# Atomic Physics 2017

For each poster contribution there will be one poster wall available (A0 size). Posters can be put up for the full duration of the workshop.

Abumwis, Ghassan

### Rydberg Positronium manipulation, towards a new measurement of the Rydberg constant

Alonso, Alberto

Positronium (Ps) is a matter-antimatter system composed of an electron and a positron. In the ground state, Ps self-annihilation is a significant limitation for high precision measurements but when excited to Rydberg states the annihilation rates become negligible, and their lifetime is dominated by fluorescence to the ground state. Additionally, Rydberg states can be manipulated using inhomogeneous electric fields due to their large dipole moment. We have recently developed techniques used to control the motion of Rydberg Ps [1,2] with quadrupole electric fields [3, 4]. This will be used in future projects aiming to measure the Rydberg constant using Ps. This will provide new insight into the ongoing proton radius puzzle [5] since Ps is a leptonic system and thus it is not susceptible to uncertainties that are inherent to all current measurements of the Rydberg constant using systems such as Hydrogen [6]. [1] \textit{Selective Production of Rydberg-Stark States of Positronium.}, T. E. Wall, A. M. Alonso, B. S. Cooper, A. Deller, S. D. Hogan, and D. B. Cassidy, Phys. Rev. Lett. \textbf{114}, 173001 (2015). [2] \textit{Measurement of Rydberg positronium fluorescence lifetimes.}, A. Deller, A. M. Alonso, B. S. Cooper, S. D. Hogan, and D. B. Cassidy, Phys. Rev. A \textbf{93}, 062513 (2016). [3] \textit{Electrostatically Guided Rydberg Positronium.}, A. Deller, A. M. Alonso, B. S. Cooper, S. D. Hogan, and D. B. Cassidy, Phys. Rev. Lett. \textbf{117}, 073202 (2016) [4] \textit{Velocity selection of Rydberg positronium using a curved electrostatic guide.}, A. M. Alonso, B. S. Cooper, A. Deller, L. Gurung, S. D. Hogan, and D. B. Cassidy, Phys. Rev. A \textbf{95}, 053409 (2017). [5] \textit{Muonic Hydrogen and the Proton Radius Puzzle.} R. Pohl, R. Gilman, G. A. Miller, and K. Pachucki. Nucl. Part. Sci. \textbf{63} 175 (2013). [6] \textit{A Precision Millimeter-Wave Measurement of the Rydberg Frequency.} J. C. De Vries PhD Thesis MIT (2001).

### Generation of 122 nm coherent radiation for Rydberg excitation of trapped Ca+ ions

Andrijauskas, Justas

A single-photon excitation of calcium ions to Rydberg states requires energies in the range of vacuum ultra-violet radiation (VUV) [1], whereas conventional lasers as well as frequency doubling techniques are not applicable. Here, we present the four wave mixing technique that has been employed to generate a VUV source at 122-123 nm. Three high power fundamental beams at 408 nm, 580 nm and 254 nm are generated and overlapped in a mercury vapor cell. Wavelengths are selected near mercury transitions to enhance the efficiency of the four wave mixing process [2,3]. We used this source to demonstrate the excitations of 22F, 52F, 53F and 66F Rydberg states of 40Ca+ ions in a radiofrequency ion trap [4,5]. J. Andrijauskas1,2, J. Vogel1, P. Bachor1,2, A. Mokhberi1, G. Jacob1, C. Gumbrich1,2, F. Schmidt-Kaler1, J. Walz1,2. 1Institut für Physik, Universität Mainz, Staudingerweg 7, D-55128 Mainz, Germany. 2 Helmholtz-Institut Mainz, D-55099, Germany. References [1] F. Schmidt-Kaler et al., New J. Phys. 13, 075014 (2011) [2] D. Kolbe, M. Scheid and J. Walz, Phys. Rev. Lett. 109, 063901(2012) [3] R. Steinborn et al., Opt. Express 21 22693 (2013) [4] T. Feldker et al., Phys. Rev. Lett. 115, 173001 (2015) [5] P. Bachor et al., J. Phys. B. 49, 154004 (2016)

### Dissipation induced steady states of Rydberg-dressed quantum gases in an optical lattice

Barbier, Mathieu

Since the realization of ultracold bosonic quantum gases trapped in optical lattices, a variety of quantum phases have been observed. However, in many systems the interaction of the trapped atoms is on-site only. Rydberg dressing in such systems is predicted to yield new exotic quantum phases, due to the additional strong long-range interaction. We analyse the effects of Rydberg excitations under the variation of experimentally tunable parameters. The system is studied in the Gutzwiller approximation (GA) which decouples the inter-site hopping, leading to selfconsistently coupled single-site Hamiltonians. We find distinctive regimes: an insulating density wave regime, a superfluid and an exotic supersolid. In the density wave regime, as well as in the supersolid, a devil$'$s staircase emerges with varying superlattice order for each step. Several crystalline structures appear with unique lattice orders for both density wave phases and supersolids. Furthermore we introduce experimentally observed dissipative effects into the description. It is anticipated that loss processes such as the spontaneous deexcitation from a Rydberg state or the decoherence of all Rydberg states will give rise to stable steady states of different structures. Those dissipations are included via the master equation in Lindblad form (LB). Using the quantum phases obtained without dissipative effects as initial states, quench-type time evolutions test their stability and possible detection in an experiment. We find that any dissipative effects causes the systems superfluidity to vanish. In addition to that the initial states structures complexity breaks down into a simpler structures such as a checkerboard or even gets homogeneous for strong dissipation. The stability of non-homogeneous quantum phases seems to be metastable on experimentally relevant timescales.

### Engineering thermal reservoirs for ultracold dipole-dipole-interacting Rydberg atoms

Bentley, Christopher

### Spectra and dynamics of heavy Rydberg states in H$_2$ and HD

Beyer, Maximilian

The binding energies of the hydrogen atom are given by the Rydberg formula $$E_n = - \frac{\mathcal{R}_{\infty}\mu/m_e}{(n-\delta)^2}$$ where the quantum defect δ vanishes in the case of a pure Coulomb potential. Heavy Rydberg systems can be realized when the electron is replaced by an anion, which leads in the case of H$^+$H$^-$ to an almost 1000 times larger Rydberg constant and to an infinite number of vibrational states [1]. In the diabatic molecular basis, these ion-pair states are described by long-range Coulomb potentials with $^1\Sigma_g^+$ and $^1\Sigma_u^+$ symmetry and by an almost energy-independent, nonzero quantum defect, reflecting the finite size of H$^-$. Strong interactions at small internuclear distances lead to large variations of $\delta$ with $n$. Gerade [2] and ungerade [3] ion-pair states have been observed previously in H$_2$ with principal quantum numbers up to $n = 240$. The quantum defects in this range were found to vary with energy, indicating the inadequacy of a pure diabatic picture. Spectra of ungerade heavy Rydberg states of H$_2$ with $n = 160 - 520$ will be presented, that show that the quantum defect only becomes energy independent for $n>350$. First observations of ion-pair states in HD will also be presented. These states form two series of heavy Rydberg states, H$^+$D$^−$ and H$^−$D$^+$ converging on different series limits. Extrapolation of high-$n$ heavy Rydberg states to the series limits were used to determine the electron affinities of the hydrogen and deuterium atoms. %\\\vspace{0.1em} \newcounter{Refcounter} \begin{list}{[\arabic{Refcounter}]\hfill}{\usecounter{Refcounter} \topsep=0mm \partopsep=2mm \itemsep=0mm \parsep=0mm \labelwidth=5mm \labelsep=2mm \leftmargin=7mm} %item\label{ref1} Author names, \textsl{Journal Abbr.} \textbf{2000}, \textsl{30}, 1111. \item\label{ref1} S. Pan, and F. H. Mies, \textsl{J. Chem. Phys.} \textbf{89}, 3096 (1988). \item\label{ref2} M. O. Vieitez, T. I. Ivanov, E. Reinhold, C. A. de Lange, and W. Ubachs, \textsl{Phys. Rev. Lett.} \textbf{101}, 163001 (2008). \item\label{ref3} R. C. Ekey, and E. F. McCormack, \textsl{Phys. Rev. A} \textbf{84}, 020501(R) (2011). \end{list}

### Spectroscopy of the atomic system driven by high intensity laser field

Bogatskaya, Anna

General approach to analyze the spontaneous emission of an atomic system driven by a strong laser field is discussed in detail. It based on the first order of perturbation theory for the interaction with quantized vacuum field modes while the interaction with the intense classical laser field is considered numerically or analytically beyond the perturbation theory [1]. Spontaneous emission from the model one-dimensional atom driven by the high-intensity Ti-Sa laser field is studied. Different types of transitions between discrete and continuum states are found to exist and distinguished. The comparison of emission spectrum data for a single atom obtained in the frames of semiclassical approach with results of quantum-electrodynamical (QED) calculations reveals the discrepancy of these data. The QED spectrum is richer showing besides peaks of fundamental frequency and its third harmonic a lot of other peaks that correspond to spontaneous emission from different populated states to low-lying unpopulated states. Some of them are formed during the laser pulse action, while other appear in the after-pulse regime. It was also demonstrated that spontaneous emission from the atom in the after-laser-regime can be used to testify the population trapping in atomic Rydberg states and interference stabilization phenomenon. The performed analysis can be effectively used for the investigation of energy level dynamics in strong laser field, i.e. the reconstruction of atomic spectra (Kramers-Henneberger atom, Rabi-oscillations etc) [2] and also is important for understanding of high order harmonic generation physics [3]. References [1] A.V. Bogatskaya, E.A. Volkova and A.M. Popov "Spontaneous transitions in atomic system in the presence of high intensity laser field", EPL 116, 14003 (2016). [2] A.V.Bogatskaya, A.M. Popov "Spontaneous emission of dressed atomic system in a strong light field", Proceedings of 5th International Conference on Photonics, Optics and Laser Technology, ISBN: 978-989-758-223-3, 129136 (2017). [3] A.V. Bogatskaya, E.A. Volkova and A.M. Popov "Prospects of odd and even harmonic generation by an atom in a high-intensity laser field", Laser Phys. Lett. 14, 055301 (2017).

