We demonstrate Rabi flopping at MHz rates between ground hyperfine states of neutral 87 Rb atoms that are trapped in two micron sized optical traps. Using tightly focused laser beams we demonstrate high fidelity, site specific Rabi rotations with crosstalk on neighboring sites separated by 8 µm at the level of 10 −3 . Ramsey spectroscopy is used to measure a dephasing time of 870 µs which is ≈ 5000 times longer than the time for a π/2 pulse.PACS numbers: 03.67. Lx, 32.80.Pj, 39.25.+k Over the last decade quantum computing has attracted much attention due to the possibility of solving certain problems much faster than a classical computer [1]. A number of different approaches are currently being pursued to build a scalable quantum computer and significant progress has been made with trapped ions [2], nuclear magnetic resonance [3], single photons [4], and solid state josephson junctions [5]. Neutral atoms trapped by optical fields are also being studied intensively as a viable approach to demonstrating quantum logic. Neutral atom approaches are attractive for a number of reasons starting with the availability of well developed techniques for laser cooling and trapping [6,7] and the potential for scalability [8]. The qubit basis states can be represented by ground state hyperfine levels which have long decoherence times and are therefore suitable for storing quantum information. The qubits can be rapidly initialized and manipulated with near resonant optical fields through optical pumping and stimulated Raman processes. A number of protocols for two-qubit gates have been proposed [9] including ground state collisions, optically induced short range dipole-dipole interactions, and dipoledipole interactions between highly excited Rydberg levels [10,11,12]. The Rydberg atom approach appears particularly attractive since it has the potential for achieving fast, MHz rate gates whose fidelity is only weakly dependent on the motional state of the atoms [13].We report here on progress towards demonstrating quantum logic operations using neutral atom qubits in optical traps. Recent achievements in neutral atom quantum computing include the implementation of a five qubit quantum register by Meschede and colleagues [14,15] and subpoissonian loading of single atoms to nearby dipole traps by the Grangier group [16,17]. Advancing on these pioneering works, we demonstrate loading and ground state manipulation of neutral 87 Rb atoms in two closely spaced microscopic optical traps. By optically addressing each of these traps, we demonstrate twophoton Rabi flopping between ground hyperfine states |0 ≡ |F = 1, m F = 0 and |1 ≡ |F = 2, m F = 0 at a rate of 1.36 MHz. This rate corresponds to a time period of 183 ns to perform a π/2 Rabi rotation. The Rabi rotations are performed with negligible cross talk between the two traps: a π rotation on one site causes less than 1.4 × 10 −3 π rotation on the other site. Using Ramsey spectroscopy, we measure a dephasing time of 870 µs. To our knowledge, our results demonstrate the best figure ...

We report an experimental study of correlated multiple light scattering in an ultracold gas of 85 Rb confined in a magneto-optic trap. Measurements are made of the polarization dependence of the spatial and spectral profile of light backscattered from the sample. The results show an interferometric enhancement sensitive to coherent multiple scattering in the atomic gas, and strong variations with the polarization of the incident and detected light. The spatial width and peak value of the enhancement are found to be dependent on the sample size. Comparison of all the measurements with realistic quantum Monte Carlo simulations yields a very good agreement.

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We report a combined theoretical and experimental study of the spectral and polarization dependence of near resonant radiation coherently backscattered from an ultracold gas of 85 Rb atoms. Measurements in a ±6 M Hz range about the 5s 2 S 1/2 → 5p 2 P 3/2 F = 3 → F ′ = 4 hyperfine transition are compared with simulations based on a realistic model of the experimental atomic density distribution. In the simulations, the influence of heating of the atoms in the vapor, magnetization of the vapor, finite spectral bandwidth, and other nonresonant hyperfine transitions are considered. Good agreement is found between the simulations and measurements.

Quantum dots (QDs) are very attractive probes for multi-color fluorescence imaging in biological applications because of their immense brightness and reported extended photostability. We report here however that single QDs, suitable for biological applications, that are subject to continuous blue excitation from a conventional 100 W mercury arc lamp will undergo a continuous blue-switching of the emission wavelength eventually reaching a permanent dark, photobleached state. We further show that β-mercaptoethanol has a dual stabilizing effect on the fluorescence emission of QDs: 1) by increasing the frequency of time that a QD is in its fluorescent state, and 2) by decreasing the photobleaching rate. The observed QD color spectral switching is especially detrimental for multi-color single molecule applications, as we regularly observe spectral blue-shifts of 50 nm, or more even after only ten seconds of illumination. However, of significant importance for biological applications, we find that even small, biologically compatible, concentrations (25 µM) of β-mercaptoethanol has a significant stabilizing effect on the emission color of QDs, but that greater amounts are required to completely abolish the spectral blue shifting or to minimize the emission intermittency of QDs.