On one side, atom-chip experiments have demonstrated to be a versatile tool to study quantum physics in cold atoms systems. On the other side, Rydberg atoms have exaggerated properties that makes them good candidates to study quantum information and quantum simulations protocols. In this thesis both system are put together with the aim to study Rydberg physics (in particular atom-atom interaction) in an atom-chip experiment. Due to the exaggerated properties of Rydberg atoms they are very sensitive to electric fields. Thus, the combination of Rydberg atoms with surfaces can be challenging. Therefore a detailed characterization of the mentioned stray electric fields via the analysis of Stark-Zeeman maps is crucial. We employ two-photon Rydberg Stark spectroscopy of a micron-sized cloud of magnetically trapped ultracold rubidium atoms to characterize the electric fields near our atom-chip. Once we characterized the local electric fields we were able to study in more detail the effect of Rydberg excitation in the ultracold cloud of atoms. For instance we observed the formation of ultralong-range Rydberg molecules in our system. In these molecules a ground state atom is weakly bound to the Rydberg electron. We also attempted to observe the effect of Rydberg blockade in the system by looking at the characteristics of Rydberg spectra. Finally we studied the possibility of inducing rf transitions within a Rydberg state manifold in order to enhance our detection method and populate states that are not accessible directly with a two-photon laser transition.
On one side, atom-chip experiments have demonstrated to be a versatile tool to study quantum physics in cold atoms systems. On the other side, Rydberg atoms have exaggerated properties that makes them good candidates to study quantum information and quantum simulations protocols. In this thesis both system are put together with the aim to study Rydberg physics (in particular atom-atom interaction) in an atom-chip experiment. Due to the exaggerated properties of Rydberg atoms they are very sensitive to electric fields. Thus, the combination of Rydberg atoms with surfaces can be challenging. Therefore a detailed characterization of the mentioned stray electric fields via the analysis of Stark-Zeeman maps is crucial. We employ two-photon Rydberg Stark spectroscopy of a micron-sized cloud of magnetically trapped ultracold rubidium atoms to characterize the electric fields near our atom-chip. Once we characterized the local electric fields we were able to study in more detail the effect of Rydberg excitation in the ultracold cloud of atoms. For instance we observed the formation of ultralong-range Rydberg molecules in our system. In these molecules a ground state atom is weakly bound to the Rydberg electron. We also attempted to observe the effect of Rydberg blockade in the system by looking at the characteristics of Rydberg spectra. Finally we studied the possibility of inducing rf transitions within a Rydberg state manifold in order to enhance our detection method and populate states that are not accessible directly with a two-photon laser transition.
We use the sensitive response to electric fields of Rydberg atoms to characterize all three vector components of the local electric field close to an atom-chip surface. We measured Stark-Zeeman maps of S and D Rydberg states using an elongated cloud of ultracold rubidium atoms (temperature $T\ensuremath{\sim}2.5\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{K}$) trapped magnetically $100\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{m}$ from the chip surface. The spectroscopy of S states yields a calibration for the generated local electric field at the position of the atoms. The values for different components of the field are extracted from the more complex response of D states to the combined electric and magnetic fields. From the analysis we find residual fields in the two uncompensated directions of $0.0\ifmmode\pm\else\textpm\fi{}0.2$ and $1.98\ifmmode\pm\else\textpm\fi{}0.09$ V/cm. This method also allows us to extract a value for the relevant field gradient along the long axis of the cloud. The manipulation of electric fields and the magnetic trapping are both done using on-chip wires, making this setup a promising candidate to observe Rydberg-mediated interactions on a chip.
