35 research outputs found
Self-organization of atoms in a cavity field: threshold, bistability and scaling laws
We present a detailed study of the spatial self-organization of laser-driven
atoms in an optical cavity, an effect predicted on the basis of numerical
simulations [P. Domokos and H. Ritsch, Phys. Rev. Lett. 89, 253003 (2002)] and
observed experimentally [A. T. Black et al., Phys. Rev. Lett. 91, 203001
(2003)]. Above a threshold in the driving laser intensity, from a uniform
distribution the atoms evolve into one of two stable patterns that produce
superradiant scattering into the cavity. We derive analytic formulas for the
threshold and critical exponent of this phase transition from a mean-field
approach. Numerical simulations of the microscopic dynamics reveal that, on
laboratory timescale, a hysteresis masks the mean-field behaviour. Simple
physical arguments explain this phenomenon and provide analytical expressions
for the observable threshold. Above a certain density of the atoms a limited
number of ``defects'' appear in the organized phase, and influence the
statistical properties of the system. The scaling of the cavity cooling
mechanism and the phase space density with the atom number is also studied.Comment: submitted to PR
Loading atoms from a large magnetic trap to a small intra-cavity dipole trap
We show that an optimized loading of a cold ensemble of rubidium-87 atoms
from a magnetic trap into an optical dipole trap sustained by a single,
far-red-detuned mode of a high-Q optical cavity can be efficient despite the
large volume mismatch of the traps. The magnetically trapped atoms are
magnetically transported to the vicinity of the cavity mode and released from
the magnetic trap in a controlled way meanwhile undergoing an evaporation
period. Large number of atoms get trapped in the dipole potential for several
hundreds of milliseconds. We monitor the number of atoms in the mode volume by
a second tone of the cavity close to the atomic resonance. While this probe
tone can pump atoms to another ground state uncoupled to the probe, we
demonstrate state-independent trapping by applying a repumper laser
Quantum stability of self-organized atomic insulator-like states in optical resonators
We investigate a paradigm example of cavity quantum electrodynamics with many
body systems: an ultracold atomic gas inside a pumped optical resonator. In
particular, we study the stability of atomic insulator-like states, confined by
the mechanical potential emerging from the cavity field spatial mode structure.
As in open space, when the optical potential is sufficiently deep, the atomic
gas is in the Mott-like state. Inside the cavity, however, the potential
depends on the atomic distribution, which determines the refractive index of
the medium, thus altering the intracavity field amplitude. We derive the
effective Bose-Hubbard model describing the physics of the system in one
dimension and study the crossover between the superfluid -- Mott insulator
quantum states. We determine the regions of parameters where the atomic
insulator states are stable, and predict the existence of overlapping stability
regions corresponding to competing insulator-like states. Bistable behavior,
controlled by the pump intensity, is encountered in the vicinity of the shifted
cavity resonance.Comment: 13 pages, 6 figures. Replaced with revised version. Accepted for
publication in New J. Phys., special issue "Quantum correlations in tailord
matter
Multimode mean-field model for the quantum phase transition of a Bose-Einstein condensate in an optical resonator
We develop a mean-field model describing the Hamiltonian interaction of
ultracold atoms and the optical field in a cavity. The Bose-Einstein condensate
is properly defined by means of a grand-canonical approach. The model is
efficient because only the relevant excitation modes are taken into account.
However, the model goes beyond the two-mode subspace necessary to describe the
self-organization quantum phase transition observed recently. We calculate all
the second-order correlations of the coupled atom field and radiation field
hybrid bosonic system, including the entanglement between the two types of
fields.Comment: 10 page
Advances in structure elucidation of small molecules using mass spectrometry
The structural elucidation of small molecules using mass spectrometry plays an important role in modern life sciences and bioanalytical approaches. This review covers different soft and hard ionization techniques and figures of merit for modern mass spectrometers, such as mass resolving power, mass accuracy, isotopic abundance accuracy, accurate mass multiple-stage MS(n) capability, as well as hybrid mass spectrometric and orthogonal chromatographic approaches. The latter part discusses mass spectral data handling strategies, which includes background and noise subtraction, adduct formation and detection, charge state determination, accurate mass measurements, elemental composition determinations, and complex data-dependent setups with ion maps and ion trees. The importance of mass spectral library search algorithms for tandem mass spectra and multiple-stage MS(n) mass spectra as well as mass spectral tree libraries that combine multiple-stage mass spectra are outlined. The successive chapter discusses mass spectral fragmentation pathways, biotransformation reactions and drug metabolism studies, the mass spectral simulation and generation of in silico mass spectra, expert systems for mass spectral interpretation, and the use of computational chemistry to explain gas-phase phenomena. A single chapter discusses data handling for hyphenated approaches including mass spectral deconvolution for clean mass spectra, cheminformatics approaches and structure retention relationships, and retention index predictions for gas and liquid chromatography. The last section reviews the current state of electronic data sharing of mass spectra and discusses the importance of software development for the advancement of structure elucidation of small molecules
Ground state bistability of cold atoms in a cavity
We experimentally demonstrate an optical bistability between two hyperfine
atomic ground states, using a single mode of an optical resonator in the
collective strong coupling regime. Whereas in the familiar case, the bistable
region is created through atomic saturation, we report an effect between states
of high quantum purity, which is essential for future information storage. The
nonlinearity of the transitions arise from cavity-assisted pumping between
ground states of cold, trapped atoms and the stability depends on the intensity
of two driving lasers. We interpret the phenomenon in terms of the recent
paradigm of first-order, driven-dissipative phase transitions, where the
transmitted and driving fields are understood as the order and control
parameters, respectively. The saturation-induced bistability is recovered for
infinite drive in one of the controls. The order of the transition is confirmed
experimentally by hysteresis in the order parameter when either of the two
control parameters is swept repeatedly across the bistability region and the
underlying phase diagram is predicted in line with semiclassical mean-field
theory