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We demonstrate the use of an anodic bonding technique for building a vacuum tight vapor cell for the use of Rydberg spectroscopy of alkali atoms with thin film electrodes on the inside of the cell. The cell is fabricated by simultaneous triple stack glass-to-glass anodic bonding at 300{\deg}C. This glue-free, low temperature sealing technique provides the opportunity to include thin film electric feedthroughs. The pressure broadening is only limited by the vapor pressure of rubidium and the lifetime is at least four months with operating temperatures up to 230{\deg}C.

The exceptionally large polarizability of highly excited Rydberg atoms—six orders of magnitude higher than ground-state atoms—makes them of great interest in fields such as quantum optics, quantum computing, quantum simulation and metrology. However, if they are to be used routinely in applications, a major requirement is their integration into technically feasible, miniaturized devices. Here we show that a Rydberg medium based on room temperature caesium vapour can be confined in broadband-guiding kagome-style hollow-core photonic crystal fibres. Three-photon spectroscopy performed on a caesium-filled fibre detects Rydberg states up to a principal quantum number of n=40. Besides small energy-level shifts we observe narrow lines confirming the coherence of the Rydberg excitation. Using different Rydberg states and core diameters we study the influence of confinement within the fibre core after different exposure times. Understanding these effects is essential for the successful future development of novel applications based on integrated room temperature Rydberg systems., Rydberg atoms are appealing for sensing, atomic and quantum information studies, if they can be suitably integrated with optical devices. Towards this end, Epple et al. show that caesium-filled kagome-lattice hollow-core photonic crystal fibres provide a platform for fibre-based spectroscopy of Rydberg states.

The coupling of electrons to matter is at the heart of our understanding of material properties such as electrical conductivity. One of the most intriguing effects is that electron-phonon coupling can lead to the formation of a Cooper pair out of two repelling electrons, the basis for BCS superconductivity. Here we study the interaction of a single localized electron with a Bose-Einstein condensate (BEC) and show that it can excite phonons and eventually set the whole condensate into a collective oscillation. We find that the coupling is surprisingly strong as compared to ionic impurities due to the more favorable mass ratio. The electron is held in place by a single charged ionic core forming a Rydberg bound state. This Rydberg electron is described by a wavefunction extending to a size comparable to the dimensions of the BEC, namely up to 8 micrometers. In such a state, corresponding to a principal quantum number of n=202, the Rydberg electron is interacting with several tens of thousands of condensed atoms contained within its orbit. We observe surprisingly long lifetimes and finite size effects due to the electron exploring the wings of the BEC. Based on our results we anticipate future experiments on electron wavefunction imaging, investigation of phonon mediated coupling of single electrons, and applications in quantum optics., 4 pages, 3 figures and supplementary information

Rydberg atoms have exaggerated properties, which is especially manifest in the optical excitation blockade. Recent experiments show that this interaction is also effective at room temperatures, opening a pathway to room temperature single photon sources.

We demonstrate our recent results on microwave field strength and polarization electrometry using a bright resonance prepared within an electromagnetically induced transparency window in a Rubidium vapor cell.

We present recent results on how Rydberg atoms are influenced inside a kagome photonic crystal fiber. Changing the core diameter alters the interactions between Rydberg atoms and the glass walls.

Mit dem diesjahrigen Nobelpreis fur Physik werden die beiden Amerikaner Eric A. Cornell und Carl E. Wieman sowie der Deutsche Wolfgang Ketterle fur ihre Experimente zur Bose-Einstein-Kondensation ausgezeichnet [1]. Der Preis wird fur die erstmalige Realisierung von Bose-Einstein-Kondensaten und erste grundlegende Studien dieses neuen Materiezustandes vergeben. Nur vier Jahre nach dem Nobelpreis fur die Entwicklung der Laserkuhlung von Atomen [2] ist dies eine weitere Auszeichnung fur ein relativ junges Forschungsgebiet, das uber die Atomphysik hinaus zunehmend in andere Gebiete der Physik ausstrahlt.

