We have created a long-lived ($\approx 40\mathrm{s}$) persistent current in a toroidal Bose-Einstein condensate held in an all-optical trap. A repulsive optical barrier across one side of the torus creates a tunable weak link in the condensate circuit, which can affect the current around the loop. Superflow stops abruptly at a barrier strength such that the local flow velocity at the barrier exceeds a critical velocity. The measured critical velocity is consistent with dissipation due to the creation of vortex-antivortex pairs. This system is the first realization of an elementary closed-loop atom circuit.

Complex chiral particles: Synthetic colloids are usually smooth, but nature can produce micrometer-scale particles with intricate structure and shape, such as the coccoliths produced by algae. Jiang et al. controlled the self-assembly of gold–cysteine nanoplatelets into a variety of chiral, hierarchically organized colloidal particles by changing the chiral fraction of cysteine and the nucleation temperature. Organic cations created electrostatic repulsions that favored edge assembly of the nanoplatelets, which in turn could create surfaces bearing twisted spikes. Science , this issue p. 642

Partial-transfer absorption imaging is a tool that enables optimal imaging of atomic clouds for a wide range of optical depths. In contrast to standard absorption imaging, the technique can be minimally destructive and can be used to obtain multiple successive images of the same sample. The technique involves transferring a small fraction of the sample from an initial internal atomic state to an auxiliary state and subsequently imaging that fraction absorptively on a cycling transition. The atoms remaining in the initial state are essentially unaffected. We demonstrate the technique, discuss its applicability, and compare its performance as a minimally destructive technique to that of phase-contrast imaging.

We demonstrate suppression of two-color photoassociative ionization in a sodium vapor magneto-optical trap by imposing an auxiliary "suppressor" light beam on the ensemble of colliding trapped atoms. We interpret the experimental results in terms of a simple picture in which the suppressor light intercepts incoming reactant particle flux and reroutes it to long-range repulsive states, effectively preventing further approach of the collision pairs.

Although diamond photonics has driven considerable interest and useful applications, as shown in frequency generation devices and single photon emitters, fundamental studies on the third-order optical nonlinearities of diamond are still scarce, stalling the development of an integrated platform for nonlinear and quantum optics. The purpose of this paper is to contribute to those studies by measuring the spectra of two-photon absorption coefficient (β) and the nonlinear index of refraction (n₂) of diamond using femtosecond laser pulses, in a wide spectral range. These measurements show the magnitude of β increasing from 0.07 to 0.23 cm/GW, as it approaches the bandgap energy, in the region from 3.18 to 4.77 eV (390–260 nm), whereas the n₂ varies from zero to 1.7 × 10⁻¹⁹ m²/W in the full measured range, from 0.83–4.77 eV (1500–260 nm). The experimental results are compared with theoretical models for nonlinear absorption and refraction in indirect gap semiconductors, indicating the two-photon absorption as the dominant effect in the dispersion of the third-order nonlinear susceptibility. These data, together with optical Kerr gate measurements, also provided here, are of foremost relevance to the understanding of ultrafast optical processes in diamond and its nonlinear optical properties.

We have measured the velocity field of a vortex lattice within a sodium Bose-Einstein condensate using Bragg scattering. The phase gradient of the macroscopic wave function was mapped into the spatial structure of the diffracted atom cloud, allowing for the single shot measurement of the rotation parameters. A combination of spectral and spatial information yields a complete description of the superfluid flow, coarse grained over the lattice structure, including direct and independent measurements of the rate and sense of rotation. Signatures of the microscopic quantum rotation have also been observed.