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Study of coherent properties of Rydberg-atom systems.

Tara Cubel Liebisch

Published Jan 1, 2007• 0 citations
OpenAlex

Abstract

Atoms excited into states of high principal quantum number, Rydberg atoms, have exaggerated properties such as large size and dipole moments, long lifetimes, and high polarizabilities. These properties can be used to realize a Rydberg excitation blockade in narrow-band photo-excitation (∼ 1 MHz bandwidth) of laser-cooled and trapped atoms. Under ideal conditions, the blockade phenomenon leads to an inhibition of more than one Rydberg excitation in dense samples of atoms. The blockade is useful in a number of applications including quantum information processing, quantum cryptography, and atomic clocks. In this thesis, I discuss experimental investigations of the dependence of the blockade effectiveness on the principal quantum number of the excited Rydberg level, and the Rydberg excitation laser's detuning and intensity. Rydberg-atom counting statistics is used as a technique for measuring the effectiveness of the Rydberg blockade. An important aspect in experiments on the blockade effect is the efficiency of the photo-excitation process used for Rydberg-atom generation. Therefore, the described work deals, in part, with methods of coherent excitation of atoms into and out of Rydberg states. I show that STIRAP (stimulated Raman adiabatic passage) allows us to transfer atoms into Rydberg levels with about 70% excitation efficiency, under the constraints of the present experiment. It is also seen that the excitation efficiency is insensitive to variations in laser pulse intensity and width, as expected for STIRAP. The experimental techniques used to measure the blockade effect using counting statistics and to achieve coherent excitation into Rydberg states, are important for future studies of small atomic systems blockaded to one Rydberg excitation.

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In this dissertation, I explore several aspects of Rydberg physics with laser spectroscopy. The experiments center around the use of a very deep, cavity-generated, one-dimensional optical lattice in a cold atom sample. The optical lattice produces elongated clouds of dense, cold atoms via dipole trapping and is able to generate extreme light shifts in the atoms due to its high intensity. I describe experiments which may be divided into two main classes. In the first, I use the dipole trap to prepare a dense sample of atoms for photoassociation of Rb atom pairs into diatomic Rydberg molecules. The Rydberg molecules are formed by low-energy scattering of the Rydberg electron of one atom from the other atom, which is a ground-state atom. This novel binding mechanism also has relevance for chemical physics. I present binding energy measurements for eight different cases of Rydberg molecules. The binding energies reveal information about electron-atom collisions, such as their scattering lengths, and about the spin couplings present within the molecular system. In the second class of experiments, I introduce two proposed measurements in which the optical lattice is ramped to very high intensity during the laser excitation, creating AC Stark shifts in the transition frequencies. In one measurement, low-lying atomic states are probed. Measurements of the light shifts are expected to yield data on the dynamic polarizability and the photoionization cross section of the 5$D_{3/2}$-state in rubidium in a 1064-nm light field. I show corresponding preliminary experimental data. The polarizability and photo-ionization cross-section measurements will provide tests of atomic structure theory, which is important both at a fundamental level and for applications such as atomic clocks. In another proposed measurement, Rydberg atoms in the lattice experience ponderomotive shifts and strong mixing of their angular-momentum states, allowing direct optical excitation of the lattice-mixed high-angular-momentum states. I describe the theoretical basis and experimental preparation for spectroscopically characterizing the lattice-mixed states. The ability to optically excite selected high-angular-momentum states without the use of static or RF fields may be useful for quantum information processing and simulation.

