A single Ca + ion in a Paul trap has been cooled to the ground state of vibration with up to 99.9% probability. Starting from this Fock state |n = 0 we have demonstrated coherent quantum state manipulation on an optical transition. Up to 30 Rabi oscillations within 1.4 ms have been observed. We find a similar number of Rabi oscillations after preparation of the ion in the |n = 1 Fock state. The coherence of optical state manipulation is only limited by laser and ambient magnetic field fluctuations. Motional heating has been measured to be as low as one vibrational quantum in 190 ms.Trapped and laser cooled ions in a Paul trap are currently considered to be promising candidates for a scalable implementation of quantum computation. Internal states of the ions serve to hold the quantum information (qubits) and an excitation of their common vibrational motion provides the coupling between qubits necessary for a quantum gate. The Cirac-Zoller proposal [1] requires that initially all ions are optically cooled to the ground state of motion and that the whole system can be coherently manipulated and controlled. This ideal situation is compromised by coupling to the environment in a real experiment. Therefore, experimental investigation of the limits of engineering a trapped ion's quantum state is an important step towards implementation of a quantum computer. In this letter, we present experiments on quantum state engineering with a single trapped 40 Ca + ion that is initially prepared in the motional and electronic ground state. We manipulate the ion's motion using an optical transition to a metastable excited level. We measure the coherence time of this process as well as motional heating rates. Ground state cooling has been achieved so far with a single 199 Hg + ion [2], and with 9 Be + [3], using resolved sideband cooling on either a quadrupole or a Raman transition. With a cooling method similar to the Hg + experiment, we reach 99.9% of motional ground state occupation within 6.4 ms. We find a motional heating rate of one phonon in 190 ms, much smaller than in the trap used for the 9 Be + experiment. The ion trap used in our experiment is a conventional 3D-quadrupole Paul trap [4]. The ring electrode is made of 0.2 mm molybdenum wire and has an inner diameter of 1.4 mm. The endcaps are formed by two pieces of the same material that are rounded at the tips. The endcap to endcap distance is 1.2 mm. The radio-frequency drive field at 20.8 MHz is fed to the ring via a helical step-up circuit with a quality factor of 100. With a drive power between 0.5 W and 2.2 W we observe motional frequencies (ω x , ω y , ω z )/(2π) between (0.96, 0.92, 2.0) MHz and (2.16, 2.07, 4.51) MHz along the respective trap axes (z denotes the axial direction of the trap, the degeneracy of x and y is lifted by small asymmetries of the setup).
Ca+ is a hydrogen-like ion with one valence electron and no hyperfine structure. All relevant transitions are easily accessible by solid state or diode lasers (see Fig. 1) [5]. In our experiment we Doppl...
Single ions in a linear string have been addressed with a tightly focused laser beam and an acousto-optic deflector. The excitation into a long-lived metastable level is detected with a quantum jump technique. Singlequantum bit operations for quantum information processing with trapped ions are shown to be feasible.
A single 40 Ca ϩ ion is confined in a linear Paul trap and Doppler-cooled on the S 1/2 to P 1/2 dipole transition. Then the narrow quadrupole S 1/2 to D 5/2 transition at 729 nm is probed. The observed spectrum is interpreted in terms of the Zeeman substructure superimposed with oscillation sidebands due to the harmonic motion in the trap. The height of the motional sidebands provides a sensitive method to determine the ion's temperature and thus allows us to test sub-Doppler laser cooling schemes needed for quantum state preparation and quantum computation. We also observe the dynamics induced by Rabi oscillations on a carrier transition and interpret it in terms of the thermal state which is reached after Doppler cooling.
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