arXiv subjects

S. Whitlock

Publications and source records attributed to S. Whitlock.

At least 37 records · Page 2Linked to original sources

arXiv2014

Semi-analytical model for nonlinear light propagation in strongly interacting Rydberg gases

Rate equation models are extensively used to describe the many-body states of laser driven atomic gases. We show that the properties of the rate equation model used to describe nonlinear optical effects arising in interacting Rydberg gases can be understood by considering the excitation of individual super-atoms. From this we deduce a simple semi-analytic model that accurately describes the Rydberg density and optical susceptibility for different dimensionalities. We identify the previously reported universal dependence of the susceptibility on the Rydberg excited fraction as an intrinsic property of the rate equation model that is rooted in one-body properties. Benchmarking against exact master equation calculations, we identify regimes in which the semi-analytic model is particularly reliable. The performance of the model improves in the presence of dephasing which destroys higher order atomic coherences.

physics.atom-ph↗

arXiv2014

Full counting statistics of laser excited Rydberg aggregates in a one-dimensional geometry

We experimentally study the full counting statistics of few-body Rydberg aggregates excited from a quasi-one-dimensional Rydberg gas. We measure asymmetric excitation spectra and increased second and third order statistical moments of the Rydberg number distribution, from which we determine the average aggregate size. Direct comparisons with numerical simulations reveal the presence of liquid-like spatial correlations, and indicate sequential growth of the aggregates around an initial grain. These findings demonstrate the importance of dissipative effects in strongly correlated Rydberg gases and introduce a way to study spatio-temporal correlations in strongly-interacting many-body quantum systems without imaging.

physics.atom-ph↗

arXiv2013

An experimental approach for investigating many-body phenomena in Rydberg-interacting quantum systems

Recent developments in the study of ultracold Rydberg gases demand an advanced level of experimental sophistication, in which high atomic and optical densities must be combined with excellent control of external fields and sensitive Rydberg atom detection. We describe a tailored experimental system used to produce and study Rydberg-interacting atoms excited from dense ultracold atomic gases. The experiment has been optimized for fast duty cycles using a high flux cold atom source and a three beam optical dipole trap. The latter enables tuning of the atomic density and temperature over several orders of magnitude, all the way to the Bose-Einstein condensation transition. An electrode structure surrounding the atoms allows for precise control over electric fields and single-particle sensitive field ionization detection of Rydberg atoms. We review two experiments which highlight the influence of strong Rydberg--Rydberg interactions on different many-body systems. First, the Rydberg blockade effect is used to pre-structure an atomic gas prior to its spontaneous evolution into an ultracold plasma. Second, hybrid states of photons and atoms called dark-state polaritons are studied. By looking at the statistical distribution of Rydberg excited atoms we reveal correlations between dark-state polaritons. These experiments will ultimately provide a deeper understanding of many-body phenomena in strongly-interacting regimes, including the study of strongly-coupled plasmas and interfaces between atoms and light at the quantum level.

physics.atom-ph↗

arXiv2013

Spontaneous avalanche ionization of a strongly blockaded Rydberg gas

We report the sudden and spontaneous evolution of an initially correlated gas of repulsively interacting Rydberg atoms to an ultracold plasma. Under continuous laser coupling we create a Rydberg ensemble in the strong blockade regime, which at longer times undergoes an ionization avalanche. By combining optical imaging and ion detection, we access the full information on the dynamical evolution of the system, including the rapid increase in the number of ions and a sudden depletion of the Rydberg and ground state densities. Rydberg-Rydberg interactions are observed to strongly affect the dynamics of plasma formation. Using a coupled rate-equation model to describe our data, we extract the average energy of electrons trapped in the plasma, and an effective cross-section for ionizing collisions between Rydberg atoms and atoms in low-lying states. Our results suggest that the initial correlations of the Rydberg ensemble should persist through the avalanche. This would provide the means to overcome disorder-induced-heating, and offer a route to enter new strongly-coupled regimes.

physics.atom-ph↗

arXiv2011

Interaction enhanced imaging of individual atoms embedded in dense atomic gases

We propose a new all-optical method to image individual atoms within dense atomic gases. The scheme exploits interaction induced shifts on highly polarizable excited states, which can be spatially resolved via an electromagnetically induced transparency resonance. We focus in particular on imaging strongly interacting many-body states of Rydberg atoms embedded in an ultracold gas of ground state atoms. Using a realistic model we show that it is possible to image individual impurity atoms with enhanced sensitivity and high resolution despite photon shot noise and atomic density fluctuations. This new imaging scheme is ideally suited to equilibrium and dynamical studies of complex many-body phenomena involving strongly interacting atoms. As an example we study blockade effects and correlations in the distribution of Rydberg atoms optically excited from a dense gas.

