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Individual electrodynamically trapped and laser cooled ions are addressed in frequency space using radio-frequency radiation in the presence of a static magnetic field gradient. In addition, an interaction between motional and spin states induced by an rf field is demonstrated employing rfoptical double resonance spectroscopy. These are two essential experimental steps towards realizing a novel concept for implementing quantum simulations and quantum computing with trapped ions.PACS numbers: 37.10. Vz, 37.10.Ty, 32.60.+i Quantum simulations addressing a specific scientific problem and universal quantum computation are expected to yield new insight into as of yet unsolved physical problems that withstand efficient treatment on a classical computer (e.g., [1]). Already a small number of qubits (i.e., a few tens) used for quantum simulations could solve problems even beyond the realm of quantum information science. Creating and investigating entanglement in large physical systems is a related important experimental challenge with implications for our understanding of the transition between the elusive quantum regime and the classical world [2].Laser cooled atomic ions confined in an electrodynamic cage have successfully been used for quantum information processing (QIP) [3] and advantages and difficulties associated with this system have been and still are subject to detailed investigations. The electromagnetic radiation used to coherently drive ionic resonances that serve as qubits needs to be stable against variations in frequency, phase, and amplitude over the course of a quantum computation or simulation. Experimentally this is particularly challenging when laser light is used for realizing quantum gates. When employing laser light additional issues need to be dealt with to allow for accurate qubit manipulation such as the intensity profile of the laser beam, its pointing stability, and diffraction effects. Furthermore, the motional state of the ion chain strongly affects the gate fidelity which requires ground state cooling and low heating rates during the gate operation [4]. Also, spontaneous scattering of laser light off excited electronic states may pose a limit for the coherence time of a quantum many-body state. The probability for scattering can be reduced by increasing the detuning from excited states (when two laser light fields are used that drive a Raman transition between hyperfine or Zeeman states) which, however, leads to an increasing demand for laser power [5].For generating Raman laser beams with a desired frequency difference, first a radio-frequency (rf) or microwave signal at this difference frequency has to be generated that is then "imprinted" on the laser light and send to the ions. Using rf or microwave radiation directly for coherent driving of qubit transitions is impeded in usual ion trap schemes, since, (i) individual addressing of qubits by focusing radiation on just one ion is difficult due to the long wavelength of rf radiation, and (ii) the required coupling between qubit stat...

The photon pair correlation in the laser-excited fluorescence of a single trapped and cooled Ba + ion shows antibunching and, in addition, novel nonclassical phenomena absent in the fluorescence of twolevel atoms. They include excessive transient values of the correlation caused by optical pumping, and temporally extended sub-Poissonian photon emission probability which arises from the transient excitation of nonabsorbing Raman coherence. The fluorescence also displays sub-Poissonian photon statistics. PACS numbers: 42.50.Dv, 32.50.+d Although light is known to carry information on its source encoded in the correlation functions of all orders, it is the second-order or intensity correlation which provides the main information on intensity fluctuations [1]. Its detection requires two measurements of the light flux. On the microscopic level, the intensity correlation is represented by the correlation of photoelectrons recorded in two events of detection [2], from which the characteristics of photon statistics have been inferred [31. Atomic resonance fluorescence is a case in point: Measurements of its intensity correlation by comparing the timeseparated photon counting signals in one or two channels of detection have revealed nonclassical properties of the light for which the atomic interaction with the excitation light and the vacuum field is responsible. In particular, sub-Poissonian photon statistics [4] have been observed [5,6], and also the rise of the correlation of the two detected photons upon the increase of their time separation T close to zero ("antibunching" [6,7]). So far, the observations have included dilute atomic beams [5,7] or a single ion in a rf trap [6]. The involved atomic particles could be well approximated as two-level systems with the monochromatic laser light cyclically exciting the resonance line.We have, for the first time, recorded the intensity correlation of the resonance fluorescence of a single ion which cannot be modeled as a two-level system. The observed intensity correlation reveals novel features that are not seen in the resonance fluorescence of two-level atoms. These features include a maximum photon correlation which is much larger than what is possible with two-level atoms, and also photon antibunching with much larger time constants of the initial photon anticorrelation.A single Ba + ion, stored in a 1-mm rf trap of 25-MHz drive frequency [8], was laser cooled [9] to less than 3 mK, and its laser-excited resonance fluorescence was recorded by two photon counting channels placed in opposite directions. The relevant levels of Ba + and the wavelengths of the two dye-laser-generated light fields are shown in Fig. 1. The 6 2 / > j/2 resonant level decays with 8-ns lifetime to the ground state 6 2 S\/2 and also to the metastable level 5 2 Z>3/2, with branching ratio 2.85 in favor of the ground state. Since the metastable level lives for 17 s [10], the two light fields are required for the elimination of optical pumping and the generation of a continuous flux of fluorescence. A ...