26 research outputs found
Mechanical effect of van der Waals interactions observed in real time in an ultracold Rydberg gas
We present time-resolved spectroscopic measurements of Rydberg-Rydberg
interactions in an ultracold gas, revealing the pair dynamics induced by
long-range van der Waals interactions between the atoms. By detuning the
excitation laser, a specific pair distribution is prepared. Penning ionization
on a microsecond timescale serves as a probe for the pair dynamics under the
influence of the attractive long-range forces. Comparison with a Monte Carlo
model not only explains all spectroscopic features but also gives quantitative
information about the interaction potentials. The results imply that the
interaction-induced ionization rate can be influenced by the excitation laser.
Surprisingly, interaction-induced ionization is also observed for Rydberg
states with purely repulsive interactions
Rabi oscillations between ground and Rydberg states and van der Waals blockade in a mesoscopic frozen Rydberg gas
We present a detailed analysis of our recent observation of synchronous Rabi
oscillations between the electronic ground state and Rydberg states in a
mesoscopic ensemble containing roughly 100 ultracold atoms [M. Reetz-Lamour
\textit{et al.}, submitted, arXiv:0711.4321]. The mesoscopic cloud is selected
out of a sample of laser-cooled Rb atoms by optical pumping. The atoms are
coupled to a Rydberg state with principal quantum number around 30 by a
two-photon scheme employing flat-top laser beams. The influence of residual
spatial intensity fluctuations as well as sources of decoherence such as
redistribution to other states, radiative lifetime, and laser bandwidth are
analysed. The results open up new possibilities for the investigation of
coherent many-body phenomena in dipolar Rydberg gases. As an example we
demonstrate the van der Waals blockade, a variant of the dipole blockade, for a
mesoscopic atom sample
A planar multipole ion trap
We report on the realisation of a chip-based multipole ion trap manufactured
using micro-electromechanical systems (MEMS) technology. It provides ion
confinement in an almost field-free volume between two planes of radiofrequency
electrodes, deposited on glass substrates, which allows for optical access to
the trap. An analytical model of the effective trapping potential is presented
and compared with numerical calculations. Stable trapping of argon ions is
achieved and a lifetime of 16s is measured. Electrostatic charging of the chip
surfaces is studied and found to agree with a numerical estimate
Measurement of the electric dipole moments for transitions to rubidium Rydberg states via Autler-Townes splitting
We present the direct measurements of electric-dipole moments for
transitions with for Rubidium atoms. The
measurements were performed in an ultracold sample via observation of the
Autler-Townes splitting in a three-level ladder scheme, commonly used for
2-photon excitation of Rydberg states. To the best of our knowledge, this is
the first systematic measurement of the electric dipole moments for transitions
from low excited states of rubidium to Rydberg states. Due to its simplicity
and versatility, this method can be easily extended to other transitions and
other atomic species with little constraints. Good agreement of the
experimental results with theory proves the reliability of the measurement
method.Comment: 12 pages, 6 figures; figure 6 replaced with correct versio
Observation of coherent many-body Rabi oscillations
A two-level quantum system coherently driven by a resonant electromagnetic
field oscillates sinusoidally between the two levels at frequency
which is proportional to the field amplitude [1]. This phenomenon, known as the
Rabi oscillation, has been at the heart of atomic, molecular and optical
physics since the seminal work of its namesake and coauthors [2]. Notably, Rabi
oscillations in isolated single atoms or dilute gases form the basis for
metrological applications such as atomic clocks and precision measurements of
physical constants [3]. Both inhomogeneous distribution of coupling strength to
the field and interactions between individual atoms reduce the visibility of
the oscillation and may even suppress it completely. A remarkable
transformation takes place in the limit where only a single excitation can be
present in the sample due to either initial conditions or atomic interactions:
there arises a collective, many-body Rabi oscillation at a frequency
involving all N >> 1 atoms in the sample [4]. This is true even
for inhomogeneous atom-field coupling distributions, where single-atom Rabi
oscillations may be invisible. When one of the two levels is a strongly
interacting Rydberg level, many-body Rabi oscillations emerge as a consequence
of the Rydberg excitation blockade. Lukin and coauthors outlined an approach to
quantum information processing based on this effect [5]. Here we report initial
observations of coherent many-body Rabi oscillations between the ground level
and a Rydberg level using several hundred cold rubidium atoms. The strongly
pronounced oscillations indicate a nearly complete excitation blockade of the
entire mesoscopic ensemble by a single excited atom. The results pave the way
towards quantum computation and simulation using ensembles of atoms
Observation of mesoscopic crystalline structures in a two-dimensional Rydberg gas
The ability to control and tune interactions in ultracold atomic gases has
paved the way towards the realization of new phases of matter. Whereas
experiments have so far achieved a high degree of control over short-ranged
interactions, the realization of long-range interactions would open up a whole
new realm of many-body physics and has become a central focus of research.
Rydberg atoms are very well-suited to achieve this goal, as the van der Waals
forces between them are many orders of magnitude larger than for ground state
atoms. Consequently, the mere laser excitation of ultracold gases can cause
strongly correlated many-body states to emerge directly when atoms are
transferred to Rydberg states. A key example are quantum crystals, composed of
coherent superpositions of different spatially ordered configurations of
collective excitations. Here we report on the direct measurement of strong
correlations in a laser excited two-dimensional atomic Mott insulator using
high-resolution, in-situ Rydberg atom imaging. The observations reveal the
emergence of spatially ordered excitation patterns in the high-density
components of the prepared many-body state. They have random orientation, but
well defined geometry, forming mesoscopic crystals of collective excitations
delocalised throughout the gas. Our experiment demonstrates the potential of
Rydberg gases to realise exotic phases of matter, thereby laying the basis for
quantum simulations of long-range interacting quantum magnets.Comment: 10 pages, 7 figure
Kinetic Monte Carlo modelling of dipole blockade in Rydberg excitation experiment
We present a method to model the interaction and the dynamics of atoms
excited to Rydberg states. We show a way to solve the optical Bloch equations
for laser excitation of the frozen gas in good agreement with the experiment. A
second method, the Kinetic Monte Carlo method gives an exact solution of rate
equations. Using a simple N-body integrator (Verlet), we are able to describe
dynamical processes in space and time. Unlike more sophisticated methods, the
Kinetic Monte Carlo simulation offers the possibility of numerically following
the evolution of tens of thousands of atoms within a reasonable computation
time. The Kinetic Monte Carlo simulation gives good agreement with
dipole-blockade type of experiment. The role of ions and the individual
particle effects are investigated.Comment: 23 pages. Submitted to New Journal of Physic