Trapped single ions provide an ideal platform for studying quantum optics, light-matter interactions, and quantum information processing. Many of these applications rely on the availability of well-defined and reproducible quantum emitters as well as precise control over the internal states of ions. To study both of these concepts, this thesis focuses on the isotope selectivity of neutral Barium (Ba) transitions as well as the state preparation of single trapped 138Ba+ ions. The first part of this thesis introduces the theoretical framework for experiments, focusing on single Ba+ ions and light-matter interactions. Therefore, the working principle of Paul traps is presented, the Ba ion is introduced and fundamental light-matter interaction tightly related to the experiments and simulations presented in this work are established. In the second part, the isotope selectivity of the neutral Ba 6s2 1S0 to 6s6p 1P1 transition at 553 nm is investigated and compared with the isotope selectivity of the 6s2 1S0 to 5d6p 3D1 transition at 413 nm. A secondary photon can be used in both cases to achieve isotope selective ionization of Ba+ ions. The isotope selectivity of the 553 nm transition was tested by carrying out spectroscopy and comparing the results with existing data. The isotope selectivity for other naturally available Ba isotopes is simulated for both transitions (553 nm and 413 nm) and for different angles between atomic beam and laser beam (90°, 85° and 80°). The simulations show, that under ideal conditions (when the atomic beam and ionization laser beam are orthogonal) both transitions perform similarly. However, the 553 nm transition exhibits greater robustness against Doppler shifts as the angle deviates from 90°. At an angle of 80° between the atomic beam and the laser beam, the maximum probability of generating 138Ba+ is 99 % using 553 nm, while the maximum probability of generating the isotope using 413 nm does not exceed 94 %. In the third part of this thesis, we focus on state preparation of Ba+ ions in the 6P1/2 state. A heralding state preparation scheme that exploits the spontaneous emission of a 389 nm photon during the decay of the excited state 6D3/2 is introduced. The detection of a photon at this wavelength projects the ion into the desired state 6P1/2 in a deterministic fashion. A simulation of this scheme shows that the preparation probability may exceed 78 %, and the heralding process ensures state preparation with 100 % fidelity, if a photon at a wavelength of 389 nm is detected. The state preparation method will be adopted in near-future experiments that are provided as an outlook at the end of this thesis.
Trapped single ions provide an ideal platform for studying quantum optics, light-matter interactions, and quantum information processing. Many of these applications rely on the availability of well-defined and reproducible quantum emitters as well as precise control over the internal states of ions. To study both of these concepts, this thesis focuses on the isotope selectivity of neutral Barium (Ba) transitions as well as the state preparation of single trapped 138Ba+ ions. The first part of this thesis introduces the theoretical framework for experiments, focusing on single Ba+ ions and light-matter interactions. Therefore, the working principle of Paul traps is presented, the Ba ion is introduced and fundamental light-matter interaction tightly related to the experiments and simulations presented in this work are established. In the second part, the isotope selectivity of the neutral Ba 6s2 1S0 to 6s6p 1P1 transition at 553 nm is investigated and compared with the isotope selectivity of the 6s2 1S0 to 5d6p 3D1 transition at 413 nm. A secondary photon can be used in both cases to achieve isotope selective ionization of Ba+ ions. The isotope selectivity of the 553 nm transition was tested by carrying out spectroscopy and comparing the results with existing data. The isotope selectivity for other naturally available Ba isotopes is simulated for both transitions (553 nm and 413 nm) and for different angles between atomic beam and laser beam (90°, 85° and 80°). The simulations show, that under ideal conditions (when the atomic beam and ionization laser beam are orthogonal) both transitions perform similarly. However, the 553 nm transition exhibits greater robustness against Doppler shifts as the angle deviates from 90°. At an angle of 80° between the atomic beam and the laser beam, the maximum probability of generating 138Ba+ is 99 % using 553 nm, while the maximum probability of generating the isotope using 413 nm does not exceed 94 %. In the third part of this thesis, we focus on state preparation of Ba+ ions in the 6P1/2 state. A heralding state preparation scheme that exploits the spontaneous emission of a 389 nm photon during the decay of the excited state 6D3/2 is introduced. The detection of a photon at this wavelength projects the ion into the desired state 6P1/2 in a deterministic fashion. A simulation of this scheme shows that the preparation probability may exceed 78 %, and the heralding process ensures state preparation with 100 % fidelity, if a photon at a wavelength of 389 nm is detected. The state preparation method will be adopted in near-future experiments that are provided as an outlook at the end of this thesis.
