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Towards Rydberg excitations in magnetic lattices on atom chips

D. Davtyan

Published Jan 1, 2019• 0 citations
OpenAlex

Abstract

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.

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Atom chips are robust and extremely powerful toolboxes for quantum optical experiments, since they make it possible to create exceedingly precise magnetic traps for neutral atoms with minimal field modulations. Accurate manipulation of trapped atoms is feasible with magnetic and electric fields created on the atom chip. Therefore atom chips with high quality surfaces and extremely well defined wires were build (roughness < 20nm). Furthermore new generations of atom chips were developed, like the multi-layer atom chip with contact-free wire crossings, the sub-micron structured atom chip and the semiconducting atom chip. Extensive characterisation measurements of atom chip wires demonstrated that the wires stand current densities of up to 10^7A/cm^2 for seconds and voltage differences of more than 500V over 10µm. From the different materials tested doped silicon with a thin silicon dioxide layer is the best qualified substrate for atom chip fabrication. The atom chips fabricated during this thesis have been used in many successful experiments, yielding numerous results. Moreover this thesis established the basics for many further experiments in atom physics and quantum optics and delivers complete instructions for the fabrication of all developed atom chips as well as an introduction to the experiments.

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High-Magnetic-Field Rydberg Atom Interactions.

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•Jan 1, 2013•0 citations
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.
Atom chips provide a convenient platform to study ultracold quantum gases and their interactions with electromagnetic fields, surfaces, micro-magnetic structures, superconducting circuits and nanomechanical systems. The research work presented in this thesis is classified in two major categories. The first part is focused on the development and construction of a permanent magnetic lattice on an atom chip and the study of trapped atoms in the permanent magnetic lattice potential. In the second part, a new experiment is proposed to create a macroscopic entanglement between the flux state of a superconducting loop and a Bose-Einstein condensate (BEC) on a superconducting atom chip. The permanent magnetic lattice potential is produced by applying a uniform bias magnetic field to a grooved silicon structure coated with a permanent magnetic film. The magnetic film consists of a multilayer structure of GdTbFeCo with perpendicular intrinsic magnetization. The permanent magnetic structure is mounted beneath a current carrying wire structure glued to a ceramic insulator. The current carrying wires assist in the preparation of the ultracold atoms and for loading into the magnetic lattice. In addition, the current carrying wires are used to produce a BEC in a Z- wire magnetic trap. The magnetic lattice potential is produced by a one-dimensional array of highly anisotropic and extremely tight magnetic microtraps of period 10 ¹m. A detailed study of the reflection dynamics of atoms in the magnetic lattice potential is presented along with numerical simulations. Finally, the trapping of atoms in the permanent magnetic lattice at a distance less than 5 ¹m from the surface of the atom chip is described. The properties of the trapped atoms including lifetime, temperature and trap frequency are discussed. A maximum radial trap frequency of about 90 kHz is measured in the magnetic lattice potential with no confinement applied along the axial direction. A new experiment which combines the quantum coherent dynamics in superconducting circuits and BEC manipulation on a superconducting atom chip is proposed and investigated. It is shown how a macroscopic entanglement can be produced between the flux state of a superconducting loop and the state of a BEC trapped in a magnetic trap. In addition, the detection of the macroscopic entanglement based on a quantum eraser and an entangled BEC interference is discussed. Thus, this proposal provides a platform to realise interferometry with an entangled BEC and to explore the quantum-classical interface.
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 describe the fabrication and construction of a setup for creating lattices of magnetic microtraps for ultracold atoms on an atom chip. The lattice is defined by lithographic patterning of a permanent magnetic film. Patterned magnetic-film atom chips enable a large variety of trapping geometries over a wide range of length scales. We demonstrate an atom chip with a lattice constant of 10 μm, suitable for experiments in quantum information science employing the interaction between atoms in highly excited Rydberg energy levels. The active trapping region contains lattice regions with square and hexagonal symmetry, with the two regions joined at an interface. A structure of macroscopic wires, cutout of a silver foil, was mounted under the atom chip in order to load ultracold (87)Rb atoms into the microtraps. We demonstrate loading of atoms into the square and hexagonal lattice sections simultaneously and show resolved imaging of individual lattice sites. Magnetic-film lattices on atom chips provide a versatile platform for experiments with ultracold atoms, in particular for quantum information science and quantum simulation.