Viele technische Errungenschaften beruhen auf der Verstarkung von Wellen wie Radio-, Licht- und Schallwellen. Die Quantenmechanik lehrt uns, dass zum einen Lichtwellen auch Teilchencharakter und zum anderen Teilchen auch eine Wellennatur besitzen. Die Tatsache, dass Teilchen als Materie- oder de-Broglie-Wellen aufgefasst werden konnen, legt die Frage nahe, ob es auch moglich ist, einen Verstarker fur Atome zu konstruieren, der ahnliche Eigenschaften hat wie ein Verstarker fur elektromagnetische Wellen. In diesem Artikel beschreiben wir ein Experiment, das dieser Frage nachgeht.

The strong interaction between Rydberg atoms can be used to control the strength and character of the interatomic interaction in ultracold gases by weakly dressing the atoms with a Rydberg state. Elaborate theoretical proposals for the realization of various complex phases and applications in quantum simulation exist. Also a simple model has been already developed that describes the basic idea of Rydberg dressing in a two-atom basis. However, an experimental realization has been elusive so far. We present a model describing the ground state of a Bose-Einstein condensate dressed with a Rydberg level based on the Rydberg blockade. This approach provides an intuitive understanding of the transition from pure twobody interaction to a regime of collective interactions. Furthermore it enables us to calculate the deformation of a three-dimensional sample under realistic experimental conditions in mean-field approximation. We compare full three-dimensional numerical calculations of the ground state to an analytic expression obtained within Thomas-Fermi approximation. Finally we discuss limitations and problems arising in an experimental realization of Rydberg dressing based on our experimental results. Our work enables the reader to straight forwardly estimate the experimental feasibility of Rydberg dressing in realistic three-dimensional atomic samples., 21 pages, 9 figures

A Bose-Einstein condensate illuminated by a single off-resonant laser beam (``dressed condensate'') shows a high gain for matter waves and light. We have characterized the optical and atom-optical properties of the dressed condensate by injecting light or atoms, illuminating the key role of long-lived matter wave gratings produced by the condensate at rest and recoiling atoms. The narrow bandwidth for optical gain gave rise to an extremely slow group velocity of an amplified light pulse (1 m/s)., 5 pages, 4 figures

In atom lithography the conventional roles played by light and matter are reversed. Instead of using a solid mask to pattern a light beam, a mask of light is used to pattern a beam of neutral atoms. In this paper we report the production of different chromium dot arrays with quadratic symmetry. The lattice period depends on the relative polarization and the phase of the two standing waves generating the light mask. A small angular misalignment of the laser beams breaks the high symmetry and a chromium superlattice is written, that is a continuous periodic change between two different quadratic lattices. The structures exhibit lines with a FWHM below 50 nm and clearly separated chromium dots with a FWHM below 70 nm.

Atomic matter waves, like electromagnetic waves, can be focused, reflected, guided and split by currently available passive atom-optical elements. However, the key for many applications of electromagnetic waves lies in the availability of amplifiers. These active devices allow small signals to be detected, and led to the development of masers and lasers. Although coherent atomic beams have been produced1,2,3,4, matter wave amplification has not been directly observed. Here we report the observation of phase-coherent amplification of atomic matter waves. The active medium is a Bose–Einstein condensate, pumped by light that is far off resonance. An atomic wave packet is split off the condensate by diffraction from an optical standing wave, and then amplified. We verified the phase coherence of the amplifier by observing interference of the output wave with a reference wave packet. This development provides a new tool for atom optics and atom interferometry, and opens the way to the construction of active matter-wave devices.

Kalte Atome an Oberflachen sind heise Kandidaten fur Physik in niederdimensionalen Systemen, fur Anwendungen in der Atomoptik und vielleicht fur den Bau von Quantencomputern. Aber wie bringt man kalte Atome an eine „heise” Oberflache (Zimmertemperatur), wie und fur wie lange kann man sie dort speichern und wie kann man sie nachweisen?

We present a new separated beam atom interferometer in which the recombination of the atomic wave packet is due to the dc-Stark interaction of an induced atomic dipole with a cylindrically symmetric electric field of a charged wire. The fringe period shows a weak power-law dependence on the de Broglie wavelength and the polarizability of the particles. We present a semiclassical theoretical model for this interferometer which resembles the measured performance of the interferometer without free parameters. A discussion of possible applications of this interferometer for atoms and molecules is given.