### Plasmonic field-enhanced high-order harmonic generation in solids

Choi, Nark Nyul

High-order harmonics generation (HHG) from an atomic gas has been studied for decades. In recent years, however, solids have attracted much attention as high-density targets. Also, by introducing plasmonic enhancement by nanostructures, HHG can be possible in solids with relatively low intensity lasers. We review the theory of HHG in solids, and investigate the localization of HHG in solids under spatially inhomogeneous fields produced by plasmonic field enhancement.

### Long-range Rydberg-Rydberg interactions: formation of macrodimers

Deiglmayr, Johannes

The large polarizability of atoms in highly excited Rydberg states leads to strong and long-ranging interactions between such atoms. Interacting pairs of Rydberg atoms represent an exotic molecular system, characterized by high internal excitation, high density of electronic states, internuclear separations exceeding one micrometer, and lifetimes beyond tens of microseconds. I will discuss the computational methods we have developed to determine the electronic structure of interacting Rydberg-atom pairs [1] and our spectroscopic approaches to verify these calculations [2]. Recently, we could observe the formation of “macrodimers” by detecting vibrational bound states of two interacting Rydberg atoms [3], as predicted in 2002 by Boisseau and coworkers [4]. The sequential photoassociation scheme, the validity of adiabatic approximations for these states, and an externsion to heteronuclear dimers will be discussed. \\ %References \hspace{10pt} \noindent{[1]} J. Deiglmayr, Phys. Scr. 91, 104007 (2016) \\ \noindent{[2]} H. Sa{\ss}mannshausen, F. Merkt, and J. Deiglmayr, Phys. Rev. A 92, 032505 (2015) \\ \noindent{[3]} H. Sa{\ss}mannshausen and J. Deiglmayr, Phys. Rev. Lett. 117, 83401 (2016) \\ \noindent{[4]} C. Boisseau, I. Simbotin, and R. C{\^o}t{\'e}, Phys. Rev. Lett. 88, 133004 (2002)\\

### Dynamical mean-field theory for dissipative ultracold Rydberg gases in optical lattices

Dhar, Arya

Rydberg-excited ultracold atoms have emerged in recent years as an efficient tool to engineer long-range interactions and simulate a variety of model Hamiltonians [1]. Theoretical studies of Rydberg-excited quantum gases in optical lattices have revealed a number of equilibrium crystalline quantum phases arising due to the competition between the different energy scales [2]. However, observation of these phases in experiments will be affected by dissipative processes such as spontaneous emission and dephasing. Our goal is to develop tools to study steady states and the corresponding non-equilibrium phase diagrams. A promising approach is to combine dynamical mean-field theory (DMFT) with the Lindblad formalism using the auxiliary master equation approach [3]. Here we report on our progress in adapting this method to study dissipative Rydberg-excited quantum gases (both bosonic and fermionic) in optical lattices. References [1] J. Zeiher et al., Nature Physics 12, 1095 (2016) [2] A. Geißler et al., PRA 95, 063608 (2017) [3] E. Arrigoni et al., PRL 110, 086403 (2013)

### Ab initio Formation of Rydberg Atoms in the Evolving Ultracold Plasmas

Dumin, Yurii

It was clearly recognized just from the first studies of ultracold plasmas that inherent property of such systems are the interrelated ionization and recombination processes: the excited atoms, on the one hand, can be spontaneously ionized and, on the other hand, they easily recombine back into the Rydberg states. However, while the ionization can be reliably simulated by the straightforward molecular-dynamic methods, modelling of the recombination turned out to be a much more difficult task, because of a huge difference in the characteristic time scales for the free and bound motion of the electrons. As a result, a simulation of the recombination was typically performed by introducing some model assumptions and, as far as we know, nobody succeeded by now to simulate the recombination ab initio. The aim of our report is to present the first successful simulation of the recombination from the first principles: namely, employing the specially-developed numerical algorithm, we were able to trace a capture of the electrons by ions and a formation of the stable bound orbits, corresponding to the Rydberg states. We hope that further utilization of this algorithm will be a valuable tool for clarification of the recombination mechanisms in the strongly-coupled plasmas and verification of the respective analytical calculations.

Festa, Lorenzo

### Triatomic Rydberg molecules and the role of spin-couplings

Fey, Christian

When neutral ground state atoms approach a Rydberg atom, they start to interact with the outer Rydberg electron and may become attracted. In an effective description, the wave function of the Rydberg electron can be viewed as an oscillatory potential landscape the ground state atoms can be “trapped in”. The associated bound states are called ultralong-range Rydberg molecules since their bond length is typically on the order of several 1000 Å [1,2,3]. On our poster we will discuss triatomic Cesium D-state Rydberg molecules (two Cs ground state atoms and one Cs Rydberg atom in an electronic D-state) and the effect of spin-couplings on the molecular structure. These couplings include the hyperfine structure of the ground state atoms, the fine structure of the Rydberg atom as well as spin-dependent interactions between the Rydberg electron and the ground state atoms (singlet vs. triplet scattering). We will present the structure of the potential energy surfaces (spatial arrangement as well as the electronic character) and compare computational results to experimental spectra. [1] C. H. Greene, A. S. Dickinson, H. R. Sadeghpour, Phys. Rev. Lett. 85, 2458 (2000). [2] V. Bendkowsky, B. Butscher, J. Nipper, J. P. Shaffer, R. Löw, T. Pfau, Nature 458, 1005 (2009). [3] C. Fey, M.Kurz, P. Schmelcher, Phys. Rev. A 94, 012516 (2016).

### Quasi-particle spectra of bosonic Rydberg-dressed many-body phases

Geißler, Andreas

As recent experiments have demonstrated the feasibilty of Rydberg dressing [1], even in a lattice system [2], the stage is set for realizing (long predicted) exotic states of matter in ultracold gases, such as the bosonic supersolid [3]. Our latest results (simulated in real-space bosonic dynamical mean-field theory RB-DMFT) have shown a rich diversity of crystalline and supersolid quantum phases, both close to resonant driving [4] and in the weak dressing limit [5]. While in the former case we predict a reduction of the Rydberg fraction compared to single atom dressing, we show in the latter case how a two-species mixture can make the realization of a supersolid more likely. Based on these results we applied a quasi-particle method based on linearized Gutzwiller dynamics (Gqp), to predict various spectral functions for both of the cases and in an experimentally feasible regime. As RB-DMFT also predicts spectral properties, it serves as a benchmark for Gqp. We furthermore characterize the various observed gapped and ungapped quasi-particle modes. [1] Y.-Y. Jau et al., Nat. Phys. 12, 71-74 (2016) [2] J. Zeiher et al., Nat. Phys. 12, 1095–1099 (2016) [3] D. J. Thouless, Ann. Phys. 51, 403-427 (1969) A. F. Andreev and I. M. Lifshitz, Sov. Phys. JETP. 29, 1107-1113 (1969) [4] A. Gei{\ss}ler et al., Phys. Rev. A 95, 063608 (2017) [5] Y. Li et al., arXiv:1705.01026

### A reaction microscope for few-body Rydberg dynamics

Geppert, Philipp

On the basis of our deterministic ion source experiment\footnote{Sahin et al 2017 \textit{New J. Phys.} \url{https://doi.org/10.1088/1367-2630/aa9461}}, we are developing a reaction microscope that is inspired by the well-known MOTRIMS technique. To this purpose, a sample of $10^6$ $^{87}$Rb atoms will be prepared in a crossed dipole trap. Using a 3-level excitation scheme, some atoms can be excited to atomic or molecular Rydberg states and photoionised by a short laser pulse from a high power CO$_2$ laser after a variable evolution time. Following small homogeneous electric fields generated by Wiley-McLaren-type ion optics, the produced ions are subsequently detected by a time and position sensitive micro channel plate detector. By analysing the trajectories of the recoil ions, we aim to meausure momentum distributions of Rydberg molecule wave functions. In this context, special focus lies on butterfly and trilobite molecules, which can be addressed efficiently due to the opportunity of exciting Rydberg p- and f-states. As a next step, stroboscopic monitoring of the internal decay of Rydberg molecules as well as measurements regarding forces between pairs of Rydberg atoms will be performed.

