Coherence effects in a Lambda system of a 40Ca+ ion are investigated. Zeeman sublevels are utilized to evoke electromagnetically induced transparencies. The characteristic dark resonance and line shape alterations are analyzed for varying light field intensities. New features arise by avoiding all coherences in a Lambda-system. This is achieved by exposing the ion to only one single laser light field, going the spectroscopy. Pulsed light sequences ensure stable cooling and spectroscopy. A quantum mechanical description of the process is given, revealing the dark-resonance-free shape of the spectrum. This enables the determination of the Rabi frequency of the applied laser light field as well as the photon collection efficiency of the imaging system. In many cases in QIPC, it is important to precisely determine the position of the ions, for this information obtained from the ions" fluorescence is utilized. The capability of the segmented Paul trap to create custom electric potentials is utilized to form time dependent potentials with multiple trapping wells. The application of feedback control allows purposeful movement of a predefined number of ions and position regulation by altering the trapping voltages. Ion transport over 1 mm with more than 99.8 % success probability is accomplished without any knowledge of the potentials. By means of the feedback control this allows automated dividing of a linear ion crystal into two distinct potentials as well as the opposite process of merging. In order to perform these experiments, several novel techniques for the operation of an ion trap have been developed. A technique for automatic compensation is introduced to minimize the RF driven micromotion of the ion crystal. Additionally, a precise method for simultaneous measuring the oscillation frequencies for individual ions in an ion crystal is presented. Hereby, the oscillation modes of multi-species ion crystals can be analyzed for the determination of dopant isotopes.
Coherence effects in a Lambda system of a 40Ca+ ion are investigated. Zeeman sublevels are utilized to evoke electromagnetically induced transparencies. The characteristic dark resonance and line shape alterations are analyzed for varying light field intensities. New features arise by avoiding all coherences in a Lambda-system. This is achieved by exposing the ion to only one single laser light field, going the spectroscopy. Pulsed light sequences ensure stable cooling and spectroscopy. A quantum mechanical description of the process is given, revealing the dark-resonance-free shape of the spectrum. This enables the determination of the Rabi frequency of the applied laser light field as well as the photon collection efficiency of the imaging system. In many cases in QIPC, it is important to precisely determine the position of the ions, for this information obtained from the ions" fluorescence is utilized. The capability of the segmented Paul trap to create custom electric potentials is utilized to form time dependent potentials with multiple trapping wells. The application of feedback control allows purposeful movement of a predefined number of ions and position regulation by altering the trapping voltages. Ion transport over 1 mm with more than 99.8 % success probability is accomplished without any knowledge of the potentials. By means of the feedback control this allows automated dividing of a linear ion crystal into two distinct potentials as well as the opposite process of merging. In order to perform these experiments, several novel techniques for the operation of an ion trap have been developed. A technique for automatic compensation is introduced to minimize the RF driven micromotion of the ion crystal. Additionally, a precise method for simultaneous measuring the oscillation frequencies for individual ions in an ion crystal is presented. Hereby, the oscillation modes of multi-species ion crystals can be analyzed for the determination of dopant isotopes.
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.
A range of experimental techniques for application in reaction studies between ionic and neutral atoms/molecules and high resolution spectroscopy experiments with sympathetically cooled molecular ions are presented. A novel ion trap loading scheme using the photo-ionisation of atoms generated by the pulsed laser ablation of a solid calcium target has been characterised. We have identified the range of ablation laser fluences that must be used in order to produce a flux of neutral calcium atoms, which is a prerequisite for isotope selective ion trap loading. Calcium ions are trapped and laser cooled in a linear radio-frequency ion trap. We have developed a spectroscopy scheme that allows the entire fluorescence spectrum of trapped ions to be rapidly collected with high precision while maintaining a low ion temperature and good ion localisation throughout interrogation. The scheme has been demonstrated by measuring the saturation intensity of the calcium ion 4S1/2?4P1/2 transition. We have developed a novel scheme to measure the secular motion of trapped ions and demonstrated the application of the technique to ion-neutral collision reaction experiments. Employing pulsed excitation and Doppler velocimetry, we have measured the centre-of-mass mode frequency of single ions as well as large ion crystals with a frequency precision better than 2x10-3 within an interrogation time on the order of seconds. This method has been used to measure the mass of ions and observe charge exchange collisions between trapped calcium isotopes. In particular, we have measured the 44Ca++40Ca!40Ca++44Ca reaction cross section and demonstrated the single-event resolution of the technique. Finally, we have developed a novel all-optical broadband scheme for exciting, amplifying and measuring the secular motion of ions in the trap. Oscillation induced by optical excitation has been coherently amplified to control and measure the ion's secular motion. Requiring only a single interrogation laser, the ion's oscillation amplitude can be precisely controlled. The application of this technique to non-destructive spectroscopy of trapped molecular ions is discussed.