This thesis is written as a part of the TRImP programme. Rare and short-lived radioactive isotopes offer new possibilities for investigating fundamental interactions and symmetries. Radium, a radioactive alkaline earth element has been identified as a new candidate for searches for a permanent electric dipole moment in neutral atoms. The TRImP facility at KVI, Groningen, The Netherlands is being setup to produce and utilize these radioactive isotopes to study fundamental interactions and symmetries. We use atom traps for these studies. We presented the various stages for the production, separation and slowing of the short-lived radioactive isotopes of our interest at the TRImP facility. We need to expand laser cooling and trapping methods to heavy alkaline earth elements barium and radium. Stable barium is well suited as a precursor for radioactive radium, because of its similar level scheme and chemical properties. Because of the radioactivity of radium we have chosen to prepare the cooling and trapping with the homologue barium. Stable isotopes of barium were used in the present study. For spectroscopy, metastable singlet and triplet D-states were populated by optical methods rather than using the conventional method of atomic discharges. The singlet D-state is populated by driving the 1S0-1P1 transition. To populate the triplet D-states efficiently the branching ratio of the 3P1 level was exploited by optical pumping with the intercombination line using the diode lasers at 791.3 nm. Long term frequency stability of these lasers were achieved by referencing the frequency to a nearby transition in molecular iodine. We performed first direct optical spectroscopy of the 1D2-1P1(1500.4 nm), 3D1-1P1(1107.8 nm) and 3D2-1p1 (1130.6 nm) transitions respectively. The hyperfine structure of the 3D1 and 3D2 levels for the Ba-135,137 isotopes were optically resolved. In addition, isotope shifts of the 3D1-1P1 transition were measured. We report the experimental observation of coherent Raman resonances in barium. The relevant Rabi frequencies were extracted for the lambda system and estimated the transition rates, relevant for the laser cooling experiment. We have achieved the first slowing of a barium atomic beam. These results can be applied for atoms, in which a closed cooling cycle can only be found with several laser fields at different wavelengths.
Trapped cold ions are tools which we used to approach two very disparate areas of physics, strong coupling between Ba+ ions and optical resonators, and investigations of a low-energy nuclear isomer of 229-Th. The first part of this thesis describes our progress towards the integration of a miniature Paul (rf) ion trap with a high finesse (F=30000) optical cavity. Ba+ ions were trapped and cooled for long periods and a new scheme for isotope selective photoionization was developed. The second part of this thesis describes our progress towards controlled excitation of the low energy nuclear isomer of 229-Th, which may provide a bridge between the techniques of cold atomic and nuclear physics. As a step towards this goal, 232-Th3+ ions were confined in rf ions traps and cooled via collisions with a buffer gas of helium. A sophisticated scanning program was developed for controlling ion trap loading, tuning lasers, and running a CCD camera to look for fluorescence. The low-lying electronic transitions of Th3+ at 984 nm, 690 nm and 1087 nm were observed via laser fluorescence.