### Ultracold heteronuclear three-body systems: How diabaticity limits the universality of recombination into shallow dimers

Giannakeas, Panagiotis

The mass-imbalanced three-body recombination process that forms a shallow dimer is shown to possess a rich Efimov-Stückelberg topology, with corresponding spectra that differ fundamentally from the homonuclear case. A semi-analytical treatment of the three-body recombination predicts an unusual spectra with intertwined resonance peaks and minima, and yields in-depth insight into the behavior of the corresponding Efimov spectra. In particular, the pattern of maxima and minima are shown to depend strongly on the degree of diabaticity, which strongly affects the universality of the heteronuclear Efimov states.

### Towards a hybrid quantum system of Rydberg atoms and superconducting coplanar waveguide resonators

Glaser, Conny

Coupling Rydberg atoms to coplanar superconducting resonators has been proposed to enable efficient state transfer between solid state systems and ultracold atoms. This coupling could be used for the generation of an atomic quantum memory or the implementation of new quantum gates [1,2]. After the successful demonstration of magnetic coupling between ultracold ground state atoms and a coplanar waveguide resonator, we progress towards coupling Rydberg atoms to the electric field of the cavity. Due to the large dipole moment of Rydberg atoms, the coupling strength to the cavity is expected to be much larger than in the case of ground state atoms. At the same time, Rydberg states are strongly affected by any detrimental fields, such as the electric field of adsorbates on the chip, leading to spatially inhomogeneous energy shifts. We report on the characterization of these fields, state selective detection of Rydberg atoms and on the progress towards strong coupling. [1] L. S\'ark\'any et al., Phys. Rev. A 92, 030303 (2015). [2] J. D. Pritchard et al., Phys. Rev. A 89, 010301 (2014).

### Two-electron nondispersive wave packets

Gonzalez Melan, Alejandro

The electron-electron interaction in the helium atom is the reason for the non integrability of this system and is also the source of strong electronic correlations in certain highly asymmetrically doubly excited states called frozen planet states, which might transform under periodic driving into nondispersive wave packets. Here, we present an efficient method to study driven frozen planet states, which allows us to identify the key ingeredients in the formation of two-electron nondispesive wave packets in helium.

### Tailoring ionization of highly Stark shifted Rubidium Rydberg states

Grimmel, Jens

Rydberg atoms are extremely sensitive to electric fields and consequently have a rich Stark spectrum. At sufficiently high electric fields these states start to ionize due to tunneling through the potential barrier as well as direct coupling to the continuum. This region is of particular interest for tailoring the ionization process to certain needs, for example in order to create cold ions and electrons for microscopy applications. In our numerical calculations we calculate the eigenvalues and eigenvectors of a matrix representation of a Hamiltonian including a complex absorbing potential (CAP), which allows us to accurately predict the ionization spectra of Rydberg states beyond the classical ionization threshold. The CAP is adjusted to the external electric field, which allows us to calculate a whole range of the spectrum with only one free parameter. We find good agreement between the results from these calculations and the experimental data of Stark maps for Rubidium Rydberg atoms with principal quantum numbers up to 70 and are able to identify Stark shifted states with ionization rates that can be controlled by fine tuning the external electric field.

### Coherent Microwave-to-Optical Conversion in Rydberg Atoms

Gross, Christian

We present an experimental demonstration of coherent microwave-to-optical conversion via six-wave mixing in a cloud of cold rubidium atoms. The microwave field strongly couples to an electric dipole transition between Rydberg states and the long lifetime of these states allows us to make use of electromagnetically induced transparency, which significantly enhances the atomic nonlinearity. With our current experimental configuration, we achieve a photon conversion efficiency of about 0.25\% with a bandwidth of more than 6 MHz. Our results are in good agreement with a theoretical model and indicate that this approach has the potential to reach a near unity conversion efficiency.

### Laser excitation of Positronium atoms into Rydberg states inside MgO cavities

Gurung, Lokesh

We have observed confinement of Positronium (Ps) atoms in cavities of MgO structures and performed laser excitation of Ps trapped in these voids of varying internal diameter using a two-color two-photon excitation scheme. A shift of approximately 5 meV in the 1S $\rightarrow$ 2P transition of Ps inside these cavities was observed. Following the excitation into 2P states by detuning the ultraviolet (UV) laser to probe the cavity, the confined atoms were promptly excited into Rydberg states (2P $\rightarrow$ nD/nS) with n = 10-17 using an infrared (IR) laser. It is observed that Rydberg atoms are able to escape the cavities after the excitation, with speed of approximately 3 x 10$^{5}$ ms$^{-1}$. Laser excitation of Ps transmitted through a MgO layer deposited on a 50 nm thick Silica Nitride (SiN) membrane was also examined. The measured FWHM of the transmitted Ps 1S $\rightarrow$ 2P in vacuum was found to be of the same order as that of reflected Ps indicating low cooling rate inside the cavities.

### Spectroscopy of Rydberg states in ultra cold ytterbium

Halter, Christian

Understanding the special features of Rydberg atoms, e.g. dipole-dipole interaction or van-der-Waals blockade, has become of utmost importance in quantum optics. Particularly ultra cold Rydberg atoms are of great interest for the investigation of long range interactions. Hence, in the present study, ultra cold ytterbium is spectroscopically investigated to gain precise knowledge on the Rydberg states. A special feature of ytterbium is that due to its two valance electrons atoms in Rydberg state can be easily manipulated and imaged using optical fields. For the above-mentioned spectroscopy an induced loss of atoms in a magneto-optical trap (MOT), that is caused by the Rydberg excitation, is used to detect the Rydberg states. Applying this method, we could measure several energy levels of Rydberg states in ultra cold ytterbium.

### High-repetition rate OPCPA for attosecond science

Harth, Anne

Time resolved applications are often based on pump probe experiments. This is also true for atto-second time resolved studies in atomic and molecular systems. The combination of such attosecond time resolved pump probe techniques with coincident, kinematically complete detection techniques opens new observation channels and allows e.g. a detailed dynamical study of dissociation processes in molecular systems. However, these experiments suffer from long data acquisition times and laser systems with > 100 kHz pulse repetition rate are required. So far, high-repetition rate XUV-IR pump-probe studies based on optical parametric amplifier systems where experimentally not feasible due to the low pulse energy of these systems. We present a compact state of the art OPCPA based XUV light source working at 200 kHz repetition rate. Due to a careful study of spatial temporal distortion on the beam and by applying a novel high- pressure gas target for high order harmonic generation, we can reach conversion efficiencies comparable to those obtained with standard 1 kHz laser systems. This system will be used for high-repetition rate XUV-IR pump probe experiments, in the future.

### Scaling behaviour and quantum chaos of Rydberg excitons in an external magnetic field

Heckötter, Julian

### Many-body physics in Rydberg-dressed spin systems

Hollerith, Simon

Off-resonant optical coupling of an atomic ground state to a Rydberg state, so called "Rydberg-dressing", has been proposed as a versatile method to implement various long-range interacting spin models with ultracold atoms. In our experiment, we realize Rydberg-dressed Ising spin interactions in an atomic Mott insulator of rubidium-87 by off-resonant optical coupling to a Rydberg P-state. First interferometric experiments in a two-dimensional sample demonstrated versatile control of the induced interactions, however collective loss processes reduced the lifetime of the system. Here, we present recent experimental results for a Rydberg-dressed 1d spin chain with long-range Ising interactions. Contrary to the 2d case, the collective loss can be avoided and lifetimes increase significantly. We substantiate the improved lifetimes by showing purely interaction driven coherent collapse and revival dynamics of the magnetization in a 1d spin chain.

### Rydberg Dimers in a Magnetic Field

Hummel, Frederic

The recent observations of ultra long-range Rydberg molecules in cold and ultra cold quantum gases are in good agreement with a theoretical treatment in Born-Oppenheimer approximation. In this picture the molecular binding mechanism emanates from maximally localized scattering of the semiclassical Rydberg electron and a neutral atom. The inclusion of electronic and nuclear spin degrees of freedom leads to a plethora of potential energy surfaces where their degeneracy is lifted upon addition of a magnetic field as it is often present in experimental realizations. In general such a system inhibits no preserved quantum numbers anymore such that angular momentum and spin will be transfered between electrons and nucleus. This is also closely related to the molecule's geometric configuration in the magnetic field. In order to calculate the spectrum of such systems we expand the theoretical tools currently used and find good agreement with experimental results.