The general features of atom lithography, such as parallel deposition, large exposure area, nanometer resolution and both direct writing and resist based patterning, make it a promising field. A summary of the present state of neutral atom lithography is presented. The methods and results from various groups are discussed. A more in-depth study of three different types of atom lithography undertaken in our group is given.

Instead of using a solid mask to pattern a light beam (optical lithography) we used a mask made of light to pattern a beam of neutral atoms (atom lithography). By making use of two special features of the atom-light interaction we wrote structures with periods below λ/2. In the first approach we inverted the focussing potentials by switching the detuning of the light field during the deposition. The second method uses the fact that atoms with a magnetic substructure in the electronic ground state are strongly sensitive to the polarization of the light field. Both techniques produce sub-100 nm chromium structures in one and two dimensions on silicon substrates.

We present the realisation of a magneto-optical trap for chromium. 52Cr atoms are loaded directly from a thermal beam. The trap lifetime is enhanced by using two red lasers to repump population that has decayed, via intercombination lines, to metastable levels back into the cooling cycle. We have measured the wavelengths of these intercombination lines and observed coherent Raman spectra. The observed density of 108 cm−3 is currently limited by collisions with the hot beam.

We have realized a new type of atom interferometer based on the refraction of matter waves in the radially symmetric electric field of a charged wire. The fringe period shows a power law dependence on the field strength and the de Broglie wavelength. Since the latter dependence is weak, the interferometer has an almost ``white light'' character.

We prepare a quasi-2D gas of argon atoms confined in a planar matter waveguide in close vicinity ( $l1\ensuremath{\mu}\mathrm{m}$) of a reflecting surface. The (anti-)nodes of a far off resonant standing light wave in front of the surface provide a confining potential for the atomic motion normal to the surface. A cloud of cold atoms that approaches the surface is compressed by deceleration in an evanescent field and loaded predominantly into one single potential well close to the surface by optical pumping. The spatial density achieved is more than 100 times higher than the density of the approaching atoms.

Imagine that a pair of coins are tossed in a black box. The box reports only one of the following three results at random: (1) the outcome of the first coin (heads or tails), (2) the outcome of the second coin (heads or tails), or (3) whether the outcomes of the two coins matched or were different. Our task is to construct a joint probability distribution of the four possible outcomes of the coins (HH, TT, HT, TH) based on many observations of the black box outputs. Now suppose that after many trials, the black box reports that each coin comes up heads two‐thirds of the time when measured individually, yet the coins never match when they are compared. (Clearly the results of the coin tosses have been correlated—perhaps a joker in the black box flips the coins and then changes the outcomes appropriately). We seek a distribution that both reflects this correlation and obeys the marginal distributions of each coin as two‐thirds heads, one‐third tails (see the three tables on page 23). The only way to satisfy...

Cr, different patterns can be generated. Possible applications using the inherent properties of atom lithography, e.g. the fabrication of photonic bandgap material, are discussed.

We report on double-slit interference of the matter waves of metastable helium atoms in the intermediate regime between Fresnel and Fraunhofer diffraction. From these diffraction data, we partially reconstruct the Wigner function of the atomic motional state immediately behind the double slit by the inverse Radon transformation technique. We were able to observe negative parts of the Wigner function, making evident the quantum nature of the atomic centre-of-mass motion.

We discuss the fabrication of various one and two-dimensional chromium nanostructures. A chromium atomic beam effusing out of a thermal oven is first transversally laser cooled. This well-collimated beam is then structured and focused by the optical dipole force in a periodic light field. By introducing a substrate at the focus of the atomic beam we are able to capture this pattern directly. We discuss a few applications and challenges of this young lithography technique.

Beams of atoms can exhibit interference and diffraction phenomena just like waves of light. For a coherent beam of helium atoms in a double-slit experiment, measurements of the quantum-mechanical analogue of the classical phase-space distribution function show that the motion of atoms behaves in a strongly non-classical manner.

