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Many-body interactions in a dissipative frozen strontium Rydberg gas

D. P. Sadler

Published Jan 1, 2016• 2 citations
OpenAlex

Abstract

This thesis describes an investigation of Rydberg excitation within a high-density sample of cold strontium atoms. This sample is prepared using a two-stage magneto-optical trap, cooling first on a broad singlet transition and then a narrow triplet transition. Rydberg atoms are then created using a two-photon, three-level ladder type scheme and detected using a novel autoionisation technique. It is shown that, in the regime of high optical thickness on the probing transition, a significant Rydberg population can be created by photons that are multiply scattered before leaving the cloud. The multiply scattered field is density-dependent and has strikingly different spectral properties from the incident laser light. This spectrum is convolved with the spectrum of the Rydberg atoms created via direct laser excitation and the two spectra can be isolated in post analysis. This technique provides a probe of the spectral distribution of the re-scattered light within the cloud, which may be qualitatively different from that of the transmitted light, and previously has not been measured directly. Additionally, this Rydberg population arising from the multiply scattered field can be seen in the spatial distribution of the Rydberg excitations within the atom sample. Finally, a careful analysis of the time dynamics of the Rydberg system reveals that multiple scattering co-exists with signatures of the Rydberg blockade in this strongly dissipative regime.

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This thesis details the first measurements of Rydberg dipole blockade in a cold ensemble of divalent atoms. Strontium atoms are cooled and trapped in a magneto-optical trap and coherently excited to Rydberg states in a two-photon, three-level ladder scheme. Owing to the divalent nature of strontium, one electron can be excited to the Rydberg state, whilst the other lower-lying electron is available to undergo resonant optical excitation to autoionising states, which ionise in sub-nanosecond timescales. The remaining ions that are recorded on a micro-channel plate are proportional to the number of Rydberg atoms. The development of a narrow linewidth laser system necessary for an additional stage of cooling is explained and characterised. Two frequency stabilisation schemes are discussed: one to address the short-term laser frequency instabilities based on the Pound-Drever-Hall technique; the other to address the long-term laser frequency instabilities based on Lamb-dip spectroscopy in an atomic beam. The cooling dynamics on the narrow cooling transition is studied experimentally and modelled via theoretical simulations.

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This thesis describes the development of a new technique for measuring the spatial distribution of Rydberg atoms in a cold strontium gas. Strontium atoms are cooled and trapped in a magneto-optical trap and coherently excited to Rydberg states in a two-photon, three-level ladder scheme. Several methods of stabilizing the frequency to the cooling transition are discussed and characterized. A frequency stabilization scheme based on electromagnetically-induced transparency for the second laser required for Rydberg excitation is also explained. The Rydberg population dynamics are studied experimentally and modeled using an optical Bloch equation simulation. The divalent nature of strontium allows doubly excited “autoionizing” states to be accessed using resonant optical excitation. These states ionize in subnanosecond timescales, with the ions recorded on a micro-channel plate being proportional to the amount of Rydberg atoms. Translation of an autoionizing laser focused to a waist of 10 μm gives a spatially resolved Rydberg signal. A two-dimensional map of the Rydberg spatial distribution has been made using this autoionizing microscopy technique.

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This thesis describes Rydberg spectroscopy and dressing experiments in an ultracold strontium gas. The strontium atoms are cooled to sub-uK temperatures in a narrowline magneto-optical trap, where Rydberg atoms are created using a two-photon excitation scheme. This required the development of a high-power ultraviolet laser system at 319 nm. The laser has a large tuning range for access to triplet Rydberg states from principal quantum numbers of 35 to > 300. By performing Rydberg spectroscopy in a magneto-optical trap, we show that narrow spectra can be obtained where the line centre is determined to ~ 10 kHz. A frequency comb is then employed to make absolute frequency measurements of optical transitions to accuracies of < 2 MHz. Techniques are outlined for improving the accuracy further, showing that overall uncertainties on the order of 10 kHz can be obtained. The reliable measurement of Rydberg levels is important for studying the variation in quantum defect across Rydberg series, and hence improving the accuracy of atomic models. In this work we also develop a novel system in which to realise Rydberg dressing. By resonantly coupling the excited state of the cooling transition to a high lying Rydberg state, we create a Rydberg-dressed magneto-optical trap. We demonstrate that the atoms acquire Rydberg properties, while undergoing continuous cooling at ~ 1 uK. The lifetime of the trap is proved to be sufficiently long to observe interactions, however the interaction strength is currently limited by the non-uniform spatial profile of the dressing beam. Straightforward methods to overcome this limitation are presented. As such this work should lead to future experiments, whereby tuneable long-range interactions can be observed in the dynamics of the dressed magneto-optical trap.