### Rydberg excitations of cold atoms inside a hollow-core fiber

Islam, Parvez

Cold atoms inside hollow-core fibers present a promising candidate to study strongly coupled light-matter systems. Combined with the long range Rydberg interaction which is controlled through an EIT process, a corresponding experimental setup should allow for the generation of a strong and tunable polariton interaction. Using this scheme, novel photonic states can be generated and studied with possible applications in quantum information and simulation. In our experimental setup, laser cooled Rubidium atoms are transported into a hollow-core fiber using an optical conveyor belt. We explain the details of optimizing this transport procedure by applying frequency and amplitude ramps. Further, we show characterizations of Kagomé-type hollow-core fibers, whose properties allow for two-photon Rydberg excitation, and use them to study Rydberg EIT in a hot atom setup. Finally, we present the first measurements of cold Rydberg excitations inside a hollow-core fiber and discuss our progress towards Rydberg quantum optics in a quasi-one-dimensional geometry.

### Observation of the motional Stark shift in low magnetic fields

Kaiser, Manuel

The motional Stark effect (MSE) originates from a Lorentz force acting in opposite directions on the ionic core and the electrons of an atom moving in a magnetic field. This introduces a coupling between the electronic structure and the center-of-mass motion of the atom, resulting in a difference between the center-of-mass motion and the pseudomomentum. The effect can be described approximately by an electric field in the frame of a moving atom and therefore the extreme polarizability of Rydberg atoms makes them the ideal candidates for extensive investigations on this topic. We present measurements of the MSE on $^{87}$Rb Rydberg atoms moving in low magnetic fields employing a velocity selective spectroscopy method in a vapor cell. For atom velocities of ~400\,m/s, principal quantum numbers of n = 100, and magnetic fields of 100\,G we measured motional Stark shifts on the order of 10\,MHz. Our experimental results are supported by numerical calculations based on a diagonalization of the effective Hamiltonian governing the valence electron of $^{87}$Rb in the presence of crossed electric and magnetic fields.

### Variational Principle for Steady States of Long-ranged Interacting Rydberg Atoms

We propose to build on the promising developments in the variational treatment of dissipative many-body systems and apply the method to the most important questions in the context of strongly interacting Rydberg atoms. While the early results obtained using the variational approach are extremely encouraging, we are trying to advance the method to describe dissipative quantum many-body systems with long-ranged interaction and/or long-ranged correlation. In this poster, we present our recent variational results for the steady state of Rydberg atoms in one-dimensional lattice including long-range interactions as well as a two-dimensional dissipative Ising model including long-range correlations.

### Calculations of Dipole-Dipole Interaction Strength and Dipole Blockade Radius using Förster Resonances in Rb Atoms

Kirova, Teodora

Rydberg atoms [1] are characterized by a large separation between the electron and the ion core, which leads to their strong polarizability and extreme long-range dipolar interactions. At short distances the strong resonant dipole-dipole (DD) interaction is of $1/R^{3}$ dependence, while the long range interaction is of van der Waals $1/R^{6}$ type. In the phenomenon of dipole blockade [2], the DD interaction causes the applied laser excitations to be off-resonant, as a result of which only a single atom can be excited within the “blockade sphere” [3] while simultaneous excitation of two/multiple Rydberg atoms will be suppressed. Our aim is to ﬁnd the best experimental parameters necessary to achieve a large (around $50 \mu m$) blockade radius. For that purpose we calculate the Rydberg-Rydberg interaction strengths for atomic Rb in the presence of strong F\”orster resonances, following the procedure outlined in [5]. We provide an overview of different F\”orster resonances for Rb-Rb, where the participating atoms are excited to $ns$ or $nd$ states with different principal quantum numbers. In certain cases these interactions can be remarkably strong, leading to dipole blockade radius $R=20 \mu m$ for $n=80$. Currently we continue the search for the best candidates among the multiple possible F\”orster resonances in Rb with large enough interaction strength in order to reach the experimentally desired dipole blockade radius. This work was supported by the Trilateral grant of the Latvian, Lithuanian, and Taiwanese Research Councils $\emph{Quantum and Nonlinear Optics with Rydberg-State Atoms}$, $FP-20338-ZF-N-100$. References [1] T. F. Gallagher, Rydberg Atoms, (Cambridge University Press, Cambridge, England, 1994). [2] M. D. Lukin et al., Phys. Rev. Lett.87, 037901 (2001). [3] D. Tong et al., Phys. Rev. Lett.93(6), 063001 (2004). [4] T. F\”orster , Ann. Phys.437, 55 (1948). [5] I. I. Beterov and M. Saffman, Phys. Rev. A 92, 042710 (2015).

### Scalable 2D arrays of individually addressable atomic qubits for quantum simulation and quantum computation

Kohfahl, Lars

Simultaneous control of quantum many-body systems in multi-site architectures is a key element for efficient quantum computation and quantum simulation. We focus on the implementation of scalable quantum register geometries based on micro-fabricated optical elements. Single-atom qubits can be stored and manipulated using multi-site focussed beam traps created by arrays of microlenses. This technique allows for the creation of large-scale multi-site trap arrays with structure sizes close to the wavelength of the light applied while simultaneously enabling single-site addressability. This contribution gives an overview on the micro-optical realization of two-dimensional arrays of individually addressable $^{85}$Rb atoms [1]. Arrays of more than 400 dipole traps feature variable trap sizes in the single micrometer regime. Furthermore, the trap separations are tunable between a few and several tens of micrometers. Utilizing light-assisted collisions exactly one atom per site can be prepared in more than 150 sites while all other sites are empty. Ways for parallelized, coherent atom transport as well as for site-selective, coherent quantum state control are presented. In addition, we report on a novel technique for the optical creation of 3D multi-layer configurations of 2D periodic quantum registers based on the Talbot effect. Recent progress towards implementing two-photon Rydberg excitations which will enable tunable Rydberg-mediated resonant dipolar and van der Waals like interactions using variable trap separations will be discussed. The Rydberg interactions shall be used in the blockade regime for two-qubit gates where as in the unblocked regime they facilitate the implementation of many-body spin Hamiltonians. In the latter case versatile geometries obtained by a SLM-based setup are intended to be used to study topological phenomena. [1] For an overview see: M. Schlosser, S.Tichelmann, J. Kruse, and G. Birkl, Quant. Inf. Proc. 10, 907 (2011)

### Rydberg excitons in Cu2O – modifying the absorption spectrum by tuning the selection rules

Konzelmann, Annika

Excitons in cuprous oxide appear in the absorption spectrum as a hydrogen-like series with modified masses due to the different effective electron and hole masses of carriers in solids as well as due to the larger dielectric permittivity. Hence, binding energies of almost 100 meV are common. As the exciton Bohr radius scales as the principal quantum number n², expected Bohr radii for n=25 are in the µm-range, which is enormous for a single quantum object. Starting from such a system, one can now demonstrate schemes that harness light-exciton interaction of these giant Rydberg excitons, and, as cuprous oxide is a semiconductor, integrate it in a solid-state system. The excitons visible in an absorption spectrum follow the dipole selection rules. One can modify these selection rules by either breaking the symmetry in cuprous oxide or by modifying the properties of the light. By using a phase plate, featuring a Riemann-type inclined surface with the phase delay around the phase plate being an integer multiple of 2π, one can construct a beam with defined angular momentum (OAM beam). An OAM beam’s phase front looks like one or more intertwined helices depending on the amount of orbital angular momentum and has vanishing intensity in the vortex center. It could be possible to transfer OAM from the beam to an electronic system, making forbidden transitions allowed. Furthermore, due to the strong E-field gradient in the vortex center of an l=1 OAM beam, quadrupole transitions might be enhanced, and, due to the vanishing E-field but remaining B-field in the vortex center of an l=2 OAM beam, magneto-optical transitions could become accessible. We want to excite excitons in cuprous oxide with OAM light and modify its absorption spectrum by tuning the selection rules.

### Multimode Bose-Hubbard model for quantum dipolar gases in confined geometries

Kraus, Rebecca

We consider ultracold dipolar bosonic molecules in a 1D optical lattice in a tight anisotropic harmonic confinement, leading to a quasi one-dimensional geometry. If the confinement along one of the directions perpendicular to the optical lattice is relaxed, the system undergoes a structural linear-zigzag transition. We show that close to the transition this system can be mapped onto a multi-orbital extended Bose-Hubbard model, where the coefficients can be determined by means of a low-energy theory. The system displays a rich phase diagram resulting from the interplay between tunneling, on-site repulsion, the external confinement and dipolar interaction. We evaluate the quantum phases of the ground state by means of a time-evolving block decimation method.

