This thesis describes experimental demonstrations of high-fidelity readout of trapped ion quantum bits ("qubits") for quantum information processing. We present direct single-shot measurement of an "optical" qubit stored in a single calcium-40 ion by the process of resonance fluorescence with a fidelity of 99.991(1)% (surpassing the level necessary for fault-tolerant quantum computation). A time-resolved maximum likelihood method is used to discriminate efficiently between the two qubit states based on photon-counting information, even in the presence of qubit decay from one state to the other. It also screens out errors due to cosmic ray events in the detector, a phenomenon investigated in this work. An adaptive method allows the 99.99% level to be reached in 145us average detection time. The readout fidelity is asymmetric: 99.9998% is possible for the "bright" qubit state, while retaining 99.98% for the "dark" state. This asymmetry could be exploited in quantum error correction (by encoding the "no-error" syndrome of the ancilla qubits in the "bright" state), as could the likelihood values computed (which quantify confidence in the measurement outcome). We then extend the work to parallel readout of a four-ion string using a CCD camera and achieve the same 99.99% net fidelity, limited by qubit decay in the 400us exposure time. The behaviour of the camera is characterised by fitting experimental data with a model. The additional readout error due to cross-talk between ion images on the CCD is measured in an experiment designed to remove the effect of qubit decay; a spatial maximum likelihood technique is used to reduce this error to only 0.2(1)x10^{-4} per qubit, despite the presence of ~4% optical cross-talk between neighbouring qubits. Studies of the cross-talk indicate that the readout method would scale with negligible loss of fidelity to parallel readout of ~10,000 qubits with a readout time of ~3us per qubit. Monte-Carlo simulations of the readout process are presented for comparison with experimental data; these are also used to explore the parameter space associated with fluorescence detection and to optimise experimental and analysis parameters. Applications of the analysis methods to readout of other atomic and solid-state qubits are discussed.

A Paul trap mass spectrometer has a three-electrode geometry mass analyzer consisting of two identical end cap electrodes and a ring electrode. Traditionally, the two end cap electrodes are electrically grounded and an RF potential is applied to the central ring electrode to generate the "trapping field". Ions of the analyte sample are formed in situ by electron bombardment and mass analysis of the fragment ions is performed by mass selectively destabilizing the ions from the trap. The inhornogeneities present in the trapping field (introduced either by misalignment of the trap geometry or by applying a dipolar auxiliary excitation across the end cap electrodes) give rise to various interesting phenomena including, resonance ejection of the trapped ions This thesis is concerned with taking a look into the experimental aspects associated with resonance ejection of ions caused by the dipolar excitation Additionally, u also reports the work undertaken to develop necessary instrumentation for resonant excitation experiments and my contribution to operational>zc the Paul trap mass spectrometer fabricated in the laboratory. The thesis is divided into 5 chapters. Chapter 1 is an introductory chapter. After discussing the conditions for stability of the trapped ions, it goes on to present a brief survey of a variety of applications in literature, which have used resonant excitation. Towards the end, the motivation of the present effort and the scope of work in the thesis have been spelt out. This includes (a) redesign of the ion detector electronics, (b) design of an auxiliary excitation generator, and (c) studies on resonance ejection. Chapter 2 outlines the design considerations, circuit description and fabrication details for the ion detector electronics. The circuits presented in this chapter include (a) electrometer amplifier and (b) -3 kV DC supply for the electron multiplier detector. The electrometer amplifier amplifies the ion current signal from the electron multiplier detector and it needs to have a high input impedance and a high slew rate. The electron multiplier detector requires -3 kV DC power supply for operation. The -3 kV DC power supply is required to have a regulated output voltage with low ripple in the output. Chapter 3 presents the design considerations, circuit description and fabrication details for the auxiliary excitation generator. The auxiliary excitation generator is a three channel DDS (Direct Digital Synthesis) oscillator with independent control of frequency amplitude, and phase of the output signal. Chapter 3 also discusses the micro controller based control sub-system that allows the user to set above mentioned output parameters. The control sub-system provides a user-friendly keyboard interface and 2-line alphanumeric LCD display per channel. It also provides various bus