We report on the design, fabrication and characterization of magnetic nanostructures to create a lattice of magnetic traps with sub-micron period for trapping ultracold atoms. These magnetic nanostructures were fabricated by patterning a Co/Pd multilayered magnetic film grown on a silicon substrate using high precision e-beam lithography and reactive ion etching. The Co/Pd film was chosen for its small grain size and high remanent magnetization and coercivity. The fabricated structures are designed to magnetically trap 87Rb atoms above the surface of the magnetic film with one-dimensional and two-dimensional (triangular and square) lattice geometries and sub-micron period. Such magnetic lattices can be used for quantum tunneling and quantum simulation experiments, including using geometries and periods that may be inaccessible with optical lattices. I. Herrera, Y Wang, P Michaux, D Nissen, P Surendran, S Juodkazis, S Whitlock, R J McLean, A Sidorov, M Albrecht, P Hannaford, Journal of Physics D: Applied Physics48, 115002 (2015)

# Category: Publications

For a full list of publications by the Exotic Quantum Matter group, please see Google scholar citations

## Radio-frequency spectroscopy of a linear array of Bose-Einstein condensates in a magnetic lattice, Phys. Rev. A

We report site-resolved radio-frequency spectroscopy measurements of Bose-Einstein condensates of 87Rb atoms in about 100 sites of a one-dimensional (1D) 10-μm-period magnetic lattice produced by a grooved magnetic film plus bias fields. Site-to-site variations of the trap bottom, atom temperature, condensate fraction, and chemical potential indicate that the magnetic lattice is remarkably uniform, with variations in the trap bottoms of only ±0.4 mG. At the lowest trap frequencies (radial and axial frequencies of 1.5 kHz and 260 Hz, respectively), temperatures down to 0.16μK are achieved in the magnetic lattice, and at the smallest trap depths (50 kHz) condensate fractions up to 80% are observed. With increasing radial trap frequency (up to 20 kHz, or aspect ratio up to ∼80) large condensate fractions persist, and the highly elongated clouds approach the quasi-1D Bose gas regime. The temperature estimated from analysis of the spectra is found to increase by a factor of about 5, which may be due to suppression of rethermalizing collisions in the quasi-1D Bose gas. Measurements for different holding times in the lattice indicate a decay of the atom number with a half-life of about 0.9 s due to three-body losses and the appearance of a high-temperature (∼1.5 μK) component which is attributed to atoms that have acquired energy through collisions with energetic three-body decay products. P. Surendran, S. Jose, Y. Wang, I. Herrera, H. Hu, X. Liu, S. Whitlock, R. McLean, A. Sidorov, P. Hannaford, Phys. Rev. A 91, 023605 (2015)

## Periodic array of Bose-Einstein condensates in a magnetic lattice, in Phys. Rev. A

We report the realization of a periodic array of Bose-Einstein condensates (BECs) of 87Rb F=1 atoms trapped in a one-dimensional magnetic lattice close to the surface of an atom chip. A clear signature for the onset of BEC in the magnetic lattice is provided by in situ site-resolved radio-frequency spectra, which exhibit a pronounced bimodal distribution consisting of a narrow component characteristic of a BEC together with a broad thermal cloud component. Similar bimodal distributions are found for various sites across the magnetic lattice. The realization of a periodic array of BECs in a magnetic lattice represents a major step towards the implementation of magnetic lattices for quantum simulation of many-body condensed matter phenomena in lattices of complex geometry and arbitrary period. S. Jose, P. Surendran, Y. Wang, I. Herrera, L. Krzemien, S. Whitlock, R. McLean, A. Sidorov, P. Hannaford, Phys. Rev. A 89, 051602(R) (2014)

## Full Counting Statistics of Laser Excited Rydberg Aggregates in a One-Dimensional Geometry, in Phys. Rev. Lett.

Full Counting Statistics (FCS) can provide valuable information on manybody systems especially if the underlying correlations cannot be directly imaged. We have used the FCS of Rydberg excitations to gain information on Rydberg interacting manybody systems. We find asymmetric excitation spectra and enhanced fluctuations of the Rydberg atom number which we attribute to the formation of Rydberg aggregates, i.e. correlated systems comprised of few excitations. We conclude that in the presence of dephasing these aggregates are formed via sequential excitation around an initial grain. Our work opens new perspectives for investigating the build-up of correlations in many-body systems. H. Schempp, G. Günter, M. Robert-de-Saint-Vincent, C. S. Hofmann, D. Breyel, A. Komnik, D. W. Schönleber, M. Gärttner, J. Evers, S. Whitlock, M. Weidemüller, Phys. Rev. Lett. 112, 013002 (2014)

## An experimental approach for investigating many-body phenomena in Rydberg-interacting quantum systems in Frontiers of Physics