physics.atom-ph↗

arXiv2011

Crossover from 2D to 3D in a weakly interacting Fermi gas

We have studied the transition from two to three dimensions in a low temperature weakly interacting $^6$Li Fermi gas. Below a critical atom number, $N_{2D}$, only the lowest transverse vibrational state of a highly anisotropic oblate trapping potential is occupied and the gas is two-dimensional. Above $N_{2D}$ the Fermi gas enters the quasi-2D regime where shell structure associated with the filling of individual transverse oscillator states is apparent. This dimensional crossover is demonstrated through measurements of the cloud size and aspect ratio versus atom number.

cond-mat.quant-gas↗

arXiv2010

Controlling spin motion and interactions in a one-dimensional Bose gas

Experiments on ultracold gases offer unparalleled opportunities to explore quantum many-body physics, with excellent control over key parameters including temperature, density, interactions and even dimensionality. In some systems, atomic interactions can be adjusted by means of magnetic Feshbach resonances, which have played a crucial role in realizing new many-body phenomena. However, suitable Feshbach resonances are not always available, and they offer limited freedom since the magnetic field strength is the only control parameter. Here we show a new way to tune interactions in one-dimensional quantum gases using state-dependent dressed potentials, enabling control over non-equilibrium spin motion in a two-component gas of 87Rb. The accessible range includes the point of spin-independent interactions where exact quantum many-body solutions are available and the point where spin motion is frozen. This versatility opens a new route to experiments on spin waves, spin-"charge" separation and the relation between superfluidity and magnetism in low-dimensional quantum gases.

cond-mat.quant-gas↗

arXiv2010

Improved detection of small atom numbers through image processing

We demonstrate improved detection of small trapped atomic ensembles through advanced post-processing and optimal analysis of absorption images. A fringe removal algorithm reduces imaging noise to the fundamental photon-shot-noise level and proves beneficial even in the absence of fringes. A maximum-likelihood estimator is then derived for optimal atom-number estimation and is applied to real experimental data to measure the population differences and intrinsic atom shot-noise between spatially separated ensembles each comprising between 10 and 2000 atoms. The combined techniques improve our signal-to-noise by a factor of 3, to a minimum resolvable population difference of 17 atoms, close to our ultimate detection limit.

physics.atom-ph↗

arXiv2010

Spatially Resolved Excitation of Rydberg Atoms and Surface Effects on an Atom Chip

We demonstrate spatially resolved, coherent excitation of Rydberg atoms on an atom chip. Electromagnetically induced transparency (EIT) is used to investigate the properties of the Rydberg atoms near the gold coated chip surface. We measure distance dependent shifts (~10 MHz) of the Rydberg energy levels caused by a spatially inhomogeneous electric field. The measured field strength and distance dependence is in agreement with a simple model for the electric field produced by a localized patch of Rb adsorbates deposited on the chip surface during experiments. The EIT resonances remain narrow (< 4 MHz) and the observed widths are independent of atom-surface distance down to ~20 \mum, indicating relatively long lifetime of the Rydberg states. Our results open the way to studies of dipolar physics, collective excitations, quantum metrology and quantum information processing involving interacting Rydberg excited atoms on atom chips.

quant-ph↗

arXiv2009

Box traps on an atom chip for one-dimensional quantum gases

We present the implementation of tailored trapping potentials for ultracold gases on an atom chip. We realize highly elongated traps with box-like confinement along the long, axial direction combined with conventional harmonic confinement along the two radial directions. The design, fabrication and characterization of the atom chip and the box traps is described. We load ultracold ($\lesssim1 μ$K) clouds of $^{87}$Rb in a box trap, and demonstrate Bose-gas focusing as a means to characterize these atomic clouds in arbitrarily shaped potentials. Our results show that box-like axial potentials on atom chips are very promising for studies of one-dimensional quantum gases.

physics.atom-ph↗

arXiv2009

Sub-Poissonian atom number fluctuations by three-body loss in mesoscopic ensembles

We show that three-body loss of trapped atoms leads to sub-Poissonian atom number fluctuations. We prepare hundreds of dense ultracold ensembles in an array of magnetic microtraps which undergo rapid three-body decay. The shot-to-shot fluctuations of the number of atoms per trap are sub-Poissonian, for ensembles comprising 50--300 atoms. The measured relative variance or Fano factor $F=0.53\pm 0.22$ agrees very well with the prediction by an analytic theory ($F=3/5$) and numerical calculations. These results will facilitate studies of quantum information science with mesoscopic ensembles.