We have used neutral chromium atoms to write periodic nanometerscale structures in a direct way. We use the force exerted on the induced dipole of the atom in the intensity gradient of an optical standing light field to focus an atomic beam onto a substrate. The generated pattern reflects the symmetry of the optical standing wave.

We have used a self-assembling monolayer of dodecanethiole molecules as the resist for a lithography technique based on a beam of metastable helium atoms. Doses as low as 3 metastable helium atoms per 10 resist molecules are enough to write patterns. The writing mechanism is based on the Penning-ionization-induced changes of the local wetting properties of the surface. An edge resolution of 30 nanometers is demonstrated.

Interacting two component Fermi gases are at the heart of our understanding of macroscopic quantum phenomena like superconductivity. Changing nature of the interaction is expected to head to novel quantum phases. Here we study the ground state of a two component fermionic gas in a harmonic potential with dipolar and contact interactions. Using a variational Wigner function we present the phase diagram of the system with equal but opposite values of the magnetic moment. We identify the second order phase transition from paramagnetic to ferronematic phase. Moreover, we show the impact of the experimentally relevant magnetic field on the stability and the magnetization of the system. We also investigate a two component Fermi gas with large but almost equal values of the magnetic moment to study how the interplay between contact and dipolar interactions affects the stability properties of the mixture. To be specific we discuss experimentally relevant parameters for ultracold $^{161}$Dy.

We present experiments measuring an interaction-induced phase shift of Rydberg atoms at Stark-tuned Förster resonances. The phase shift features a dispersive shape around the resonance, showing that the interaction strength and sign can be tuned coherently. We use a pair-state interferometer to measure the phase shift. Although the coupling between pair states is coherent on the time scale of the experiment, a loss of visibility occurs as a pair-state interferometer involves three simultaneously interfering paths and only one of them is phase shifted by the mutual interaction. Despite additional dephasing mechanisms, a pulsed Förster coupling sequence allows for observation of coherent dynamics around the Förster resonance.

We study a quasi-two-dimensional dipolar gas at finite, but ultralow, temperatures using the classical field approximation. The method, already used for a contact interacting gas, is extended here to samples with a weakly interacting long-range interatomic potential. We present statistical properties of the system for the current experiment with chromium [M\"uller, Billy, Henn, Kadau, Griesmaier, Jona-Lasinio, Santos, and Pfau, Phys. Rev. A 84, 053601 (2011)] and compare them with statistics for atoms with larger magnetic dipole moments. Significant enhancement of the third-order correlation function, relevant for the particle losses, is found.

Demagnetization cooling relies on spin-orbit coupling that brings motional and spin degrees of freedom into thermal equilibrium. In the case of a gas, one has the advantage that the spin degree of freedom can be cooled very efficiently using optical pumping. We investigate demagnetization cooling of a chromium gas in a deep optical dipole trap over a large temperature range and reach high densities up to $5\times 10^{19} m^{-3}$. We study the loss mechanism under such extreme conditions and identify excited-state collisions as the main limiting process. We discuss that in some systems demagnetization cooling has a realistic potential of reaching degeneracy by optical cooling only., Comment: 6 pages, 4 figures

We study normal modes propagating on top of the stable uniform background in arrays of dipolar Bose-Einstein condensate (BEC) droplets trapped in a deep optical lattice. Both the on-site mean-field dynamics of the droplets and their displacement due to the repulsive dipole-dipole interactions (DDIs) are taken into account. Dispersion relations for two modes, \textit{viz}., high- and low- frequency counterparts of optical and acoustic phonon modes in condensed matter, are derived analytically and verified by direct simulations, for both cases of the repulsive and attractive contact interactions. The (counterpart of the) optical-phonon branch does not exist without the DDIs. These results are relevant in the connection to emerging experimental techniques enabling real-time imaging of the condensate dynamics and direct experimental measurement of phonon dispersion relations in BECs., Comment: Physical Review A, in press