Trapped ions, as one of the pillars of progress in frequency metrology and quantum optics, require a complex experimental environment with well-defined conditions. We present that a feature called dark resonance, provided by the trapped ion itself, can be used as a versatile sensor for enhanced in-situ analysis of interacting fields. The dark resonance is formed in the lambda-type energy level scheme of a laser cooled 40Ca+ ion and corresponds to a fluorescence quenching. The method uses an analysis of the detection times of photons emitted from the upper energy level, which is excited via two optical dipole transitions. The two excitation lasers are phase locked to an optical frequency comb to reduce their linewidths and for precise control of their optical frequencies within the dark resonance. The amplitudes of interacting fields are obtained using the Fourier transform of the ion fluorescence or photon correlation measurements. This paper shows that the method can be applied for sensing of electric, magnetic and electromagnetic fields. Firstly, we present the potential for frequency analysis of the secular motion of a few-ion Coulomb crystal, which corresponds to the axial static electric field of a linear ion trap. Secondly, we demonstrate the optical frequency analysis of the employed lasers driving the two transitions. In the last case we show the analysis of an alternating magnetic field at the position of single ion.
We report transport operations with linear crystals of 40Ca+ ions performed by applying complex electric time-dependent potentials. For their control we use the information obtained from the ions’ fluorescence. We demonstrate that by means of this feedback technique, we can transport a predefined number of ions and also split and unify ion crystals. The feedback control allows for a robust scheme, compensating for experimental errors, as it does not rely on a precisely known electrical modeling of the electric potentials in the ion trap beforehand. Our method allows us to generate a self-learning voltage ramp for the required process. With an experimental demonstration of a transport with more than 99.8% success probability, this technique may facilitate the operation of a future ion-based quantum processor.
Scalability and the implementation of fault tolerant quantum gates are the two main challenges which must be overcome in order to unlock the vast potential of quantum computing. This thesis describes work with calcium ions trapped in both microscopic and macroscopic linear Paul traps addressing both of these issues. We describe the assembly of a microstructured multi-zone ion trap which forms part of our group's contribution to the European "Microtrap" collaboration. We report the successful trapping of ions and characterization of the trap as well as a measurement of the heating rate. In miniaturized trap structures such as this one, background scattered light from the cooling beam causes difficulties. We introduce and demonstrate experimentally two techniques to overcome this problem. The first achieves background-free detection of ions using different repumping methods to enable the filtering out of the excitation wavelength. The second makes possible background-free readout of trapped ion qubits by separating in time the excitation and detection steps The second half of the thesis describes our experimental efforts towards implementing a two-qubit entangling gate with a mixed-species crystal. We describe the setup and characterization of a new macroscopic trap including the trapping and coherent manipulation of the internal states of both 40Ca+ and 43Ca+ ions. We accomplish the simultaneous independent readout of two qubits implemented in a 40Ca+ - 43Ca+ ion pair. We also present the setup and characterization of two injection-locked frequency-doubled Raman lasers and demonstrate coherent manipulation as well as a measurement of the off-resonant photon scattering error they introduce. Finally, we use them to achieve sideband cooling to the motional ground state of a mixed species ion crystal.
Coulomb crystal yields a wide range of applications including quantum computing and simulation, phase transitions, quantum logic spectroscopy, nonlinear dynamics and chaos, chemical reaction process, etc. The structure of the Coulomb crystal and the trajectory of each trapped ion are typically determined by the parameters of the trap and the ion species. However, dark ions are often inevitable in experiments, which introduces uncertainty to the desired crystal structures and ion trajectories. Few research has been conducted to investigate the configuration change of the crystal in the presence of dark ions and the influence of a dark ion on its surrounding ion trajectories in a multi-ion system. In this paper, we utilize the molecular dynamics simulation software LAMMPS and the (py)Lion package (modified to adapt the semi-classical theory of laser cooling) for the simulation of the 3D ion trajectories of Coulomb crystals. The formation process of 40Ca+ Coulomb crystals in a linear trap is simulated. With the micromotion and secular motion trajectories of each ion, we calculate the temperature of Coulomb crystal and the average velocity of specific ions. It’s observed that the crystal structure exhibits obvious layering phenomenon when the trapped ions yield a large difference in their charge-to-mass ratio (CMR), however, layering is not obvious with a small difference in the CMR. In addition, we simulate and compare the Coulomb crystal structure formed by pure 40Ca+ ions and that formed by 40Ca+ ions mixed with a small number of dark ions including isotopic ions (44Ca+) and impurity ions (CaH+). Three different cases are investigated, namely the 1D ion string, 2D plane structure and 3D helical structure. Results show that ions in the neighborhood of a dark ion exhibit around micron-order position changes compared to their positions before the dark ion is formed. Such a change can be measured in experiment through microscopic imaging, providing a way to identify dark ions in Column crystals with a large number of ions.