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Rydberg tomography of an ultracold atomic cloud

María Martínez Valado, Nicola Malossi, Stefano Scotto +3 more

One of the most striking features of the strong interactions between Rydberg atoms is the Rydberg blockade effect, which allows only a single excitation to the Rydberg state within the volume of the blockade sphere. Here we present a method that spatially visualizes this phenomenon in an inhomogeneous gas of ultracold rubidium atoms. In our experiment we scan the position of one of the excitation lasers across the cold cloud and determine the number of Rydberg excitations detected as a function of position. Comparing this distribution to the one obtained for the number of ions created by a two-photon ionization process via the intermediate $5P$ level, we demonstrate that the blockade effect modifies the width of the Rydberg excitation profile. Furthermore, we study the dynamics of the Rydberg excitation and find that the time scale for the excitation depends on the atomic density at the beam position.

arXiv:1308.0205•Physical Review A•Oct 21, 2013•14 citations
This thesis describes an investigation of Rydberg excitation within a high-density sample of cold strontium atoms. This sample is prepared using a two-stage magneto-optical trap, cooling first on a broad singlet transition and then a narrow triplet transition. Rydberg atoms are then created using a two-photon, three-level ladder type scheme and detected using a novel autoionisation technique. It is shown that, in the regime of high optical thickness on the probing transition, a significant Rydberg population can be created by photons that are multiply scattered before leaving the cloud. The multiply scattered field is density-dependent and has strikingly different spectral properties from the incident laser light. This spectrum is convolved with the spectrum of the Rydberg atoms created via direct laser excitation and the two spectra can be isolated in post analysis. This technique provides a probe of the spectral distribution of the re-scattered light within the cloud, which may be qualitatively different from that of the transmitted light, and previously has not been measured directly. Additionally, this Rydberg population arising from the multiply scattered field can be seen in the spatial distribution of the Rydberg excitations within the atom sample. Finally, a careful analysis of the time dynamics of the Rydberg system reveals that multiple scattering co-exists with signatures of the Rydberg blockade in this strongly dissipative regime.
This thesis details the first measurements of Rydberg dipole blockade in a cold ensemble of divalent atoms. Strontium atoms are cooled and trapped in a magneto-optical trap and coherently excited to Rydberg states in a two-photon, three-level ladder scheme. Owing to the divalent nature of strontium, one electron can be excited to the Rydberg state, whilst the other lower-lying electron is available to undergo resonant optical excitation to autoionising states, which ionise in sub-nanosecond timescales. The remaining ions that are recorded on a micro-channel plate are proportional to the number of Rydberg atoms. The development of a narrow linewidth laser system necessary for an additional stage of cooling is explained and characterised. Two frequency stabilisation schemes are discussed: one to address the short-term laser frequency instabilities based on the Pound-Drever-Hall technique; the other to address the long-term laser frequency instabilities based on Lamb-dip spectroscopy in an atomic beam. The cooling dynamics on the narrow cooling transition is studied experimentally and modelled via theoretical simulations.
This thesis describes the development of a new technique for measuring the spatial distribution of Rydberg atoms in a cold strontium gas. Strontium atoms are cooled and trapped in a magneto-optical trap and coherently excited to Rydberg states in a two-photon, three-level ladder scheme. Several methods of stabilizing the frequency to the cooling transition are discussed and characterized. A frequency stabilization scheme based on electromagnetically-induced transparency for the second laser required for Rydberg excitation is also explained. The Rydberg population dynamics are studied experimentally and modeled using an optical Bloch equation simulation. The divalent nature of strontium allows doubly excited “autoionizing” states to be accessed using resonant optical excitation. These states ionize in subnanosecond timescales, with the ions recorded on a micro-channel plate being proportional to the amount of Rydberg atoms. Translation of an autoionizing laser focused to a waist of 10 μm gives a spatially resolved Rydberg signal. A two-dimensional map of the Rydberg spatial distribution has been made using this autoionizing microscopy technique.
This thesis describes Rydberg spectroscopy and dressing experiments in an ultracold strontium gas. The strontium atoms are cooled to sub-uK temperatures in a narrowline magneto-optical trap, where Rydberg atoms are created using a two-photon excitation scheme. This required the development of a high-power ultraviolet laser system at 319 nm. The laser has a large tuning range for access to triplet Rydberg states from principal quantum numbers of 35 to > 300. By performing Rydberg spectroscopy in a magneto-optical trap, we show that narrow spectra can be obtained where the line centre is determined to ~ 10 kHz. A frequency comb is then employed to make absolute frequency measurements of optical transitions to accuracies of < 2 MHz. Techniques are outlined for improving the accuracy further, showing that overall uncertainties on the order of 10 kHz can be obtained. The reliable measurement of Rydberg levels is important for studying the variation in quantum defect across Rydberg series, and hence improving the accuracy of atomic models. In this work we also develop a novel system in which to realise Rydberg dressing. By resonantly coupling the excited state of the cooling transition to a high lying Rydberg state, we create a Rydberg-dressed magneto-optical trap. We demonstrate that the atoms acquire Rydberg properties, while undergoing continuous cooling at ~ 1 uK. The lifetime of the trap is proved to be sufficiently long to observe interactions, however the interaction strength is currently limited by the non-uniform spatial profile of the dressing beam. Straightforward methods to overcome this limitation are presented. As such this work should lead to future experiments, whereby tuneable long-range interactions can be observed in the dynamics of the dressed magneto-optical trap.