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Atoms excited into states of high principal quantum number, Rydberg atoms, have exaggerated properties such as large size and dipole moments, long lifetimes, and high polarizabilities. These properties can be used to realize a Rydberg excitation blockade in narrow-band photo-excitation (∼ 1 MHz bandwidth) of laser-cooled and trapped atoms. Under ideal conditions, the blockade phenomenon leads to an inhibition of more than one Rydberg excitation in dense samples of atoms. The blockade is useful in a number of applications including quantum information processing, quantum cryptography, and atomic clocks. In this thesis, I discuss experimental investigations of the dependence of the blockade effectiveness on the principal quantum number of the excited Rydberg level, and the Rydberg excitation laser's detuning and intensity. Rydberg-atom counting statistics is used as a technique for measuring the effectiveness of the Rydberg blockade. An important aspect in experiments on the blockade effect is the efficiency of the photo-excitation process used for Rydberg-atom generation. Therefore, the described work deals, in part, with methods of coherent excitation of atoms into and out of Rydberg states. I show that STIRAP (stimulated Raman adiabatic passage) allows us to transfer atoms into Rydberg levels with about 70% excitation efficiency, under the constraints of the present experiment. It is also seen that the excitation efficiency is insensitive to variations in laser pulse intensity and width, as expected for STIRAP. The experimental techniques used to measure the blockade effect using counting statistics and to achieve coherent excitation into Rydberg states, are important for future studies of small atomic systems blockaded to one Rydberg excitation.
In this dissertation, I explore several aspects of Rydberg physics with laser spectroscopy. The experiments center around the use of a very deep, cavity-generated, one-dimensional optical lattice in a cold atom sample. The optical lattice produces elongated clouds of dense, cold atoms via dipole trapping and is able to generate extreme light shifts in the atoms due to its high intensity. I describe experiments which may be divided into two main classes. In the first, I use the dipole trap to prepare a dense sample of atoms for photoassociation of Rb atom pairs into diatomic Rydberg molecules. The Rydberg molecules are formed by low-energy scattering of the Rydberg electron of one atom from the other atom, which is a ground-state atom. This novel binding mechanism also has relevance for chemical physics. I present binding energy measurements for eight different cases of Rydberg molecules. The binding energies reveal information about electron-atom collisions, such as their scattering lengths, and about the spin couplings present within the molecular system. In the second class of experiments, I introduce two proposed measurements in which the optical lattice is ramped to very high intensity during the laser excitation, creating AC Stark shifts in the transition frequencies. In one measurement, low-lying atomic states are probed. Measurements of the light shifts are expected to yield data on the dynamic polarizability and the photoionization cross section of the 5D3/2D_{3/2}D3/2​-state in rubidium in a 1064-nm light field. I show corresponding preliminary experimental data. The polarizability and photo-ionization cross-section measurements will provide tests of atomic structure theory, which is important both at a fundamental level and for applications such as atomic clocks. In another proposed measurement, Rydberg atoms in the lattice experience ponderomotive shifts and strong mixing of their angular-momentum states, allowing direct optical excitation of the lattice-mixed high-angular-momentum states. I describe the theoretical basis and experimental preparation for spectroscopically characterizing the lattice-mixed states. The ability to optically excite selected high-angular-momentum states without the use of static or RF fields may be useful for quantum information processing and simulation.
In this dissertation we report the results of two experimental projects with laser-cooled rubidium atoms: I. Application of Bessel beams for atom optics, and II. Spectroscopic measurements of Rydberg blockade effect. The first part of the thesis is devoted to the development of new elements of atom optics based on blue-detuned high-order Bessel beams. Properties of a 4th order Bessel beam as an atomic guide were investigated for various parameters of the hollow beam, such as the detuning from an atomic resonance, size and the order of the Bessel beam. We extended its application to create more complicated interferometer-type structures by demonstrating a tunnel lock, a novel device that can split an atomic cloud, transport it, delay, and switch its propagation direction between two guides. We reported a first-time demonstration of an atomic beam switch based on the combination of two crossed Bessel beams. We achieved the 30% efficiency of the switch limited by the geometrical overlap between the cloud and the intersection volume of the two tunnels, and investigate the heating