interfaces (such as I2C and SPI) to interface with DDS oscillator ICs, amplitude control DAC, and LCD displays. The chapter then goes on to describe the implementation details of the software written for the control sub-system. The hardware design is simplified by using a micro controller as heart of the control sub-system and employing the software to handle the complex functions. As an example, the design of the keyboard interface is simplified by directly connecting a matrix keyboard to the input/output port of the micro controller. The software is used to scan the keyboard, detect key press and find out the key pressed. Nonetheless, in order to meet specific performance required for the present work, the software needs to have a sense of time, be portable and scalable. Details of the "layered" architecture adopted by as to meet these specific requirements, the lower level "driver" functions implemented for various interfaces of the control sub-system, and the higher level or the "application" software, are described. The application software uses the driver functions to accomplish various tasks required to be executed by the control sub-system. Finally, the chapter presents the design consideration and fabrication details of the coupling transformer used to couple the output of the auxiliary excitation generator to the Paul trap Chapter 4 describes the resonant excitation experiments performed as part of the present work. First of all the chapter presents the improvement in the performance of the Paul trap mass spectrometer as a result of redesigned ion detector electronics It is seen that the resolution is improved significantly due to the improved response time of the electrometer amplifier. The chapter then describes the effect of the resonant excitation on the ions and also that the frequency of the applied auxiliary excitation should be between 500 kHz to 125 kHz. Next, a number of mass spectra for different frequencies of the applied auxiliary excitation are presented. These mass spectra indicate that the resonant ejection sets in for lower masses even at lower amplitude of the auxiliary excitation where as higher amplitude is required for the resonant ejection of the higher masses. It is seen that the resonant excitation of ions improves resolution of the mass spectrum. Moreover, the auxiliary excitation results in ejection of the ions at lower amplitude of the RF voltage and thus allows extending the mass range of the mass spectrometer. We present the mass spectrum of CCI4 which is not possible to normally record in our instrument. We also present results intended to understand the relation between frequency and amplitude of the auxiliary excitation on the mass spectra of benzene. Finally, results of an interesting experiment are presented which indicates the presence of the non-linear resonance points in the Paul trap. Chapter 5 presents the concluding remarks. References cited in the thesis are attached in their alphabetical order at the end of the thesis.

This thesis reports on the setup of a new ion trap apparatus designed for experiments with single ⁴⁰Ca⁺ ions to perform molecular spectroscopy. The calcium ion is laser cooled, allowing for sympathetic cooling of the nonfluorescing molecular ion. The aim of these experiments is to explore loading and identifying molecular ions in RF-Paul traps, as well as developing new spectroscopic tools to measure transitions of molecular ions via the fluorescence of co-trapped ⁴⁰Ca⁺ ions. Ground state cooling of a mixed ion pair is implemented as a first step towards increasing the sensitivity of our technique to the level necessary to measure transitions with low scattering rates (like those present in molecular ions). Doppler cooling on the S(1/2)->P(1/2) transition of the calcium ion results in the formation of a Coulomb crystal, the behavior of which may be used to infer properties of the molecular ion. Following cooling, sideband spectroscopy on the narrow S(1/2)->D(5/2) quadrupole transition of calcium may be used to identify the mass of single molecular ions. This method is verified via a non-destructive measurement on ⁴⁰CaH⁺ and ⁴⁰Ca¹⁶O⁺. The normal modes of the Coulomb crystal can also be used to extract information from the target ion to the control ion. By driving the blue side of a transition, laser induced heating can be put into the two ion system, which leads to changes in fluorescence of the ⁴⁰Ca⁺ ion, first demonstrated with two Ca⁺ isotopes. Increasing the sensitivity of this technique requires ground state cooling of both the ⁴⁰Ca⁺ ion and the ion of interest, enabling the transfer of the ion's motional state into the ground state with high probability. This thesis demonstrates ground state cooling of the atomic ion and sympathetic cooling of a second ion (⁴⁴Ca⁺). Once in the ground state, heating of the Coulomb crystal by scattering photons off of the spectroscopy ion can be measured by monitoring the resolved motional sidebands of the S(1/2)->D(5/2) transition of ⁴⁰Ca⁺, allowing for spectral lines to be inferred. Future experiments will investigate this technique with molecular ions.

