523 research outputs found
Ultracold atoms in optical lattices
Bosonic atoms trapped in an optical lattice at very low temperatures, can be
modeled by the Bose-Hubbard model. In this paper, we propose a slave-boson
approach for dealing with the Bose-Hubbard model, which enables us to
analytically describe the physics of this model at nonzero temperatures. With
our approach the phase diagram for this model at nonzero temperatures can be
quantified.Comment: 29 pages, 10 figure
Quantum phases in a resonantly-interacting Bose-Fermi mixture
We consider a resonantly-interacting Bose-Fermi mixture of K and
Rb atoms in an optical lattice. We show that by using a red-detuned
optical lattice the mixture can be accurately described by a generalized
Hubbard model for K and Rb atoms, and K-Rb
molecules. The microscopic parameters of this model are fully determined by the
details of the optical lattice and the interspecies Feshbach resonance in the
absence of the lattice. We predict a quantum phase transition to occur in this
system already at low atomic filling fraction, and present the phase diagram as
a function of the temperature and the applied magnetic field.Comment: 4 pages, 3 figure
Theory for Bose-Einstein condensation of light in nano-fabricated semiconductor microcavities
We construct a theory for Bose-Einstein condensation of light in
nano-fabricated semiconductor microcavities. We model the semiconductor by one
conduction and one valence band which consist of electrons and holes that
interact via a Coulomb interaction. Moreover, we incorporate screening effects
by using a contact interaction with the scattering length for a Yukawa
potential and describe in this manner the crossover from exciton gas to
electron-hole plasma as we increase the excitation level of the semiconductor.
We then show that the dynamics of the light in the microcavities is damped due
to the coupling to the semiconductor. Furthermore, we demonstrate that on the
electron-hole plasma side of the crossover, which is relevant for the
Bose-Einstein condensation of light, this damping can be described by a single
dimensionless damping parameter that depends on the external pumping.
Hereafter, we propose to probe the superfluidity of light in these
nano-fabricated semiconductor microcavities by making use of the differences in
the response in the normal or superfluid phase to a sudden rotation of the
trap. In particular, we determine frequencies and damping of the scissors modes
that are excited in this manner. Moreover, we show that a distinct signature of
the dynamical Casimir effect can be observed in the density-density
correlations of the excited light fluid
Mott insulators in an optical lattice with high filling factors
We discuss the superfluid to Mott insulator transition of an atomic Bose gas
in an optical lattice with high filling factors. We show that also in this
multi-band situation, the long-wavelength physics is described by a single-band
Bose-Hubbard model. We determine the many-body renormalization of the tunneling
and interaction parameters in the effective Bose-Hubbard Hamiltonian, and
consider the resulting model at nonzero temperatures. We show that in
particular for a one or two-dimensional optical lattice, the Mott insulator
phase is more difficult to realize than anticipated previously.Comment: 5 pages, 3 figures, title changed, major restructuring, resubmitted
to PR
On the Role of Penning Ionization in Photoassociation Spectroscopy
We study the role of Penning ionization on the photoassociation spectra of
He(^3S)-He(^3S). The experimental setup is discussed and experimental results
for different intensities of the probe laser are shown. For modelling the
experimental results we consider coupled-channel calculations of the crossing
of the ground state with the excited state at the Condon point. The
coupled-channel calculations are first applied to model systems, where we
consider two coupled channels without ionization, two coupled channels with
ionization, and three coupled channels, for which only one of the excited
states is ionizing. Finally, coupled-channel calculations are applied to
photoassociation of He(^3S)-He(^3S) and good agreement is obtained between the
model and the experimental results.Comment: 14 pages, 18 figures, submitted to the special issue on Cold
Molecules of J. Phys.
Free fermion antibunching in a degenerate atomic Fermi gas released from an optical lattice
Noise in a quantum system is fundamentally governed by the statistics and the
many-body state of the underlying particles. Whereas for bosonic particles the
correlated noise observed for e.g. photons or bosonic neutral atoms can still
be explained within a classical field description with fluctuating phases, the
anticorrelations in the detection of fermionic particles have no classical
analogue. The observation of such fermionic antibunching is so far scarce and
has been confined to electrons and neutrons. Here we report on the first direct
observation of antibunching of neutral fermionic atoms. Through an analysis of
the atomic shot noise in a set of standard absorption images, of a gas of
fermionic 40K atoms released from an optical lattice, we find reduced
correlations for distances related to the original spacing of the trapped
atoms. The detection of such quantum statistical correlations has allowed us to
characterise the ordering and temperature of the Fermi gas in the lattice.
Moreover, our findings are an important step towards revealing fundamental
fermionic many-body quantum phases in periodic potentials, which are at the
focus of current research.Comment: (Nature, in press
Ultrafast rerouting of light via slow modes in a nanophotonic directional coupler
We demonstrate that two coupled photonic-crystal waveguides can route two subsequently arriving light pulses to different output ports even though the pulses are only 3 ps apart. This rerouting of light is due to an ultrafast shift in the transmittance spectrum triggered by the generation of electrons and holes in the Si base material by a femtosecond laser pulse. The use of slow-light modes allows for a coupler length of only 5.2 μm. Since these modes are not directly involved, the 3 ps dead time is solely determined by the duration of the input pulse rather than its transit time through the device.We acknowledge funding through the EU FP6-FET
“SPLASH” project. This work is also part of the research
program of FOM, which is financially supported by the
NWO
Cross-talk between signaling pathways leading to defense against pathogens and insects
In nature, plants interact with a wide range of organisms, some of which
are harmful (e.g. pathogens, herbivorous insects), while others are beneficial
(e.g. growth-promoting rhizobacteria, mycorrhizal fungi, and predatory
enemies of herbivores). During the evolutionary arms race between plants
and their attackers, primary and secondary immune responses evolved to
recognize common or highly specialized features of microbial pathogens
(Chisholm et al., 2006), resulting in sophisticated mechanisms of defense
Insulator-Superfluid transition of spin-1 bosons in an optical lattice in magnetic field
We study the insulator-superfluid transition of spin-1 bosons in an optical
lattice in a uniform magnetic field. Based on a mean-field approximation we
obtained a zero-temperature phase diagram. We found that depending on the
particle number the transition for bosons with antiferromagnetic interaction
may occur into different superfluid phases with spins aligned along or opposite
to the field direction. This is qualitatively different from the field-free
transition for which the mean-field theory predicts a unique (polar) superfluid
state for any particle number.Comment: 10 pages, 2 eps figure
- …