processes induced by the switch. We also showed other applications of crossed Bessel beams, such as a 3-D optical trap for atoms confined in the intersection volume of two hollow beams and a splitter of the atomic density. The second part of this dissertation is devoted to the spectroscopic measurements of the Rydberg blockade effect, a conditional suppression of Rydberg excitations depending on the state of a control atom. We assembled a narrow-linewidth, tunable, frequency stabilized laser system at 480 nm to excite laser-cooled rubidium atoms to Rydberg states with a high principal quantum number n ~ 50 through a two-photon transition. We applied the laser system to observe the Autler-Townes splitting of the intermediate 5p state and used the broadening of the resonance features to investigate the enhancement of Rydberg-Rydberg interactions in the presence of an external electric field.
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.
When cold atoms are laser-excited to Rydberg states, the strong interactions between Rydberg excitations can lead to complex many-body entanglement in the system. The excitation process results in spatial correlations between excitations in the system. In most cases, Rydberg excitation positions are anticorrelated such that no two excitations are within a "blockade radius" of each other. Such systems are well-suited for studying basic many-body physics, as well as for future technological applications such as quantum computation. This thesis describes an experimental apparatus we constructed to perform spatially-sensitive detection of Rydberg atoms. We use the apparatus to perform measurements of the Rydberg-Rydberg pair-correlation function. This results in the first direct spatial images of the Rydberg blockade effect. We measure the blockade radius for a variety of S and D Rydberg states in rubidium. We investigate the dependence of the blockade radius on laser detunings and energy level shifts induced by optical potentials due to trapping lasers. Our results have some implications for atom traps used for neutral atom quantum computation. We also present simulations results used to characterize the performance of the imaging system.
This thesis focusses on the development of the experimental system used to study the eff ect of Rydberg blockade interactions in clouds of ultracold atoms and quantum degenerate gases. Initial measurements have been performed quantifying these interactions, like the Rydberg blockade radius and its variation\n\n\n\nwith the density of sample, which can be very useful in devising eff cient systems for quantum information processing in quantum gates and/or as probes for detecting impurities in a sample as has been recently proposed [1]. The e ffects of the Rydberg atom interactions are probed via electromagnetically-induced\n\n\n\ntransparency (EIT). The experimental system and apparatus required for the production and detection of ultracold atoms and quantum degenerate gases is developed. Furthermore, the laser systems used for the production of Rydberg atoms via the two-photon transitions are also described. To highlight the density dependent eff ects of the Rydberg atom interactions and the\n\n\n\nunique advantages o ered to us by the Rydberg blockade eff ect, we compare the EIT experiments performed on a Λ-system using Raman lasers and the experiments performed with a ladder system involving the ground state, the fi rst excited state and a Rydberg state of Rb-87. The manifestation of the blockade eff ect with atom number density, with temperature of the atoms and with the principal quantum number of the Rydberg state to which the\n\n\n\natoms are excited, is discussed. The usefulness of the Rydberg blockade interaction as a phenomenon to be exploited in quantum information processing and an outline of the future directions of our experiments concludes the thesis.
The advances in laser cooling and trapping techniques allow samples of ultra-cold atoms to be created and controlled for the study of quantum physics. In this thesis, we focus on using ensembles of cold atoms to investigate novel quantum effects and their applications in quantum optics. Realization of a scalable intercontinental quantum system, e.g. a global quantum internet, relies on quantum memories with long lifetimes on the order of seconds. We have reduced the differential light shifts of the atomic ground states to a sub-Hz level and achieved a lifetime of 16 seconds. Achieving fast and efficient quantum operations, on the other hand, requires strongly-interacting systems. We explore highly excited Rydberg atoms towards this goal and demonstrate Rydberg excitation blockade, many-body Rabi oscillations, trapping of Rydberg atoms, deterministic single photon source, and atom-photon entanglement. Finally, we integrate the two essential capabilities for quantum information processing, fast quantum state generation and long-term storage, by simultaneously exploiting Rydberg levels for interactions and ground atomic levels for the storage of quantum state. Our work advances the control of coherence properties and interactions in cold atomic ensembles to a new level and opens new opportunities for studies of complex quantum systems.