Controlling strongly interacting many-body systems enables the creation of tailored quantum matter, with properties transcending those based solely on single particle physics. Atomic ensembles which are optically driven to a Rydberg state provide many examples of this, such as atom-atom entanglement [1,2], many-body Rabi oscillations [3], strong photon-photon interaction [4] and spatial pair correlations [5]. In its most basic form, Rydberg quantum matter consists of an isolated ensemble of strongly interacting atoms spatially confined to the blockade volume -a so-called superatom. Here we demonstrate the controlled creation and characterization of an isolated mesoscopic superatom by means of accurate density engineering and excitation to Rydberg p-states. Its variable size allows to investigate the transition from effective two-level physics for strong confinement to many-body phenomena in extended systems. By monitoring continuous laser-induced ionization we observe a strongly anti-bunched ion emission under blockade conditions and extremely bunched ion emission under off-resonant excitation. Our experimental setup enables in vivo measurements of the superatom, yielding insight into both excitation statistics and dynamics. We anticipate straightforward applications in quantum optics and quantum information as well as future experiments on many-body physics.Rydberg superatoms combine single and many-body quantum effects in a unique way and have been proposed as fundamental building blocks for quantum simulation and quantum information [6]. Due to the phenomenon of Rydberg blockade [7], the ensemble collectively forms a system with only two levels of excitation. Provided a range of interaction larger than the sample size, the presence of one excitation shifts all other atoms out of resonance and therefore only one excitation can be created at a time. Changing the size or the driving conditions revives the underlying many-body nature and the presence of several excited atoms with pronounced correlations becomes possible. This tunability and the possibility of multiple usage within a single experimental sequence make superatoms a promising complement to single-atom-based quantum technology. It is therefore important to understand the significance of the superatom concept, the implications of its finite spatial extent and its many-body level structure. We here investigate the latter by measuring the mean Rydberg excitation as well as its time-resolved two-particle correlations in an optically excited, mesoscopic superatom for varying excitation strength and under resonant and non-resonant conditions, revealing very different excitation dynamics.The realization of superatom-based quantum systems requires the implementation of arbitrary arrangements of isolated mesoscopic atomic ensembles in a scalable way. We here prepare an individual superatom by carefully shaping the density distribution of a Bose-Einstein condensate of 87 Rb atoms. We first load the condensate into a one-dimensional optical lattice with a spacin...

We investigate the possibility of a bistable phase in an open many-body system. To this end we discuss the microscopic dynamics of a continuously off-resonantly driven Rydberg lattice gas in the regime of strong decoherence. Our experimental results reveal a prolongation of the temporal correlations exceeding the lifetime of a single Rydberg excitation and show strong evidence for the formation of finite-sized Rydberg excitation clusters in the steady state. We simulate the dynamics of the system using a simplified and a full many-body rate-equation model. The results are compatible with the formation of metastable states associated with a bimodal counting distribution as well as dynamic hysteresis. Yet, a scaling analysis reveals that the correlation times remain finite for all relevant system parameters, which suggests a formation of many small Rydberg clusters and finite correlation lengths of Rydberg excitations. These results constitute strong evidence against the presence of a global bistable phase previously suggested to exist in this system.

Engineering molecules with a tunable bond length and defined quantum states lies at the heart of quantum chemistry. The unconventional binding mechanism of Rydberg molecules makes them a promising candidate to implement such tunable molecules. A very peculiar type of Rydberg molecules are the so-called butterfly molecules, which are bound by a shape resonance in the electron–perturber scattering. Here we report the observation of these exotic molecules and employ their exceptional properties to engineer their bond length, vibrational state, angular momentum and orientation in a small electric field. Combining the variable bond length with their giant dipole moment of several hundred Debye, we observe counter-intuitive molecules which locate the average electron position beyond the internuclear distance.

We have studied the associative ionization of a Rydberg atom and a ground state atom in an ultracold Rydberg gas. The measured scattering cross section is three orders of magnitude larger than the geometrical size of the produced molecule. This giant enhancement of the reaction kinetics is due to an efficient directed mass transport which is mediated by the Rydberg electron. We also find that the total inelastic scattering cross section is given by the geometrical size of the Rydberg electron's wavefunction.PACS numbers: 32.80. Rm, 34.50.Fa, 82.45.Jn Molecule formation in dilute gases or plasmas usually happens via two-body collisions and is relevant, e.g., for the production of molecules in interstellar space [1] or during the early stage of the universe [2]. One prominent class of such collisions is the associative ionization between a ground state atom and a (highly) excited atom. The latter can result from electron capture in a plasma or photon absorption. The formation of a bound molecule requires the release of binding energy which is realized by the ejection of an electron [3]. Associative ionization involving Rydberg atoms has been studied in detail in hot atomic beam experiments and cross sections on the order of the geometrical size of the formed molecules have been found [4][5][6].Ultracold Rydberg gases allow to extend the study of this fundamental chemical reaction to the low-energy limit, where a new interaction mechanism [7] has drawn researchers' attention in the last years: The scattering between the electron in a Rydberg state and a ground state atom creates a potential for the atom that reaches far from the Rydberg atoms core. This potential gives rise to ultra long range Rydberg molecules [8], trilobite molecules [9] and was found to induce phonons inside of a Bose-Einstein-Condensate [10]. The understanding of this interaction and it's role in associative ionization is also important to fully exploit the potential of ultracold Rydberg systems to study many-body quantum phenomena [11][12][13] beyond the frozen gas approximation.Here, we study the associative ionization of a rubidium atom in a Rydberg p-state (principal quantum number n = 30 − 60) and a ground state atom at ultracold temperatures. The measured scattering cross section is three orders of magnitude larger than the geometrical size of the produced molecular ion. We attribute this enhancement to a directed mass transport of the ground state atom towards the ionic core of the Rydberg atom. This transport mechanism is mediated by the scattering between the Rydberg electron and the ground state atom. The formation of the molecular ion happens, when the two collision partners are close enough that the released binding energy suffices to eject the excited electron via resonant dipole interaction [3]. The appearance of a . .