A new method to drive a system of neutral dipolar fermions into the lowest Landau level regime is proposed. By employing adiabatic spin-flip processes in combination with a diabatic transfer, the fermions are pumped to higher orbital angular momentum states in a repeated scheme that allows for the precise control over the final angular momentum. A simple analytical model is derived to quantify the transfer and compare the approach to rapidly rotating systems. Numerical simulations of the transfer process have been performed for small, interacting systems., Comment: 6 pages, 3 figures

We demonstrate continuous Sisyphus cooling combined with a continuous loading mechanism used to efficiently slow down and accumulate atoms from a guided beam. While the loading itself is based on a single slowing step, applying a radio frequency field forces the atoms to repeat this step many times resulting in a so-called Sisyphus cooling. This extension allows efficient loading and cooling of atoms from a wide range of initial beam conditions. We study the interplay of the continuous loading and simultaneous Sisyphus cooling in different density regimes. In the case of a low density flux we observe a relative gain in phase-space density of nine orders of magnitude. This makes the presented scheme an ideal tool for reaching collisional densities enabling evaporative cooling - in spite of unfavourable initial conditions., Comment: 8 pages, 8 figures

We have demonstrated that a cesium atomic beam can be used to pattern a gold surface using a self assembling monolayer (SAM) as a resist. A 12.5 μm period mesh was used as a proximity mask for the atomic beam. The cesium atoms locally change the wetability of the SAM, which allows a wet etching reagent to remove the underlying gold in the exposed regions. An edge resolution of better than 100 nm was obtained. The experiment suggests that this method can either be used as a sensitive position detector with nanometer resolution in atom optics, or for nanostructuring in a resist technique.

We report on an improved version of Young's double-slit experiment for thermal atoms, that allows two-particle correlation experiments to be performed with atom - photon pairs. We propose the use of single-photon scattering for the controlled preparation of highly entangled states of more than two particles. The principle is discussed for an entangled atom - photon pair, which offers the possibility of realizing a virtual double-slit experiment for atoms. Then the scheme is extended in an obvious way to prepare a so-called GHZ state made up of two photons and one atom. In general, highly entangled states can be built up involving the centre-of-mass wavefunction of an atom and the occupation of radiation modes by sequentially scattered photons.

We report on an experimental investigation of coherent optical excitation processes for an atomic two-level system in metastable helium. A simple absolute measure of the excitation probability is obtained from the deflection of an atomic beam, as excited atoms receive an additional momentum corresponding to a single photon recoil. With this method, we investigate the excitation process as a function of laser power, wavefront curvature and interaction time. Rabi oscillations are seen in the center-of-mass motion of an atomic ensemble for small wavefront curvatures, which change to adiabatic transfer produced by a Doppler detuning chirp for large wavefront curvatures. The adiabatic transfer provides a simple method for complete excitation of an atomic two-level system.

We discuss the prospects for a laser-like source of atoms where — in close analogy to the laser, and in close analogy to the recently observed Bose-Einstein condensation—many atoms of an atomic gas occupy a single center-of-mass motional state. We develop a theoretical description borrowing concepts from many-body quantum field theory and laser theory. Our results show threshold behavior, mode competition and Poissonian atom statistics above threshold.

Atom lithography provides a new technology that enables one to write directly nanometer scale structures onto a substrate in a parallel process. With this technique the trajectories of atoms are manipulated by light forces that efficiently focus the atoms in a standing light field produced by a laser. By chosing different standing light field configurations one can write different periodic patterns. Theoretical calculations predict a linewidth of the deposited structures in the range of 10–20 nm. The technique is a parallel, direct and species selective process involving low particle energy that prevents any damage of the substrate.

We present evidence for Rydberg-Rydberg interaction in a gas of rubidium atoms above room temperature. Rabi oscillations on the nanosecond time scale to different Rydberg states are investigated in a vapor cell experiment. Analyzing the atomic time evolution and comparing to a dephasing model, we find a scaling with the Rydberg quantum number n that is consistent with van der Waals interaction. Our results show that the interaction strength can be larger than the kinetic energy scale (Doppler width), which is the requirement for realization of thermal quantum devices in the GHz regime.