This thesis investigates the effects of strong Rydberg-Rydberg interactions in the presence of three-level coherent phenomena such as electromagnetically induced transparency (EIT), Autler-Townes splitting (ATS) and coherent population trapping (CPT). As a result of their remarkable properties, highly excited Rydberg atoms have great potential for applications in diverse areas. The interaction-induced dipole blockade between the Rydberg atoms has been proposed as a fundamental tool in quantum information processing with neutral atoms. Yet, they require an increasing level of understanding and control. A many-body theory is developed for the Rydberg excitation dynamics in various atomic systems with different densities and velocity distributions such as for atoms in a vapour cell, ultracold atoms in magneto-optical and optical dipole traps, or a system of optical lattices or dipole trap arrays. The systems were investigated by solving the optical Bloch equations numerically for a two-photon ladder excitation scheme, taking into consideration various experimentally relevant conditions for the emergence of the effects of Rydberg-Rydberg interactions. The lineshape properties of the EIT/ATS/CPT spectra are observed and reviewed while the physical arguments for how the lineshape properties change as a function of various parameters are provided. To account for the differing atomic distributions in the atomic systems, a many-body model based on Monte-Carlo simulation is developed to describe the optical response of the multiatoms systems to two-photon excitation laser lights in the regimes of EIT, ATS and CPT and strong atomic interactions. The study of the lineshape properties allows one to understand possible ways to detect the emergence of interaction-induced excitation suppression, asymmetry, line shift and excitation "antiblockade" enhancement effects. Besides these, the main result of this study is the demonstration that the linewidth properties of the Rydberg EIT/ATS/CPT spectra can reveal collective many-body behaviour associated with the creation of many-body entangled states. The linewidth properties provide compelling evidence of the creation of collective states by confirming the predicted dependence of the generalised Rabi frequency on the square root of the number of atoms in the mesoscopic ensemble. This study could help to quantify and control the conditions necessary for creating many-body entanglement within atomic ensembles. The thesis also describes the step towards experimentally investigating the response of an atomic system to excitation lasers and studying the excitation dynamics of interacting atoms in such systems. Significant work was done to set up the two-photon excitation system for preparing atoms in high-lying Rydberg states and performing Rydberg-based vapour cell experiments. Qualitative analysis of the Rydberg states transparency features allows to obtain laser parameters such as the laser linewidths and Rabi frequencies that are used in the theoretical model to create the similarity between the experimental and theoretical parameters. Further works were carried out towards realising the building blocks for experimentally investigating the interactions between Rydberg atoms in ultracold atom systems. This is by setting-up a trapping system capable of producing a highly dense ultracold atom ensemble within a magneto-optical trap, cold enough to serve as a reservoir from which the optical dipole trap can be loaded efficiently. The optical dipole trap is designed and set up to achieve a micron-size trapping potential capable of producing a sub-micron sized atomic cloud, with a controllable number of atoms, where a full blockade effect can be achieved and exploited to implement a qubit system for quantum gate operations.
Dipole-dipole interactions, the strongest, longest-range interactions possible between two neutral atoms, cannot be better manifested anywhere else than in a Rydberg atomic system. Rydberg atoms, having high principal quantum numbers n>>1 and dipole moments that scale as n^2, provide a powerful tool to examine dipole-dipole interactions. Therefore, we have studied the production and production rates of strontium Rydberg atoms created using two-photon excitation and have explored their properties in two distinct experiments. In the first experiment, very-high-n (n~300) Rydberg atoms are produced in a tightly collimated atomic beam allowing spectroscopic studies of their energy levels and their Stark effects. Simulations using a two-active-electron model, developed by our theoretical collaborators, allow detailed analysis of the results and are in remarkable agreement with the experimental results. The high density of Rydberg atoms achieved ~ 5*10^5 cm^(-3), in this experiment will allow studies of strongly interacting Rydberg-Rydberg systems. The second experiment, in which a cold strontium Rydberg gas is excited in a magneto-optic trap, features an imaging technique offering both spatial and temporal resolution. We use this technique to observe and study the evolution of an ultra-cold strontium Rydberg gas which reveals the importance of Rydberg-Rydberg interactions in the early stages of this evolution. Strongly interacting Rydberg gas provides an opportunity to realize a very strongly-correlated ultra-cold plasma.
One of the most striking features of the strong interactions between Rydberg atoms is the Rydberg blockade effect, which allows only a single excitation to the Rydberg state within the volume of the blockade sphere. Here we present a method that spatially visualizes this phenomenon in an inhomogeneous gas of ultracold rubidium atoms. In our experiment we scan the position of one of the excitation lasers across the cold cloud and determine the number of Rydberg excitations detected as a function of position. Comparing this distribution to the one obtained for the number of ions created by a two-photon ionization process via the intermediate 5P5P5P level, we demonstrate that the blockade effect modifies the width of the Rydberg excitation profile. Furthermore, we study the dynamics of the Rydberg excitation and find that the time scale for the excitation depends on the atomic density at the beam position.