### Cold Rydberg atoms near an optical nanofibre

Krishnapriya, Subramonianrajasree

Optical nanofibers (ONF) are used to confine light in subwavelength dimensions. In ONFs, a large evanescent field component extends beyond the fiber surface into the surroundings. These fibers also have the distinct advantage of being readily integrated into experimental setups, due to the ease with which light can be coupled into and out of them using standard fiber optical components. One of the attractions of these fibers is their sensitivity to very low particle numbers in their vicinity and the ability to use them as wavelength-dependent “detectors” for atoms or molecules. Probe light passing through an ONF is sensitive to the presence of atoms/molecules near the fiber and the particles can be detected via an absorption dip. Alternatively, in the absence of probe light, fluorescing particles can be detected by coupling the emitted light into the optical ONF and observing the light signal using single photon counting techniques. Cold atomic gases are suited for exploring the quantum physics of many-body systems and for investigating quantum matter and exotic quantum phenomena. In this paper, we will discuss advances in our current work on using cold Rydberg atoms trapped next to an optical nanofiber for the development of quantum networks. Non-destructive detection methods via nonlinear processes, such as electromagnetically induced transparency (EIT), are used to detect the presence of the Rydberg states. Rydberg atoms can be viewed as extra-large, neutral atoms which show many interesting properties. For example, the high polarizability of Rydberg states gives rise to stronger optical nonlinearities. We can make use of the extraordinary properties of Rydberg atoms in dense atomic gases to explore the realm of strongly-correlated, many-body physics. By combining laser cooling and trapping of atoms with the coherent excitation of Rydberg atoms from dense atomic gases, and interfacing the system using an ONF, we will be able to explore new aspects of Rydberg physics heretofore experimentally inaccessible. To date, effort on creating highly excited Rydberg atoms in the vicinity of an optical nanofiber has been very limited, despite the advantages such a system can have in developing neutral atom-based quantum networks. Here, we present our proof-of-principle results on the formation of Rydberg atoms next to a nanofiber. A 2-photon absorption process is used as evidence of the formation of the Rydberg states, as was previously demonstrated by Mohapatra et al. [3] in an atomic vapor. We use a cascaded two photon absorption process to create the Rydberg states. Limitations on the achievable Rydberg state due to the presence of the dielectric nanofiber surface are explored as it is crucial that the outer electron not be ripped from the atom, thereby ionizing the system References [1] L. Russell, R. Kumar, V. B. Tiwari, and S. Nic Chormaic, “Measurements on release-recapture of cold Rb-85 atoms using an optical nanofibre”, Opt. Commun. 309, 313 (2013). [2] L. Russell, R. Kumar, V. B. Tiwari, and S. Nic Chormaic, “Investigation of a 85Rb dark MOT using an optical nanofiber”, Meas. Sci. Technol. 25, 055203 (2014). [3] A. K. Mohapatra, T. R. Jackson, and C. S. Adams, “Coherent optical detection of highly excited Rydberg states using electromagnetically induced transparency”, Phys. Rev. Lett. 98, 113003 (2007).

### Rydberg excitons in strain-induced waveguide traps in cuprous oxide

Krüger, Sjard Ole

Excitonic trapping potentials derived from inhomogeneous strain fields are an established experimental technique in the study of the collective behavior of semiconductor excitons. In recent years several new excitonic species have been observed in cuprous oxide, among them highly excited states showing signs of the Rydberg blockade [1]. We present calculations of the excitonic waveguide potentials induced by cylindrical stressors for states inside and outside the Rydberg regime. The contributions to the strain potential -- a band-gap shift affecting all excitonic states equally and a binding-energy shift -- are calculated independently from a cubic strain Hamiltonian [2] and a Wannier-type two-band model. For states in the Rydberg regime, the band-gap shift dominates the strain potential while the binding-energy shift can reach the same order of magnitude for low principal quantum numbers. [1] T. Kazimierczuk et al., Nature 514, 343 (2014) [2] K. Suzuki and J. Hensel, Physical Review B 9, 4184 (1974)

### Growing of quantum Hall states for Rydberg cavity polaritons

Letscher, Fabian

In a recent experiment, the realization of synthetic Landau levels for photons has been demonstrated in a continuum setup of twisted cavities (see Nature 534, 671–675). Coupling these photonic modes to a Rydberg state in a EIT configuration may path the way to realize fractional quantum Hall states. Here, we discuss a scheme for growing interesting quantum Hall states for Rydberg polaritons in a setup of twisted cavities. In a first step, we show the adiabatic transfer of orbital angular momentum into the system (flux insertion). This is done by employing a STIRAP scheme in dense atomic gas coupling to the photonic modes and a classical laser beam carrying orbital angular momentum. Adding flux quanta to the system creates a quasi-hole excitation in the center. In a second step we coherently refill the hole excitation with a single photon. This may be realized by employing the blockade mechanism of Rydberg polaritons. Repeating the sequence may allow to grow interesting quantum Hall states of light within the lowest Landau level of the twisted cavity setup.

### Rydberg optical Feshbach resonances

Lippe, Carsten

In a cloud of ultracold atoms the scattering interaction between a ground state atom and the highly excited Rydberg electron gives rise to an oscillatory potential that supports molecular bound states. We use high resolution photoassociation spectroscopy over a range of several 10 GHz to precisely determine binding energies and we especially give the first experimental evidence of Butterfly Rydberg molecules. The rotational structure in an external electric field allows us to extract the bond length and huge dipole moments originating from a strong admixture of high angular momentum states. In addition we show, that by coupling two ground state atoms to a Rydberg molecular state via a laser field we realize a Rydberg optical Feshbach resonance. As the laser field is tuned, the Rydberg optical Feshbach resonance results in a changed interatomic interaction, that we detect as different revival times in collapse and revival experiments in an optical lattice. Long lifetimes of Rydberg molecular states allow us to maintain long sample lifetimes on the order of milliseconds while changing the scattering length by up to 50 Bohr radii. So far optical Feshbach resonances were observed near intercombination transitions in strontium and ytterbium. We believe that Rydberg optical Feshbach resonances open up a whole new field: They are feasible with arbitrary Rydberg molecular states and all atomic species that are able to create Rydberg molecules. Especially this plenitude of molecular states allows to optimize the ratio between the change in scattering length and loss rates in further research.

### Stochastic dynamics of charged particles in a confined magnetic cluster

Lukashenko, Anastasiia

We consider dynamics of charged particles frozen into the magnetic field of two linked circular loops localized in the perpendicular planes. At large distances, the magnetic field of this system is well equivalent to the dipolar one, so that the corresponding field lines wrap the surfaces topologically equivalent to tori covering the entire system. At the small distances, the field lines approximately wrap the deformed toroids covering each loop separately. The most interesting region is the space between two above-mentioned domains of regular dynamics. This is a stochastic region, involving the "magnetic islands".

### Transitions in the triangle between GOE-GUE-Poisson statistics for magnetoexcitons

Main, Jörg

Until now only for specific transitions between Poissonian statistics (P), the statistics of a Gaussian orthogonal ensemble (GOE), or the statistics of a Gaussian unitary ensemble (GUE) analytical formulas for the level spacing distribution function have been derived within random matrix theory. We investigate arbitrary transitions in the triangle between all three statistics. To this aim we propose an according formula for the level spacing distribution function depending on two parameters. Recent investigations on the Hamiltonian of excitons revealed that the combined presence of a cubic band structure and external fields breaks all antiunitary symmetries [1], and thus the nearest-neighbor spacing distribution of magnetoexcitons can exhibit all three statistics depending on the system parameters. Evaluating the numerical results for magnetoexcitons in dependence on the excitation energy and on a parameter connected with the cubic valence band structure and comparing the results with the formula proposed allows us to investigate the level spacing dynamics in the triangle between Poissonian, GOE, and GUE statistics and to distinguish between regular and chaotic behavior as well as between existent or broken antiunitary symmetries. [1] F. Schweiner et al., Phys. Rev. Lett. 118, 046401 (2017)

### Electron-Nuclear Coupling through Autoionizing Rydberg States after Strong-field Excitation of Molecules

Mi, Yonghao

Channel-selective electron emission from strong-field photo-ionization of ${\mathrm{H}}_{2}$ molecules is experimentally investigated by using ultrashort laser pulses and a Reaction Microscope. The electron momenta and energy spectra in coincidence with bound and dissociative ionization channels are compared. Surprisingly, we observed an enhancement of the photoelectron yield in the low-energy region for the bound ionization channel. By further investigation of asymmetrical electron emission using two-color laser pulses, this enhancement is understood as the population of the autoionizing states of ${\mathrm{H}}_{2}$ molecules in which vibrational energy is transferred to electronic energy. This general mechanism provides access to the vibrational-state distribution of molecular ions produced in a strong-field interaction.

### Creation and detection of photonic molecules in Rydberg gases

Moos, Matthias

We consider the propagation of photons in a gas of Rydberg atoms under conditions of EIT, where they form interacting massive quasi-particles, termed Rydberg polaritons. Under off-resonant EIT-conditions, either bunching or antibunching of polaritons can be observed depending on the strength of the interactions. The bunching is associated with the formation of bound states. We employ a Green’s function approach and numerical wave-function simulations to analyze the conditions for the creation and the dynamics of these photonic molecules and their interplay with the scattering continuum which can also induce bunching. Analytic solutions of the pair-propagation problem obtained from a pseudopotential approximation provide a detailed understanding of bound and scattering states. We find that the scattering contributions asymptotically acquire a robust phase allowing to separate bound state and scattering contributions by a homodyne detection scheme.