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. C. S. Hofmann, G. Günter, H. Schempp, N. M. L. Müller, A. Faber, H. Busche, M. Robert-de-Saint-Vincent, S. Whitlock, M. Weidemüller, Frontiers of Physics 9, 571-586 (2014)

## Observing the Dynamics of Dipole-Mediated Energy Transport by Interaction-Enhanced Imaging, in Science

By synthesising an artificial quantum system, we have simulated key processes of photosynthesis on a quantum level with high spatial and temporal resolution and discovered new properties of energy transport. This work is an important step towards answering the question how quantum physics can contribute to the efficiency of energy conversion in synthetic systems, for example in photovoltaics. From a gas of ground state atoms we excited some atoms to highly excited Rydberg states. Similar to the light-harvesting complexes of photosynthesis, energy is transported from Rydberg atom to Rydberg atom, similar to a radio transmitter. To observe the transport of energy we use an electromagnetically induced transparency resonance, which makes up to 50 atoms absorb laser light within a characteristic radius around each Rydberg atom, making it possible to precisely measure the Rydberg atom distribution as a function of time. We were surprised to see that the Rydberg atoms quickly diffused from their original positions. Aided by a mathematical model we could show that the background gas of atoms crucially influences the energy transport dynamics, and the dynamics can be controlled by tuning the Rydberg-Rydberg interactions or the interaction with the laser fields. G. Günter, H. Schempp, M. Robert-de-Saint-Vincent, V. Gavryusev, S. Helmrich, C.S. Hofmann, S. Whitlock, M. Weidemüller, Science 342, 954-956 (2013)

## Sub-Poissonian statistics of Rydberg-interacting dark-state polaritons, in Phys. Rev. Lett.

Electromagnetically-induced transparency (EIT) and the associated appearance of hybrid quasi-particles (dark-state polaritons) in ultracold Rydberg gases have opened intriguing perspectives to create new atom-light interfaces operating at the quantum level and with fully tunable interactions. For the first time we give a complete picture of Rydberg interacting dark state polaritons by probing both the photonic and atomic degrees of freedom in a single experiment. Strong long-range interactions between Rydberg atoms give rise to an effective interaction blockade for dark-state polaritons, which results in large optical nonlinearities and modified polariton number statistics. Our work provides a better understanding of strongly-interacting dark-state polaritons and creates new avenues in the area of single photon nonlinear optics and for the generation of nonclassical states of light and matter. C. S. Hofmann, G. Günter, H. Schempp, M. Robert-de-Saint-Vincent, M. Gärttner, J. Evers, S. Whitlock, M. Weidemüller, Phys. Rev. Lett. 110, 203601 (2013)

## Spontaneous avalanche ionization of a strongly blockaded Rydberg gas, in Phys. Rev. Lett.

We have observed the sudden and spontaneous evolution of an initially correlated gas of repulsively interacting Rydberg atoms to an ultracold plasma. 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. The Rydberg blockade effect is observed to strongly affect the dynamics of plasma formation, and the initial correlations of the Rydberg distribution should persist through the avalanche. This may provide the means to overcome disorder-induced-heating, and offer a route to enter new strongly-coupled regimes. M. Robert-de-Saint-Vincent, C. S. Hofmann, H. Schempp, G. Günter, S. Whitlock, M. Weidemüller, Phys. Rev. Lett. 110, 045004 (2013)

## Detection of small atom numbers through image processing, in Phys. Rev. A

We demonstrate improved detection of small trapped atomic ensembles through advanced postprocessing 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 in well-localized ensembles 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 ratio by a factor of 3, to a minimum resolvable population difference of 17 atoms, close to our ultimate detection limit. C. F. Ockeloen, A. F. Tauschinsky, R. J. C. Spreeuw, S. Whitlock, Phys. Rev. A 82, 061606(R) (2010)

## Optimized magnetic lattices for ultracold atomic ensembles, in New J. Phys.

We introduce a general method for designing tailored lattices of magnetic microtraps for ultracold atoms on the basis of patterned permanently magnetized films. A fast numerical algorithm is used to automatically generate patterns that provide optimal atom confinement while respecting desired lattice symmetries and trap parameters. The algorithm can produce finite and infinite lattices of any plane symmetry; we focus specifically on square and triangular lattices, which are of interest for future experiments. Typical trap parameters, as well as the impact of realistic imperfections such as finite lithographic resolution and magnetic inhomogeneity, are discussed. The designer lattices presented open new avenues for quantum simulation and quantum information processing with ultracold atoms on atom chips. R. Schmied, D. Leibfried, R. J. C. Spreeuw, S. Whitlock, New J. Phys. 12, 103029 (2010)