Orientador: Jose Antonio Roversi / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-11T05:38:26Z (GMT). No. of bitstreams: 1 Nohama_FabianoKenji_D.pdf: 1986556 bytes, checksum: dce935416c59935cf9e7d868f74c5653 (MD5) Previous issue date: 2008 / Resumo: Nesta tese estudamos um sistema de duas cavidades acopladas e sua interação com átomos de dois níveis bem como com íons aprisionados. Para o acoplamento entre as cavidades consideramos dois mecanismos distintos: (1) acoplamento pela sobreposição dos campos e; (2) acoplamento via fibra óptica. Considerando a interação dos campos acoplados com átomos de dois níveis nós observamos o emaranhamento em um sistema tripartite (quando as cavidades estão interagindo com apenas um átomo). Também foi possível obter a transferência do estado quântico entre dois átomos localizado em cavidades diferentes. Além disso elaboramos uma proposta relativamente simples para a geração de estados maximamente emaranhados (estados de Bell) entre dois átomos utilizando as duas cavidades acopladas. Por último, estudamos dois íons aprisionados, cada um deles localizado no interior de cavidades diferentes. As duas cavidades sendo conectadas por uma fibra óptica. Neste caso foi possível observar a transferência de um conjunto de estados de dois-qubits a partir dos graus de liberdade (de movimento e dos estados internos) de um dos íons para o outro, localizado em uma cavidade diferente. Nas propostas envolvendo a transferência de estados quânticos e a geração de estados de Bell foram incluídos os efeitos de dissipação devido à presença de um reservatório de temperatura T = 0K. Com isso pudemos concluir que as propostas são confiáveis para as reais taxas de dissipação observadas em experimentos / Abstract: In this thesis we studied a system of two coupled cavities and its interaction with two-level atoms and trapped ions. For the coupling between the cavities we considered two situations: (1) coupling due the overlap between the fields and; (2) coupling by optical fiber. Considering the interaction of the coupled fields with two-level atoms we observed the entanglement in a tripartite system (when the cavities are interacting with only one atom). When both cavities are interacting with an atom it was possible to obtain the quantum state transfer between two atoms located in different cavities. Besides, we conceived a relatively simple proposal to the generation of maximally entangled states (EPR states) between two atoms using the two coupled cavities. At last we studied two trapped ions, each one located inside different cavities. The two cavities are connected by an optical fiber, where it was possible to observe the transfer of a two-qubits set from the movement and internal states degrees of freedom of one of the ions to the other one, located in a different cavity. In the proposals involving the quantum state transfer and the EPR state genera-tion we included the dissipation effects due the presence of a reservoir at temperature T = 0. With this we concluded that the proposals are reliable considering dissipation rates observed in experiments / Doutorado / Física Geral / Doutor em Ciências

This thesis presents the investigations of Rubidium atoms in magneto-optical traps and triply charged Thorium ions in electrodynamic traps for future advances in long-distance quantum telecommunication, next generation clocks, and fundamental tests of current physical theories. Experimental realizations of two core building blocks of a quantum repeater are described: a multiplexed quantum memory and a telecom interface for long-lived quantum memories. A color change of single-photon level light fields by several hundred nanometers in an optically thick cold gas is demonstrated, while preserving quantum entanglement with a remotely stored matter excitation. These are essential elements for long-distance quantum telecommunication, fundamental tests of quantum mechanics, and applications in secure communication and computation. The first trapping and laser cooling of Thorium-229 ions are described. Thorium-229 nuclear electric quadrupole moment is revealed by hyperfine spectroscopy of triply charged Thorium-229 ions. A system to search for the isomer nuclear transition in Thorium-229 is developed and tested with the excitation of a forbidden electronic transition at 717 nm. Direct excitation of the nuclear transition with laser light would allow for an extremely accurate clock and a sensitive test bed for variations of fundamental physical constants, including the fine structure constant.