### Tunneling exit characteristics from classical backpropagation of an ionized electron wave packet

Ni, Hongcheng

We investigate tunneling ionization of a single active electron with a strong and short laser pulse, circularly polarized. With the recently proposed backpropagation method, we can compare different criteria for the tunnel exit as well as popular approximations in strong-field physics on the same footing. Thereby, we trace back discrepancies in the literature regarding the tunneling time to inconsistent tunneling exit criteria. The main source of error is the use of a static ionization potential, which is, however, time dependent for a short laser pulse. A vanishing velocity in the instantaneous field direction as tunneling exit criterion offers a consistent alternative, since it does not require the knowledge of the instantaneous binding energy. Finally, we propose a mapping technique that links observables from attoclock experiments to the intrinsic tunneling exit time.

### Splitting and decelerating beams of Rydberg atoms above two-dimensional electrode arrays

Palmer, James

The very large static electric dipole moments of high Rydberg states of atoms and molecules (|μelec| > 10000 D for states with principle quantum numbers n > 51) can be utilized to control the motion of gas-phase samples using inhomogeneous electric fields [1-3]. This has recently led to the development of sets of chip-based devices to guide [4,5], decelerate [5-8] and trap [6,7,9] Rydberg atoms and molecules. The electrode structures employed by these devices are such that they can be integrated with microwave waveguides and resonators, an important consideration when aiming to develop hybrid approaches to quantum infor- mation processing. They also allow for the realisation of beam splitters for Rydberg atoms and molecules [10]. Such devices will be useful in a wide range of experiments including (i) splitting Rydberg atom beams to distribute atoms among arrays of superconducting resonators, (ii) the development of Rydberg-atom interferometers, and (iii) using one part of a split beam as an intensity reference in collision and spectroscopy measurements. The design and experimental implementation of a Rydberg-atom beam splitter will be presented. In this work beams of helium Rydberg atoms were transversely split into pairs of spatially separated components using static electric fields. Upon exciting the device the resulting bunches of atoms were separated by up to ∼13 mm. The operation of the beam splitter was characterised by position sensitive electric field ionisation of the Rydberg atoms. The interpretation of the experimental data was aided by comparison with the results of numerical particle trajectory simulations. Also presented are the design and results of the first experimental tests of transmission line decelerators for Rydberg atoms that are constructed in a way to minimise losses from collisions with background gas in the vacuum apparatus. These devices are expected to be well suited to the preparation of cold samples of highly excited molecules in cryogenic environments for studies of collision and decay processes. [1] Y. Yamakita, S. R. Procter, A. L. Goodgame, T. P. Softley, and F. Merkt, J. Chem. Phys. 121, 1419 (2004). [2] E. Vliegen, H. J. Worner, T. P. Softley, and F. Merkt, Phys. Rev. Lett. 92, 033005 (2004). [3] S. D. Hogan, EJP Techniques and Inst. 3, 1 (2016). [4] P. Lancuba and S. D. Hogan, Phys. Rev. A 88, 043427 (2013). [5] P. Allmendinger, J. Deiglmayr, J. A. Agner, H. Schmutz, and F. Merkt, Phys. Rev. A 90, 043403 (2014). [6] S. D. Hogan, P. Allmendinger, H. Saßmannshausen, H. Schmutz, and F. Merkt, Phys. Rev. Lett. 108, 063008 (2012). [7] P. Allmendinger, J.A. Agner, H. Schmutz, and F. Merkt, (2013), Phys. Rev. A., 88, 043433 (2013). [8] P. Lancuba and S. D. Hogan, Phys. Rev. A 90, 053420 (2014). [9] P. Lancuba and S. D. Hogan, J. Phys. B: At. Mol. Opt. Phys. 49, 074006 (2016). [10] J. Palmer and S. D. Hogan, Phys. Rev. A 95, 053413 (2017).

### Long-range Rydberg molecules bound by electron-atom scattering

Peper, Michael

In 1934, Amaldi and Segré [1] observed pressure-dependent shifts and broadenings in Rydberg spectra. The pressure shifts result from interactions of the Rydberg electron with ground-state atoms lying within the Rydberg electron's orbit. Fermi [2] explained this interaction as arising from the s-wave scattering of the slow Rydberg electron with the ground-state atom. As first predicted by Greene et al. [3] and first observed by Bendkowsky et al. [4], the interaction can be treated using oscillatory potentials that may support bound states of long-range diatomic molecules in case of a negative s-wave scattering length. For the alkali metal atoms, the triplet scattering length is negative whereas the singlet scattering length is very small or even positive. Singlet and triplet scattering channels are, however, mixed by the hyperfine interaction in the ground-state atom [5]. This mixing allowed for a first determination of the zero-energy singlet s-wave scattering length of caesium [6]. The often neglected p-wave contribution is known to lower the potential barrier towards smaller internuclear separations [7,8]. The excited molecules may therefore decay by tunnelling through the potential barrier. Measurements of the lifetimes of the molecules can provide a sensitive probe of the potential-energy curve. We will present studies on the formation and dynamics of long-range Rydberg molecules in an ultracold caesium Rydberg gas using high-resolution photoassociation spectroscopy and an outlook towards future experiments with heteronuclear molecules. [1] E. Amaldi, and E. Segré, Il Nuovo Cimento 11, 145 (1934). [2] E. Fermi, l Nuovo Cimento 11, 157 (1934). [3] C. H. Greene, A. S. Dickinson, and H. R. Sadeghpour, Phys. Rev. Lett. 85, 2458 (2000). [4] V. Bendkowsky et al., Nature 458, 1005 (2009). [5] D. A. Anderson, S. A. Miller, and G. Raithel, Phys. Rev. A 90, 062518 (2014). [6] H. Saßmannshausen, F. Merkt, and J. Deiglmayr, Phys. Rev. Lett. 114, 133201 (2015). [7] M. T. Eiles, and C. H. Greene, Phys. Rev. A 95, 042515 (2017). [8] S. Markson et al., ChemPhysChem 17, 3683 (2016).

### Quantum and classical dynamics of frustrated tunneling

Popruzhenko, Sergey

Experimental observation of the frustrated tunneling ionization (FTI) [1] followed by an extensive work on excitation of Rydberg states in the field of intense laser radiation (see [2,3] and references therein) has triggered the interest to multiquantum bound-bound transitions in strong electromagnetic fields. It is now commonly accepted that the excitation of Rydberg atomic or molecular states in short intense laser pulses is a counterpart of the above-threshold ionization (ATI). In contrast to the ATI process, which is being routinely described by the strong field approximation [4], the strong field excitation of bound states and the FTI effect lack a comparatively simple and efficient quantum-mechanical theory. Up to now, either numerical solutions to the time-dependent Schroedinger equation (TDSE) or classical Monte Carlo modeling have been mostly employed in studies of frustrated tunneling. Here we present a theoretical approach to describe bound-bound transitions in a strong laser field within a fairly simple but fully quantum-mechanical treatment [5]. It is based on an analytic generalization of the strong field approximation where the photoelectron momentum is considered as a complex-valued quantity connected to the quantum numbers of final bound states via the classical equations of motion in complex time- and position-momentum space. Application of this formalism shows the presence of interference effects in spectra of excited Rydberg states. This interference is similar to the one well known in ATI spectra. However, in the case of bound states the role of the Coulomb interaction appears to be much more important than for ATI. We compare predictions of our model to results of the exact TDSE solution and to those obtained within the classical modeling. These comparisons reveal the interplay between quantum and classical dynamics in the strong field excitation of bound states. [1] T. Nubbemeyer et al., Phys. Rev. Lett. 101, 233001 (2008). [2] U. Eichmann et al., Nature 461, 1261 (2009). [3] H. Zimmermann et al., Phys. Rev. Lett. 118, 013003 (2017). [4] L.V. Keldysh, Sov. Phys. ­– JETP 20, 1307 (1965); F.H.M. Faisal, J. Phys. B 6, L89 (1973); H.R. Reiss, Phys. Rev. A 22, 1786 (1980). [5] S.V. Popruzhenko, J. Phys. B 50 (2017), in press.

### Exciton-phonon interaction breaking all antiunitary symmetries in external magnetic fields

Rommel, Patric

If the complete valence band structure of a direct-band-gap cubic semiconductor is considered, the application of an external magnetic field to highly excited Rydberg excitons breaks all antiunitary symmetries in the system, leading to the appearance of GUE statistics [1]. However, this effect only occurs if the magnetic field is not oriented in one of the symmetry planes of the cubic lattice. Recent experimental investigations by M. Aßmann et al. [2] on the spectrum of magnetoexcitons in cuprous oxide nevertheless revealed the statistics of a Gaussian unitary ensemble (GUE) for all orientations of the field. We investigate the effect of quasiparticle interactions or especially the exciton-phonon interaction on the level statistics of magnetoexcitons and show that the motional Stark field induced by the exciton-phonon interaction leads to the occurrence of GUE statistics for arbitrary orientations of the magnetic field in agreement with experimental observations [3]. Importantly, the breaking of all antiunitary symmetries can be explained only by considering both the exciton-phonon interaction and the cubic crystal lattice. [1] F. Schweiner et al. Magnetoexcitons Break Antiunitary Symmetries. Phys. Rev. Lett. 118, 046401 (2017) [2] M. Aßmann et al. Quantum chaos and breaking of all anti-unitary symmetries in Rydberg excitons. Nat. Mater. 15, 741 (2016) [3] F. Schweiner et al. Exciton-phonon interaction breaking all antiunitary symmetries in external magnetic fields. Phys. Rev. B 96, 035207 (2017)

Rubisch, Andreas

Saalmann, Ulf

### Rydberg optical Feshbach resonances in cold gases

Sándor, Nóra

We present a novel optical scheme to tune the scattering length of two colliding ground-state atoms. The scheme is based on off-resonantly coupling the scattering-state of the atomic pair to an excited Rydberg-molecular state using laser light. The efficiency of the process can be described by the effective optical length and pole strength of this Rydberg optical Feshbach resonance. I demonstrate for the s-wave scattering of two colliding 87 Rb atoms, that these quantities can be tuned over several orders of magnitude, while incoherent processes and losses are minimised. Given the ubiquity of Rydberg molecular states, this technique should be generally applicable to homonuclear atomic pairs as well as to atomic mixtures with s-wave (or even p-wave) scattering, although the details of the calculation are different.