We present a scheme for a laser-like source of atoms, starting from atomic de Broglie wave resonators, for which we seek to overcome losses by gain. The resonators are blue-detuned optical dipole-force traps, which can be loaded continuously or repetitively with laser-cooled atoms. This is achieved by trapping the atoms in a different internal state than that used for laser cooling. The transfer of cold atoms into the trapped states involves emission of a photon and can be stimulated by the presence of identical bosonic atoms in the final atomic state. Numerical simulations show threshold behaviour, mode competition and Poissonian atom statistics above threshold.

Strongly interacting atom-cavity systems within a network with many nodes constitute a possible realization for a quantum internet which allows for quantum communication and computation on the same platform. To implement such large-scale quantum networks, nanophotonic resonators are promising candidates because they can be scalably fabricated and interconnected with waveguides and optical fibers. By integrating arrays of ring resonators into a vapor cell we show that thermal rubidium atoms above room temperature can be coupled to photonic cavities as building blocks for chip-scale hybrid circuits. Although strong coupling is not yet achieved in this first realization, our approach provides a key step towards miniaturization and scalability of atom-cavity systems., Comment: 5 pages, 4 figures

We report on the observation of the collapse of a dipolar Bose-Einstein condensate (BEC) upon release from a one-dimensional optical lattice. In strong contrast with the typical behavior of quantum gases under time of flight (TOF), we show here that a strongly dipolar BEC, stabilized in the lattice, may become unstable during the TOF dynamics due to the combined effect of the anisotropy of the dipolar interaction and intersite coherence. Such TOF-triggered collapse is a peculiar case of a collapse induced by the deconfinement of a dipolar many-body system at a fixed two-body interaction strength.

We present experimental results on nondegenerate four-wave mixing in a thermal vapor cell of rubidium atoms via highly excited Rydberg states. We observe sub-Doppler spectral features and a large sensitivity of the mixed light mode to small electric fields due to the large polarizabilities of the involved Rydberg state. We observe a saturation of the optical response due to the nonlinear four-level physics. The strong interaction between Rydberg atoms and the associated Rydberg blockade could potentially increase the nonlinearity dramatically.

Density-wave patterns in (quasi-) discrete media with local interactions are known to be unstable. We demonstrate that \emph{stable} double- and triple- period patterns (DPPs and TPPs), with respect to the period of the underlying lattice, exist in media with nonlocal nonlinearity. This is shown in detail for dipolar Bose-Einstein condensates (BECs), loaded into a deep one-dimensional (1D) optical lattice (OL), by means of analytical and numerical methods in the tight-binding limit. The patterns featuring multiple periodicities are generated by the modulational instability of the continuous-wave (CW) state, whose period is identical to that of the OL. The DPP and TPP emerge via phase transitions of the second and first kind, respectively. The emerging patterns may be stable provided that the dipole-dipole (DD) interactions are repulsive and sufficiently strong, in comparison with the local repulsive nonlinearity. Within the set of the considered states, the TPPs realize a minimum of the free energy. Accordingly, a vast stability region for the TPPs is found in the parameter space, while the DPP\ stability region is relatively narrow. The same mechanism may create stable density-wave patterns in other physical media featuring nonlocal interactions, such as arrayed optical waveguides with thermal nonlinearity., Comment: 7 pages, 4 figures, Phys. Rev. Lett., in press

We present an optimal protocol to implement a room temperature Rydberg single photon source within an experimental setup based on micro cells filled with thermal vapor. The optimization of a pulsed four wave mixing scheme allows to double the effective Rydberg blockade radius as compared to a simple Gaussian pulse scheme, releasing some of the constrains on the geometry of the micro cells. The performance of the optimized protocol is improved by about 70% with respect to the standard protocol., Comment: 5 pages, 6 figures

We present a very sensitive and scalable method to measure the population of highly excited Rydberg states in a thermal vapor cell of rubidium atoms. We detect the Rydberg ionization current in a 5 mm electrically contacted cell. The measured current is found to be in excellent agreement with a theory for the Rydberg population based on a master equation for the three level problem including an ionization channel and the full Doppler distributions at the corresponding temperatures. The signal-to-noise ratio of the current detection is substantially better than purely optical techniques., Comment: 4 pages, 4 figures