physics.atom-ph↗

arXiv2009

Two-dimensional array of microtraps with atomic shift register on a chip

Arrays of trapped atoms are the ideal starting point for developing registers comprising large numbers of physical qubits for storing and processing quantum information. One very promising approach involves neutral atom traps produced on microfabricated devices known as atom chips, as almost arbitrary trap configurations can be realised in a robust and compact package. Until now, however, atom chip experiments have focused on small systems incorporating single or only a few individual traps. Here we report experiments on a two-dimensional array of trapped ultracold atom clouds prepared using a simple magnetic-film atom chip. We are able to load atoms into hundreds of tightly confining and optically resolved array sites. We then cool the individual atom clouds in parallel to the critical temperature required for quantum degeneracy. Atoms are shuttled across the chip surface utilising the atom chip as an atomic shift register and local manipulation of atoms is implemented using a focused laser to rapidly empty individual traps.

physics.atom-ph↗

arXiv2008

Fully permanent magnet atom chip for Bose-Einstein condensation

We describe a self-biased, fully permanent magnet atom chip used to study ultracold atoms and to produce a Bose-Einstein condensate (BEC). The magnetic trap is loaded efficiently by adiabatic transport of a magnetic trap via the application of uniform external fields. Radio frequency spectroscopy is used for in-trap analysis and to determine the temperature of the atomic cloud. The formation of a Bose-Einstein condensate is observed in time of flight images and as a narrow peak appearing in the radio frequency spectrum.

cond-mat.other↗

arXiv2008

Three-dimensional character of atom-chip-based rf-dressed potentials

We experimentally investigate the properties of radio-frequency-dressed potentials for Bose-Einstein condensates on atom chips. The three-dimensional potential forms a connected pair of parallel waveguides. We show that rf-dressed potentials are robust against the effect of small magnetic-field variations on the trap potential. Long-lived dipole oscillations of condensates induced in the rf-dressed potentials can be tuned to a remarkably low damping rate. We study a beam-splitter for Bose-Einstein condensates and show that a propagating condensate can be dynamically split in two vertically separated parts and guided along two paths. The effect of gravity on the potential can be tuned and compensated for using a rf-field gradient.

cond-mat.other↗

arXiv2007

A lattice of microtraps for ultracold atoms based on patterned magnetic films

We have realized a two dimensional permanent magnetic lattice of Ioffe-Pritchard microtraps for ultracold atoms. The lattice is formed by a single 300 nm magnetized layer of FePt, patterned using optical lithography. Our magnetic lattice consists of more than 15000 tightly confining microtraps with a density of 1250 traps/mm$^2$. Simple analytical approximations for the magnetic fields produced by the lattice are used to derive relevant trap parameters. We load ultracold atoms into at least 30 lattice sites at a distance of approximately 10 $μ$m from the film surface. The present result is an important first step towards quantum information processing with neutral atoms in magnetic lattice potentials.

physics.atom-ph↗

arXiv2007

Scanning magnetoresistance microscopy of atom chips

Surface based geometries of microfabricated wires or patterned magnetic films can be used to magnetically trap and manipulate ultracold neutral atoms or Bose-Einstein condensates. We investigate the magnetic properties of such atom chips using a scanning magnetoresistive (MR) microscope with high spatial resolution and high field sensitivity. We show that MR sensors are ideally suited to observe small variations of the magnetic field caused by imperfections in the wires or magnetic materials which ultimately lead to fragmentation of ultracold atom clouds. Measurements are also provided for the magnetic field produced by a thin current-carrying wire with small geometric modulations along the edge. Comparisons of our measurements with a full numeric calculation of the current flow in the wire and the subsequent magnetic field show excellent agreement. Our results highlight the use of scanning MR microscopy as a convenient and powerful technique for precisely characterizing the magnetic fields produced near the surface of atom chips.

physics.atom-ph↗

arXiv2006

Effect of Magnetization Inhomogeneity on Magnetic Microtraps for Atoms

We report on the origin of fragmentation of ultracold atoms observed on a permanent magnetic film atom chip. A novel technique is used to characterize small spatial variations of the magnetic field near the film surface using radio frequency spectroscopy of the trapped atoms. Direct observations indicate the fragmentation is due to a corrugation of the magnetic potential caused by long range inhomogeneity in the film magnetization. A model which takes into account two-dimensional variations of the film magnetization is consistent with the observations.

physics.atom-ph↗

arXiv2006

Condensate splitting in an asymmetric double well for atom chip based sensors

We report on the adiabatic splitting of a BEC of $^{87}$Rb atoms by an asymmetric double-well potential located above the edge of a perpendicularly magnetized TbGdFeCo film atom chip. By controlling the barrier height and double-well asymmetry the sensitivity of the axial splitting process is investigated through observation of the fractional atom distribution between the left and right wells. This process constitutes a novel sensor for which we infer a single shot sensitivity to gravity fields of $δg/g\approx2\times10^{-4}$. From a simple analytic model we propose improvements to chip-based gravity detectors using this demonstrated methodology.

cond-mat.other↗