### An optogalvanic flux sensor for trace gases

Schmidt , Johannes

We demonstrate the applicability of a new kind of gas sensor based on Rydberg excitations. From an arbitrary probe gas the molecule in question is excited to a Rydberg state, by succeeding collisions with all other gas components this molecule gets ionized and the emerging electron and ion can then be measured as a current, which is the clear signature of the presence of this particular molecule. As a first test we excite Alkali Rydberg atoms in an electrically contacted vapor cell [1,2] and demonstrate sensitivities down to 100 ppb. We investigate different amplification circuits, ranging from solid state devices on the cell to thin film technology based transimpedance amplifiers inside the cell [3,4]. For a real life application, we employ our gas sensing scheme to the detection of nitric oxide in a background gas at thermal temperatures and atmospheric pressure. References [1] D. Barredo, et al., Phys. Rev. Lett. 110, 123002 (2013) [2] R. Daschner, et al., Opt. Lett. 37, 2271 (2012) [3] P. Schalberger, et al., JSID 19, 496-502 (2011) [4] J. Schmidt, et al., AMFPD 24, 296-298 (2017) J. Schmidt1,2, M. Fiedler1, R. Albrecht1, P. Schalberger2, H. Baur2, R. Löw1, T. Pfau1, Edward Grant3, N. Frühauf2, H. Kübler1 Integrated Quantum Science and Technology, Universität Stuttgart 15. Physikalisches Institut 2 Institut für Großflächige Mikroelektronik 3 University of British Columbia, Chemistry Department

### A Photon-Photon Quantum Gate Based on Rydberg Polaritons

Schmidt-Eberle, Steffen

Rydberg polaritons offer a unique way to create strong interactions for photons. We utilize these interactions to demonstrate a photon-photon quantum gate. To achieve this, a photonic control qubit is stored in a quantum memory consisting of a superposition of a ground state and a Rydberg state in an ultracold atomic gas. This qubit interacts with a photonic target qubit in the form of a propagating Rydberg polariton to generate a conditional pi phase shift, as in Ref. [1]. Finally, the control photon is retrieved. We measure two controlled-NOT truth tables and the two-photon state after an entangling-gate operation. This work is an important step toward applications in optical quantum information processing, such as deterministic photonic Bell-state detection which is crucial for quantum repeaters. [1] D. Tiarks et al., Science Advances 2, 1600036 (2016).

### Exitonic spectra in high external fields

Schweiner, Frank

Excitonic spectra are often described within the simple band model by a hydrogen-like series [1]. However, to understand and interpret excitonic absorption spectra of real semiconductors [2] it is indispensable to incorporate the complete valence band structure in a quantitative theory. This is all the more important for magnetoexcitons, where the external magnetic field reduces the symmetry of the system even further. We present the theory of excitons in cuprous oxide in an external magnetic field and especially discuss the dependence of the spectra on the direction of the external magnetic field, which cannot be understood from a simple hydrogen-like model. Due to the specific material parameters in, the exciton radius is much larger than the Bohr radius. This makes excitons attractive for investigations in external fields since the region of "high magnetic fields" can be reached within several Tesla. We compare our theoretical results with high-resolution experimental spectra by the group of M. Bayer, D. Fröhlich and M. Aßmann of the TU Dortmund and obtain an excellent agreement as regards not only the energies but also the relative oscillator strengths [3]. Furthermore, this comparison allows for the determination of the fourth Luttinger parameter of this semiconductor. A closer investigation of the level statistics of mangetoexcitons reveals that all antiunitary symmetries are broken in this system. Hence, we give theoretical evidence for a spatially homogeneous system breaking all antiunitary symmetries [4]. [1] T. Kazimierczuk et al., Nature 514, 343 (2014) [2] J. Thewes et al., Phys. Rev. Lett. 115, 027402 (2015) [3] F. Schweiner et al., Phys. Rev. B 95, 035202 (2017) [4] F. Schweiner et al., Phys. Rev. Lett. 118, 046401 (2017)

### Theoretical approaches to complex many-body systems: Application to ultracold plasma

Sous, John

The dynamics of interacting many-body systems have attracted immense interest. Various fields have approached the problem from different angles. AMO physics experimentalists prepare ultracold atoms in optical lattices as model systems to study engineered localization and explore the phenomenon of many-body localization. A new approach has classified a hydrodynamic behavior in closed quantum many-body systems that arises from quantum chaos. Investigations of classical amorphous solids find glassy systems characterized by very slow dynamics. Spin glasses represent a special case with unique properties owing to the presence of an order parameter. We take all of these approaches into consideration in an effort to explain the arrested relaxation observed experimentally in an ultracold molecular plasma. Pairwise interactions over the broad energy landscape arising from a highly disordered Hamiltonian apparently give rise to a very slow relaxation in the energy coordinate. Future work will explore related phenomena in this experimental setting in a context of quantum dynamics.

### Giant Dipole Systems: Atoms, Molecules, and Excitons

Stielow, Thomas

An exotic species of Rydberg atoms in crossed eletric and magnetic fields are so-called giant-dipole atoms, which are characterized by a huge permanent electric dipole momentof several hundred thousand Debye. Recently, diatomic molecular states formed by the binding of a giant-dipole atom with a neutral ground-state perturber, similar to Rydberg molecules, have been analyzed. We present a refined theory of ultra-long range giant dipole molecules including higher order binding interactions as well as angular momentum couplings. Within Born--Oppenheimer theory, we obtain a rich topology of novel PESs. We analyze the vibrational structure which provides evidence for the existance of stable bound molecular states. We further discuss the possible formation of exitonic giant dipole states in cuprous oxide. We analyze potential surfaces analogous to the atomic system and show the possible existance of bound states.

Syed, Zaheer U.

### Towards imaging nonlocal photon interactions in a Rydberg gas

Tebben, Annika

The transmission of a light field under conditions of electromagnetically induced transparency in an ultracold atomic gas is modified by Rydberg interactions. Here, we develop an analytical theory of the nonlinear and nonlocal optical response, without employing the adiabatic elimination of the intermediate state. In the vicinity of the single-photon resonance we find an enhanced absorption due to resonant Rydberg dressing of the atoms. Simulations show the capability of experimentally measuring this feature.

### Many-body quantum spin dynamics with ultracold Rydberg gases

Dipolar interacting Rydberg spin systems have been ideal platforms to study non-equilibrium phenomena of isolated quantum systems. Their tunable strong, long-range interactions provide new opportunities to investigate the dynamics of strongly correlated many-body quantum systems with beyond nearest-neighbor coupling. In this work, we present an experimental realization of a dipolar spin-1/2 model by coupling two strongly interacting Rydberg states utilizing a microwave field. By using a spin-locking technique to have a full phase control of the driving field, we study spin dynamics by letting the systems evolve under designated interactions. The resulting magnetizations extracted from the systems utilizing a state-tomography technique and a selective ionization after the dynamics will be discussed.

### Excitation of terahertz currents by two-color laser pulses interacting with atomic gases.

Tulsky, Vasily

One of the promising schemes an electromagnetic source operating in a broad-band terahertz (THz) domain is based on the irradiation of gaseous media by two-color intense laser pulses consisting of an infrared or optical fundamental component and its second harmonic. Most of the previous experimental and theoretical work was focused on the case when both components are linearly polarized (see [1-5] and references therein). A recent experiment [6] shows that using circularly polarized fields leads (at all other parameters fixed) to a significant increase in the THz signal. In the present work we discuss the underlying mechanism for this effect and develop a theory which covers both the quantum-mechanical ionization process and the subsequent classical plasma dynamics leading to the THz radiation emission. Results for ionization rates are obtained within Strong Field Approximation and compared to numerical solutions to the time-dependent Schroedinger equation; for the analysis of the macroscopic plasma dynamics Vlasov and particle-in-cell simulations are applied. Parameters of the laser pulse and of the gas target maximizing the THz current are suggested. [1] Y. S. You, T. I. Oh, and K. Y. Kim, Phys. Rev. Lett. 109, 183902 (2012). [2] D.S. Bagulov, I.A. Kotelnikov Zh. _Exp. Teor. Fiz. 143, 26 (2013). [3] L. A. Johnson, J. P. Palastro, T. M. Antonsen, and K.Y. Kim, Phys. Rev. A 88, 063804 (2013). [4] N.V. Vvedenskii, A.I. Korytin, V.A. Kostin, A.A. Murzanev, A.A. Silaev, and A.N. Stepanov, Phys. Rev. Lett. 112, 055004 (2014). [5] S.V. Popruzhenko, V.A. Tulsky, Phys. Rev. A 92, 033414 (2015). [6] C. Meng, W. Chen, X. Wang, Z. Lu, Y. Huang, J. Liu, D. Zhang, Z. Zhao, and J. Yuan, Appl. Phys. Lett. 109, 131105 (2016).