We have performed high resolution photoassociation spectroscopy of rubidium ultra long-range Rydberg molecules in the vicinity of the 25P state. Due to the hyperfine interaction in the ground state perturber atom, the emerging mixed singlet-triplet potentials contain contributions from both hyperfine states. We show that this can be used to induce remote spin-flips in the perturber atom upon excitation of a Rydberg molecule. When furthermore the spin-orbit splitting of the Rydberg state is comparable to the hyperfine splitting in the ground state, the orbital angular momentum of the Rydberg electron is entangled with the nuclear spin of the perturber atom. Our results open new possibilities for the implementation of spin-dependent interactions for ultracold atoms in bulk systems and in optical lattices. [5,6]. Longrange interactions beyond the pure contact interaction are more challenging to achieve. Possible realizations include second order tunneling [7], cavity-mediated interactions [8], magnetic dipolar interactions in high spin atomic species [9][10][11] and electric dipolar interactions between heteronuclear molecules [12]. Exciting atoms to Rydberg states is another way to induce long-range interactions, as evidenced by the demonstration of the Rydberg blockade [13][14][15][16] and anti-blockade [17,18]. Recently, these concepts were transferred to the realm of ultracold quantum gases [19]. First experimental results with off-resonant excitation schemes show that for short times, coherent interactions between ground state atoms can be generated [20]. In most such "Rydberg dressing" schemes the interaction is based on admixing Rydberg excitations to two particles, resulting in energy shifts which scale quadratically with the driving laser intensity. This narrows the parameter window for coherent effects drastically [20,21].The discovery of Rydberg macrodimers [22,23] and Rydberg molecules [24] has opened up an increasing field of research, combining ultracold chemistry with manybody physics and low energy electron scattering. Rydberg molecules are bound by the contact interaction between the Rydberg electron and a ground state perturber atom. The large extension of the Rydberg electron wave function (50 − 1000 nm) makes it possible to induce longrange interactions between two spatially separated (remote) ground state atoms that otherwise interact solely through contact interaction on a typical length scale of 5 nm in the case of rubidium. In contrast to the usual Rydberg dressing of single species gases [21], only one excitation is required, thus leading to a more favorable first order process, which scales linearly with the laser intensity.For alkali atoms, one can distinguish three different types of molecules: ultra-long range Rydberg molecules [24,25], trilobite molecules [26] and butterfly molecules [27][28][29]. While sharing a similar binding mechanism, they differ in the degree of perturbation, which is imposed by the ground state perturber to the Rydberg electron wave function. Here, we change th...

We report on the realization of high resolution electron microscopy of Rydbergexcited ultracold atomic samples. The implementation of an ultraviolet laser system allows us to excite the atom, with a single-photon transition, to Rydberg states. By using the electron microscopy technique during the Rydberg excitation of the atoms, we observe a giant enhancement in the production of ions. This is due to l-changing collisions, which broaden the Rydberg level and therefore increase the excitation rate of Rydberg atoms. Our results pave the way for the high resolution spatial detection of Rydberg atoms in an atomic sample. − 500 V cm 1 ) or electric field gradients ( − 200 V cm 2 ) in the center of the chamber, which are of 2 New J. Phys. 16 (2014) 083034 T Manthey et al

We show that the excitation of long-range Rydberg molecules in a three-dimensional optical lattice can be used as a position-and time-sensitive probe for doubly occupied sites in the system. To this end, we detect the ions which are continuously generated by the decay of the formed Rydberg molecules. While a superfluid gas shows molecule formation for all parameters, a Mott insulator with n = 1 filling reveals a strong suppression of the number of formed molecules. In the limit of weak probing, the technique can be used to probe the superfluid to Mott-insulator transition in real-time. Our method can be extended to higher fillings and has various applications for the real-time diagnosis and manipulation of ultracold lattice gases.

We characterize the two-photon excitation of an ultracold gas of rubidium atoms to Rydberg states analyzing the induced atomic losses from an optical dipole trap. Extending the duration of the Rydberg excitation to several milliseconds, the ground-state atoms are continuously coupled to the formed positively charged plasma. In this regime we measure the n dependence of the plasma-induced blockade effect and we characterize the interaction of the excited states and the ground state with the plasma. We also investigate the influence of the quasielectrostatic trapping potential on the system, confirming the validity of the ponderomotive model for states with 20 n 120.