This thesis describes experimental work towards developing a trapped ion quantum information processor. An existing ion trap apparatus was capable of trapping and laser-cooling single ions or small ion strings of 40 Ca+, and had been used for studies of quantum jumps and natural lifetime measurements in Ca. This thesis describes improvements in this apparatus, which have allowed the stability and the flexibility of experimental control of the ions to be greatly increased. This enabled experiments to read out the spin state of a single trapped ion, and to load ions with isotope selectivity through photoionization. The optical systems were improved by installation of new lasers, optical reference cavities, and a system of acousto-optic modulators for laser intensity switching and frequency control. The photon counting for fluorescence detection was improved, and a new photon time-of-arrival correlation circuit developed. This has permitted rapid and more sensitive detection of micromotion, and hence cancellation of stray fields in the trap. A study of resonant circuits in the low RF, high voltage (10 MHz, 1 kV) regime was carried out with a view to developing a new RF supply for the Paul trap with reduced noise and increased power. A new supply based on a helical resonator was built and used to trap ions. This technique has reduced noise and will permit higher secular frequencies to be attained in the future. A magnetic field B in the ion trap is used to define a quantization axis, and in one series of experiments was required to be of order 100 G to provide a substantial Zeeman splitting. A set of magnetic field coils to control the size and direction of B is described. The design of these posed some problems owing to an unforseen issue with the vacuum chamber. In short, it is magnetizable and acts to first approximation like a magnetic shield. The field coils had to be sufficiently substantial to produce the desired field at the ion even in the presence of this shielding effect, and dark resonance (and other) spectra with Zeeman splitting were obtained to calibrate the field using the ion as a probe. Finally, the thesis describes the successful loading of the ion trap by laser photoionization from a weak atomic beam. This involved two new lasers at 423 nm and 389 nm. Saturated absorption spectroscopy of neutral calcium is first described, then transverse excitation of an atomic beam in our vacuum chamber is used to identify all the main isotopes of calcium and confirm their abundances in our source (a heated sample of natural calcium). Finally, photoionization is used to load the trap. This has three advantages over electron-impact ionization. By avoiding an electron gun, we avoid charging of insulating patches and subsequent electric field drift as they discharge; the flux in the atomic beam and hence calcium (and other) deposits on the electrodes can be greatly reduced; and most importantly, the photoionization is isotope selective. Evidence is presented which suggests that even with an non-enriched source, the rare isotope 43 Ca can be loaded with reasonable efficiency. This isotope is advantageous for quantum information experiments for several reasons, but chiefly because its ground state hyperfine structure can act as a stable qubit.
Trapped ions are currently one of the most promising architectures for realising the quantum information processor. The long lived internal states are ideal for representing qubit states and, through controlled interactions with electromagnetic radiation, ions can be manipulated to execute coherent logic operations. In this thesis an experiment capable of trapping Yb+ ions, including 171Yb+, is presented. Since ion energy can limit the coherence of qubit manipulations, characterisation of an ion trap heating rate is vital. Using a trapped 174Yb+ ion a heating rate consistent with previous measurements of other ion species in similar ion traps is obtained. This result shows abnormal heating of Yb+ does not occur, further solidifying the suitability of this species for quantum information processing. Efficient creation, and cooling of trapped ions requires exact wavelengths for the ionising, cooling and repump transitions. A simple technique to measure the 1S0 ? 1P1 transition wavelengths, required for isotope selective photoionisation of neutral Yb, is developed. Using the technique new wavelengths, accurate to 60 MHz, are obtained and differ from previously published results by 660 MHz. Through a simple modification the technique can also predict Doppler shifted transition frequencies, which may be required in non-perpendicular atom-laser interactions. Using trapped ions, the 2S1=2 ? 2P1/2 Doppler cooling and 2D3/2 ? 2D[3/2]1/2 repump transitions are also measured to a greater accuracy than previously reported. Many experiments require wavelengths which can only be obtained using complex expensive laser systems. To remedy this a simple cost effective laser is developed to enable laser diodes to be operated at sub zero temperatures, extending the range of obtainable wavelengths. Additional diode modulation capabilities allow for the manipulation of atoms and ions with hyperfine structures. The laser is shown to be suitable for manipulating Yb+ ions by cooling a diode from 372 nm to 369 nm and simultaneously generating 2.1 GHz frequency sidebands. Coherent manipulation such as arbitrary qubit rotations, motional coupling and ground state cooling, are required for trapped ion quantum computing. Two photon stimulated Raman transitions are identified as a suitable technique to implement all of these requirements and an investigation into implementing this technique with 171Yb+ is conducted. The possibility of exciting a Raman transition via either a dipole or quadrupole transitions in 171Yb+ is analysed, with dipole transitions preferred because quadrupole transitions are found to be too demanding experimentally. An inexpensive setup, utilising a dipole transition, is designed and tested. Although currently limited the setup shows potential to be an inexpensive, high fidelity method of exciting a Raman transition.