### Fully programmable all-to-all interactions with Rydberg-dressed atoms

van Bijnen, Rick

Rydberg atoms provide a promising toolbox to engineer interactions and quantum magnetism in ultracold atomic experiments, on behalf of their extremely strong interactions. We show how these huge interaction strengths can be harvested by employing off-resonant laser-dressing of ground state atoms to Rydberg states, highlighting recent experimental progress. We theoretically explore the possibility to realise fully programmable all-to-all interactions between ground state atoms in optical lattices. Such a system would be realisable with current experimental capabilities, and would be capable of realising arbitrary spin models. In addition, such a system could serve as a prototype quantum annealer and has applications in quantum machine learning.

### Noninterfering superpositions of up to six light waves

van Kruining, Koen

We present a set of superpositions of plane waves that have a homogeneous electric field density. For most physical processes, including human eyesight, these superpositions can be considered noninterfering. The homogeneity of the electric field allows effects beyond the electric dipole interaction to be relevant. In particular, our superpositions act as optical lattices with inverted effective potentials for different enantiomers of a chiral molecule. We estimate the power and alignment requirements needed for this proposal.

### Rydberg excitation of cold, trapped ions

Vogel, Jonas

Exciting cold, trapped ions to electronically high-laying Rydberg states offers a unique opportunity for observing novel effects arising from the interplay between the Coulomb interaction and their giant dipole moments [1,2]. We have employed a coherent VUV radiation source at 122.04nm for Rydberg excitation of 40Ca+ ions in a linear radiofrequency ion trap [1]. In our experiment, the excitation to 52F, 53F, 66F [3] and 22F [4] states was achieved. Large polarizability of these states might lead to a parasitic coupling of the electronic levels to the vibrational excitations in the ion trap, which causes large decoherences [5]. For this reason, we have employed a new segmented, microfabricated ion trap featuring shielded experimental zone and low RF axial component. Using sideband spectroscopy on the 4S1/2-3D5/2 transition, we have characterized the micromotion effect on the line shape of the 22F transition. Furthermore, to eliminate the Stark shift on Rydberg states, the use of a digital ion trap based on fast switching of the trap derive has been studied. The ultimate goal of this experiment is to exploit strong state-dependent interactions between Rydberg ions for manipulating the many-body properties of cold ionic ensembles. This technology is of interest for the exploration of non-equilibrium dynamics in structural phase transitions [6] and symmetry-breaking defect formations [7,8], as well as for applications in quantum simulation and quantum information processing [9,10]. A. Mokhberi1, J. Vogel1, J. Andrijauskas1,2, P. Bachor1,2, G. Jacob1, P. Islam1, J. Walz1,2, F. Schmidt-Kaler1 1Institut für Physik, Universität Mainz, Staudingerweg 7, D-55128 Mainz, Germany. 2 Helmholtz-Institut Mainz, D-55099, Germany. References [1] F. Schmidt-Kaler et al., New J. Phys. 13, 075014 (2011) [2] M. Meuller et al., New J. Phys. 10, 093009 (2008) [3] T. Feldker et al., Phys. Rev. Lett. 115, 173001 (2015) [4] P. Bachor et al., J. Phys. B. 49, 154004 (2016) [5] G. Higgins et al., Phys. Rev. X 7, 021038 (2017) [6] W. Li and I. Lesanovsky, Phys. Rev. Lett. 108, 023003 (2012) [7] S. Ulm et al., Nat. comm. 4, 2290 (2013) [8] P. Silvi et al., Phys. Rev. Lett. 116, 225701 (2016) [9] W. Li and I. Lesanovsky, Appl. Phys. B 114 (1), 37 (2014) [10] W Li et al., Phys. Rev. A 87, 052304 (2013)

### Giant optical nonlinearities from Rydberg-excitons in semiconductor microcavities

Walther, Valentin

The experimental demonstration of exciton-polaritons -- hybrid excitations of semiconductor quantum well excitons and cavity photons -- has led to both scientific developments and technological advances. Short-range collisional exciton interactions provide access to a plethora of driven-dissipative and hydrodynamical effects that arise from the weak nonlinearities of polariton condensates. However, only enhanced optical nonlinearities would enable quantum photonics applications and open up a new realm of photonic many-body physics in a scalable and engineerable solid-state environment. Here we present a possible path to such capabilities in cavity-coupled semiconductors by harnessing the enormous interactions between recently observed Rydberg-excitons. We demonstrate that optical nonlinearities in these semiconductor systems can be enhanced by several orders of magnitude and induce nonlinear processes at the ultimate quantum level of single photons.

### Controlling cold atom-ion collisions using a Rydberg state

Wang, Limei

Substantial progress in the preparation of cold atom-ion hybrid systems has been achieved [1,2]. With respect to quantum computation and quantum simulation [3], control of collisions is required. In particular, for a variety of experiments unwanted chemical reactions between atoms and ions such as charge exchange or the formation of molecular ions need to be suppressed. We present a method to control the cold collision between an ultracold atom and a trapped ion. A laser is used to excite the ground state atom to a repulsive Rydberg potential level once it approaches the ion to a certain distance. In this way the ion is effectively surrounded by a potential wall that the atom cannot cross. Once the atom leaves the interaction area, it is de-excited back to its original level. The adiabaticity of the scheme is analyzed as a function of different parameters such as laser frequency, laser power, initial atom-ion collision energy, as well as the direction of the collisional process with respect to the light field. By controlling e.g. the laser power and the laser frequency, as well as by addressing different Rydberg states, the properties of this shielding effect can be widely tuned. In particular, unwanted chemical reactions between atoms and ion can efficiently be suppressed, which is an important step towards realization of diverse quantum technological applications for hybrid atom-ion systems. [1] A. Härter and J. Hecker Denschlag, Contemporary Physics 55(1), 33 (2014). [2] M. Tomza, K. Jachymski, R. Gerritsma, A. Negretti, T. Calarco, Z. Idziaszek and P.S. Julienne, arXiv:1708.07832 (2017). [3] T. Secker, R. Gerritsma, A. W. Glaetzle and A. Negretti, Phys. Rev. A 94, 013420 (2016).

### Two-color phase-of-the-phase spectroscopy in the multiphoton regime

Zabel, Michael

Applying phase-of the-phase spectroscopy, recently introduced by Skruszewicz, et al., we investigated the momentum-resolved photoelectron emission from xenon in colinearly polarized two-color laser fields at above-threshold ionization both experimentally and theoretically. A characteristic checkerboard pattern occurs in the resulting phase-of-phase spectra.

### Electric field controlled collisions between polar molecules and Rydberg atoms

Zeppenfeld, Martin

The combination of polar molecules and Rydberg atoms represents an intriguing new hybrid system for applications in quantum science. Possible applications include controlling motional[1] and internal[2] molecular degrees of freedom, nondestructive detection of molecules[3], and quantum information processing[4]. As a first experiment, we have investigated state-changing collisions between polar molecules and Rydberg atoms due to Förster resonant energy transfer in a room temperature thermal ensemble. We precisely map out the Rydberg state via state sensitive field ionization combined with mm-wave state transfer and observe large collision cross-sections for energy transfer between ammonia molecules and rubidium Rydberg atoms. The dependence of the energy transfer on a resonance condition between molecular and atomic states allows the collision rates to be tuned via electric fields. [1] B. Zhao et al., PRL 108, 193007 (2012) [2] S.D. Huber et al., PRL 108, 193006 (2012) [3] M Zeppenfeld, EPL 118, 13002 (2017) [4] E. Kuznetsova et al., PCCP 13, 17115 (2012)

### Studying ion-atom scattering in the ultracold regime with Rydberg molecules

Zuber, Nicolas

We are building up a two species experiment with rubidium 87 and lithium 6 and the ability for Rydberg excitation to study atom-ion collisions in the ultracold quantum regime~[1]. We present details about our experimental setup including a spatially and temporally resolving delay-line detector with a single particle rate up to several MHz. To detect the scattered wave packet an ion microscope with a magnification of over 1000 is currently integrated into the setup. The atom-ion pair is created by fast photoionization of a Rydberg molecule, which provides precise starting conditions for the scattering event. [1] T. Schmid et al, arXiv:1709.10488