Research Council, grant number AH/E009573/1.Peer reviewedPostprin
Observation of sub-Poisson photon statistics in the cavity-QED microlaser
We have measured the second-order correlation function of the cavity-QED
microlaser output and observed a transition from photon bunching to
antibunching with increasing average number of intracavity atoms. The observed
correlation times and the transition from super- to sub-Poisson photon
statistics can be well described by gain-loss feedback or enhanced/reduced
restoring action against fluctuations in photon number in the context of a
quantum microlaser theory and a photon rate equation picture. However, the
theory predicts a degree of antibunching several times larger than that
observed, which may indicate the inadequacy of its treatment of atomic velocity
distributions.Comment: 4 pages, 4 figure
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