Trapped ions in linear Paul traps are largely isolated from interaction with the environment. This property of trapped ions make them a primary choice for quantum technology. \nOver the last decade, trapped atomic ions in linear radio frequency Paul traps have shown to be an important tool to implement quantum algorithms. The scalability of linear ion \ntraps is required to handle large numbers of qubits, in order to implement useful quantum computation. Advance micro-fabrication technology allows the realisation of scalable ion traps. Further developments of micro-trap designs, for the purpose of scalable quantum technology, requires inter-disciplinary investigations of ion traps. Micro-scale ion trap designs typically require a versatile experimental setup. The first part of this thesis describes such an experimental setup including a chip bracket that can host macroscopic ion traps as well as advanced symmetric and asymmetric ion trap chips with up to 90 control electrodes. The system provides versatile optical access for both type of traps and the vacuum chamber is designed in a way so the ion traps can be replaced within a short amount of time. To test the working of the setup, a macroscopic ion trap with an ion-electrode distance of 310±10 μm is used to trap ytterbium ions (Yb+). The trap is characterised by measuring the heating rate, (n•), and spectral noise density SE(ω). A \nphotoionisation technique is used to ionise the different isotopes of Yb in our trap. Isotope selective photoionisation requires exact measurements of 1So↔1P1 transition for the different Yb isotopes. A technique to measure these resonant frequencies is described. \nThis technique works by observing and aligning fluorescence spots and by using this technique, the 1So↔1P1 transition frequencies for stable isotopes of Yb were measured with an accuracy of 60 MHz. These new measured transition frequencies for stable Yb isotopes differ from previously published work by 660 MHz. Furthermore, this technique can also be used to obtain the transition frequencies at different laser-atomic beam angles, typical for non-perpendicular laser-atomic beam angles. The second part of this thesis discusses the optimisation of surface trap geometries as they are being used to implement scalable ion trap designs which consist of a large number of trapping zones. The trap depth in surface traps is low compared to symmetric traps of similar dimensions. How to optimise the trap geometry to achieve maximum trap depth for a given ion height above the trap electrodes, is discussed. Fast and adiabatic ion shuttling operations in one dimension as well as ion separation and recombination processes are important for many quantum information implementations. The maximum speed of separation of trapped ions for adiabatic shuttling operations depends on the secular frequencies, the trapped ion experiences in the process. It will be shown how such ion trap structures have to be designed for fast ion separation process and linear shuttling. Numerical results of adiabatic shuttling operations for trapped ions in such trap structures are also presented.
Summary form only given. Barium (Ba) is a versatile atom for basic quantum optical measurements because it possesses multiple transitions at conveniently accessible wavelengths which allow the creation of a variety of useful systems. This atom has been used as a three-level /spl Lambda/, cascade, and V system for driven atom and lasing without inversion experiments. Other work relied on the (6s/sup 2/)/sup 1/S/sub 0/-(6s6p)/sup 1/P/sub 1/ 553.5 nm transition as a two-level system for cavity quantum electrodynamics, two-photon laser, and single-atom laser experiments. However, these observations, which utilize natural Ba, are complicated by the presence of multiple isotopes with small frequency shifts. These isotopes introduce additional features and under certain conditions mask small structures, making interpretation of the results non-trivial.