Rydberg atoms are often proposed as the basis of quantum computing and quantum information protocols. One of the central reasons for this is that they provide a strong and long-ranged interaction that can be coherently switched on and off. This thesis details two techniques which use the exaggerated properties of Rydberg atoms to manipulate both the quantum state of the atom itself and that of the external light field.\n\nThe first proposal initially focuses on the creation of many-body quantum states from two-level atoms trapped in a two-dimensional lattice. This approach uses the van der Waals interaction present between alkali metal atoms in highly excited Rydberg s-states. The approximate solution of the corresponding Hamiltonian is detailed in the regime where the laser driving is the largest energy scale of the system. The states which are most likely achieved using an oscillating laser detuning are then determined. These states are then taken as the basis for the creation of deterministic single-photons, whose properties are shown to rely on the interplay between interatomic spacing and the geometry of the lattice.\n\nThe second technique described uses the coupling between a Rydberg atom and a moving electron to manipulate the atomic state. In this system, the atom is initially excited to a Rydberg s-state and trapped at a finite distance from an electron waveguide. Two analytical methods are used to show that the final state of the atom depends strongly on the direction and modulus of the electron momentum. A complementary numerical simulation shows that the atoms may be left in a polarised state, suggesting the possibility of using this setup to ‘switch on’ permanent electric dipoles in the atoms. This investigation leads naturally to a system where multiple interacting atoms are trapped close to the waveguide, allowing various many-body states to be accessed.
Atom chips are robust and extremely powerful toolboxes for quantum optical experiments, since they make it possible to create exceedingly precise magnetic traps for neutral atoms with minimal field modulations. Accurate manipulation of trapped atoms is feasible with magnetic and electric fields created on the atom chip. Therefore atom chips with high quality surfaces and extremely well defined wires were build (roughness < 20nm). Furthermore new generations of atom chips were developed, like the multi-layer atom chip with contact-free wire crossings, the sub-micron structured atom chip and the semiconducting atom chip. Extensive characterisation measurements of atom chip wires demonstrated that the wires stand current densities of up to 10^7A/cm^2 for seconds and voltage differences of more than 500V over 10µm. From the different materials tested doped silicon with a thin silicon dioxide layer is the best qualified substrate for atom chip fabrication. The atom chips fabricated during this thesis have been used in many successful experiments, yielding numerous results. Moreover this thesis established the basics for many further experiments in atom physics and quantum optics and delivers complete instructions for the fabrication of all developed atom chips as well as an introduction to the experiments.
In this thesis I examine high-magnetic-field Rydberg spectra, with the goal of discerning the viability of using such states as an interaction mechanism for quantum computation processes. In the presence of a strong magnetic field of B0 = 2.6 T, high-lying states of E ~ -55 cm(-1) are non-degenerate and show a wide range of magnetic dipole moments. Near-degenerate pairs of states of the same mJ manifold but of different axial parity Pi_z will strongly interact in the presence of an applied weak parallel electric field F, generating a pair of highly polar states. I show experimental results that, in agreement with theoretical work presented here, illustrate the presence of strong permanent dipole moments p ~ 1500 ea0 under these conditions. Such Rydberg states can also be tuned into resonance through careful control of the magnetic field, and are suitable for developing quantum phase gates and performing Rydberg-Rydberg interaction experiments. In order to perform such high-field Rydberg excitation experiments, a stable, dense and well-controlled cold atom cloud is needed within a high-magnetic-field environment. Toward this end, I present a full characterization of the utilized high-field trapping apparatus, including measurements of the trapped atom sample size, shape, and temperature. Analysis of the loss rate from the high-magnetic-field atom trap, both as a function of the applied magnetic field B0 and transverse confinement are presented. Experimental results for the high-field confinement of 85Rb are presented, as well as the first demonstration of a high-field 87Rb trap. Current trap limitations and possible improvements to the trapping apparatus are discussed. Other applications of the high-magnetic-field trap, such as the investigation of ultracold two-component plasmas and three-body recombination experiments, are also presented.