Collisions involving Rydberg atoms reveal detailed information on the state of a background medium and can be used as diagnostic probes of temperature and density distributions in a neutral or ionized gas. Spectroscopy of Rydberg atoms in highly excited states reveals the interaction of the Rydberg electron with core electrons, including relativistic effects, and can be used for precise determination of fundamental constants. The advent of ultracold trapping and cooling methods in the last three decades has ushered in a new paradigm in Rydberg physics control and manipulation. The concept of the Rydberg blockade, for instance, al- lows for precise control of long-range dipolar interaction between atoms, creation of correlated many-body wave functions, and realization of macroscopic quantum entanglement and quan- tum logic operations. The formation of a new class of Rydberg molecules arising from ground and Rydberg atom collisions can be used to manipulate electron-atom scattering phase shifts, form and manipulate molecules with enormous permanent electric dipole moments, study Ry- dberg chemistry at the ultracold, and realize macroscopic quantum polaronic systems. In this thesis, I will investigate charge transfer from covalent ground-Rydberg collisions to form heavy ion pair states. In another related study, I explore the formation of spin-mixed ultralong range Rydberg molecules, by accounting for spin-dependent relativistic fine and hyperfine interac- tion. Such studies help to not only explain experimental observations, but also point to how molecular reactions can be controlled using small electric or magnetic fields.
This thesis contributes to the quantum platform based on an atom chip. In our atom chip, atoms are trapped by means of a magnetic field. This magnetic field is created by a very thin layer of a permanently magnetized material, which is etched to create a periodic structure. Together with an applied homogeneous bias field this structure creates a periodic lattice of magnetic field minima. Cold atoms are trapped in these minima (so called microtraps). In our experiment we want to induce interaction between atoms in different lattice sites by exciting them into so-called Rydberg states: atomic states with a very large principal quantum number. These states have some extreme properties: very strong van der Waals interaction, polarizability, and long lifetimes. This makes Rydberg atoms promising candidates to build a quantum platform. In this thesis we investigate different challenges that one encounters on the way to a quantum platform based on an atom chip with Rydberg atoms. In chapter 2 we present our work on a spatial light modulator (SLM). In chapter 3 we present our experimental results on measuring and controlling stray electric fields on our atom chip. In chapter 4 we construct a comprehensive model, describing such mechanisms in order to describe the measured fields as well as its dynamical change. In chapter 5 we present a setup for our new Magchips Nano experiment. Finally, in chapter 6 we present our first steps, taken to bring the new Magchips Nano machine to life.
Rydberg atoms, i.e. atoms with one or more highly excited electrons, show exaggerated properties compared to ground state atoms. For example, their energy levels are very sensitive to external electric fields (Stark effect) and they exhibit strong interatomic interactions, which can lead to a suppression of excitation known as the Rydberg blockade. The latter property makes Rydberg atoms promising candidates for quantum simulation and quantum computing. This work describes the direct observation of the excitation blockade for highly Stark-shifted Rydberg atoms induced by controllable dipole-dipole interactions. We investigate the electric field region around and above the classical ionization threshold, which is typically not considered suitable for blockade experiments. We excite Rydberg atoms in the energy region corresponding to the unperturbed 43S1/2 state at electric fields around 127V/cm out of a magneto-optically trapped cloud of 87Rb atoms with a temperature of 150 μK. We tune the dipole-dipole interaction by the applied electric field thus changing the electric dipole moment of the excited state. The position of the Rydberg atoms is detected by ionization and subsequent ion imaging; the controlled ionization of the atoms is achieved by adiabatic transfer of the Rydberg state to an ionizing state. For the spatially resolved detection of the atoms, a high resolution ion microscope for cold atoms has been developed and experimentally characterized. It features a maximum magnification of 1000 and sub-micrometer spatial resolution. A theoretical treatment of highly Stark-shifted Rydberg states is developed introducing a complex absorbing potential. It is capable of predicting the ionization rates of the Rydberg states for a wide range of electric fields and energies with only one free parameter. Together with the numerical calculations, measurements of the ionization spectra for different Rydberg states are presented. The 43S1/2 state is measured in an electric-field range from 125V/cm to 165V/cm (classical ionization limit 127V/cm), the 70S1/2 state in an electric-field range from 15V/cm to 35V/cm (classical ionization limit 16.1V/cm) and the 100S1/2 state in an electric-field range from 7V/cm to 19V/cm (classical ionization limit 3.7V/cm). Our results show an intricate energy level structure with broad, strongly ionizing states as well as sharp, nearly non-ionizing states. The numerical calculations are also used for the prediction of selective field ionization spectra and the results are used to identify Förster resonances in the Stark spectrum of the 45D5/2 state. Complementary to the spatially resolved measurements in high electric fields, the Rydberg blockade effect is also observed via the excitation number statistics at Förster resonances.
