The alternating- and direct-current (a.c. and d.c.) Josephson effects were first discovered in a system of two superconductors, the macroscopic wavefunctions of which are weakly coupled via a tunnelling barrier. In the a.c. Josephson effect, a constant chemical potential difference (voltage) is applied, which causes an oscillating current to flow through the barrier. Because the frequency is proportional to the chemical potential difference only, the a.c. Josephson effect serves as a voltage standard. In the d.c. Josephson effect, a small constant current is applied, resulting in a constant supercurrent flowing through the barrier. In a sense, the particles do not 'feel' the presence of the tall tunnelling barrier, and flow freely through it with no driving potential. Bose-Einstein condensates should also support Josephson effects; however, while plasma oscillations have been seen in a single Bose-Einstein condensate Josephson junction, the a.c. Josephson effect remains elusive. Here we observe the a.c. and d.c. Josephson effects in a single Bose-Einstein condensate Josephson junction. The d.c. Josephson effect has been observed previously only in superconducting systems; in our study, it is evident when we measure the chemical potential-current relation of the Bose-Einstein condensate Josephson junction. Our system constitutes a trapped-atom interferometer with continuous readout, which operates on the basis of the a.c. Josephson effect. In addition, the measured chemical potential-current relation shows that the device is suitable for use as an analogue of the superconducting quantum interference device, which would sense rotation.

The prospect of quantum networks, in which quantum information is carried by single photons in photonic circuits, has long been the driving force behind the effort to achieve all-optical routing of single photons. We realized a single-photon-activated switch capable of routing a photon from any of its two inputs to any of its two outputs. Our device is based on a single atom coupled to a fiber-coupled, chip-based microresonator. A single reflected control photon toggles the switch from high reflection (R ~ 65%) to high transmission (T ~ 90%), with an average of ~1.5 control photons per switching event (~3, including linear losses). No additional control fields are required. The control and target photons are both in-fiber and practically identical, making this scheme compatible with scalable architectures for quantum information processing.

When two Bose-Einstein condensates (BEC's) collide with high collisional energy, the celebrated matter wave interference pattern results. For lower collisional energies the repulsive interaction energy becomes significant, and the interference pattern evolves into an array of grey solitons. The lowest collisional energy, producing a single pair of solitons, has not been probed. We use density engineering on the healing length scale to produce such a pair of solitons. These solitons then evolve periodically between vortex rings and solitons, which we image in-situ on the healing length scale. The stable, periodic evolution is in sharp contrast to the behavior of previous experiments, in which the solitons decay irreversibly into vortex rings via the snake instability. The evolution can be understood in terms of conservation of mass and energy in a narrow condensate. The periodic oscillation between two qualitatively different forms seems to be a rare phenomenon in nature.

In a recent Letter, Brunner and Simon proposed an interferometric scheme using imaginary weak values with a frequency-domain analysis to outperform standard interferometry in longitudinal phase shifts [Phys. Rev. Lett105, 010405 (2010)]. Here we demonstrate an interferometric scheme combined with a time-domain analysis to measure longitudinal velocities. The technique employs the near-destructive interference of non-Fourier limited pulses, one Doppler shifted due to a moving mirror in a Michelson interferometer. We achieve a velocity measurement of 400 fm/s and show our estimator to be efficient by reaching its Cramér-Rao bound.

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