The subject of this thesis’s work is research of cold molecules on a microchip. On the one hand, cold and ultracold molecules might allow high- precision measurements of molecular properties and the determination of their behavior at the quantum level. On the other hand, one could try to study highly controlled chemical reactions or to build a quantum computer with their help. In the experiments presented here, electrically neutral, polar molecules in low-field-seeking states are trapped and decelerated on a microchip with the help of electric fields. This exploits the Stark effect: the energy level shift of atoms and molecules in an electric field. After manipulation on the chip the molecules can be detected quantum-state specifically via the REMPI method. At first, CO molecules were prepared in the upper Λ-doublet component of the a3Π1, v = 0, J = 1 state, trapped on the chip and get vibrationally excited by an infrared laser in the middle of the chip to the J = 1 or J = 2 rotational state of the vibrational state v = 1. This could be shown experimentally and was analyzed theoretically for molecules which were trapped at all times. Secondly, CO molecules were detected on the chip and the structure of previously trapped molecule clouds of several of the only 120 μm spaced traps was spatially resolved. For this purpose the chip was extended with an ion lens setup, which enabled the determination of the temperature of the clouds by following the free ballistic expansion of the molecules in time; either by comparing the data to trajectory simulations (T = 11 ± 1 mK) or by applying a ballistic expansion model (T = 13 ± 3.5 mK), comparable to the method used by the ultracold atom community. The spatial imaging enabled the observation of a phasespace manipulation scheme, which adiabatically expanded the molecules in the trap and significantly reduces their temperature to approximately one third of their initial value. The third goal was to trap ground state ammonia molecules, which should extend the trapping time and would show that also other molecules could be trapped on the chip. The ammonia isotopologues 14NH3, 14ND3, and 15ND3 in the |vInv = 1, J = 1,K = 1> state of the electronic ground state X were used. No signal could be measured from trapped molecules, which is attributed to large nonadiabatic transitions to untrappable quantum states, leading to severe trap loss. At last, a new chip- based Stark decelerator is presented, which is a miniaturized version of the so-called “traveling-wave” ring decelerator. This chip-based decelerator should be able to trap more molecules compared to the chip that was used so far, and due to improved electronics it should be able to decelerate molecules from molecular beam velocities of approximately 360 m/s down to a standstill, in contrast to large-scale traveling-wave decelerators so far. The chip is still in development to achieve operation with the full specification (voltage). Performance simulations show the usefulness of this design, which needs to be tested in experiments.
Interactions between single atoms are fundamental to physics and to control them is an ultimate goal. The exaggerated properties of Rydberg atoms offer to met the technical challenges to isolate and control single interaction channels in ultracold gases. Here, I present experiments on two subjects related to interactions of Rydberg atoms in dense ultracold clouds. One subject concerns coherence in strongly interacting ensembles of atoms, where the interaction between Rydberg atoms is induced via Stark-tuned Förster resonances. Pulsed experiments, following the idea of Ramsey experiments, are used for high resolution spectroscopy of the Förster defect and phase sensitive detection. Coherent oscillations between pair states and an interaction-induced phase shift of Rydberg atoms are measured. These experiments are accompanied by calculations of the interaction strength and by simulations using the concept of a pair state interferometer. The simulations nicely reproduce the experimental findings and support the observation that the ensemble of atoms in the presence of interactions can be described and controlled coherently. The second subject of this thesis is the measurement of a permanent dipole moment in a homonuclear diatomic molecule that arises by the interaction between a Rydberg atom and a ground state atom. Usually parity symmetry prohibits a permanent dipole moment in diatomic molecules, but here the strong asymmetry between the constituents of the ultralong-range Rydberg molecule allows breaking parity symmetry. These molecules consist of one ground state atom bound inside the Rydberg electron wavefunction of a highly excited atom. Calculations predict dipole moments on the order of 1 Debye. Experimental proof is reported on the measurement of a linear Stark effect of these molecules, in excellent agreement with the calculations.