High-Rate, High-Fidelity Entanglement of Qubits Across an Elementary Quantum Network.

We demonstrate remote entanglement of trapped-ion qubits via a quantum-optical fiber link with fidelity and rate approaching those of local operations. Two ^{88}Sr^{+} qubits are entangled via the polarization degree of freedom of two spontaneously emitted 422 nm photons which are coupled by high-numerical-aperture lenses into single-mode optical fibers and interfere on a beam splitter. A novel geometry allows high-efficiency photon collection while maintaining unit fidelity for ion-photon entanglement. We generate heralded Bell pairs with fidelity 94% at an average rate 182 s^{-1} (success probability 2.18×10^{-4}).

A short response time atomic source for trapped ion experiments.

Ion traps are often loaded from atomic beams produced by resistively heated ovens. We demonstrate an atomic oven which has been designed for fast control of the atomic flux density and reproducible construction. We study the limiting time constants of the system and, in tests with 40Ca, show that we can reach the desired level of flux in 12 s, with no overshoot. Our results indicate that it may be possible to achieve an even faster response by applying an appropriate one-off heat treatment to the oven before it is used.

Fast quantum logic gates with trapped-ion qubits.

Quantum bits (qubits) based on individual trapped atomic ions are a promising technology for building a quantum computer. The elementary operations necessary to do so have been achieved with the required precision for some error-correction schemes. However, the essential two-qubit logic gate that is used to generate quantum entanglement has hitherto always been performed in an adiabatic regime (in which the gate is slow compared with the characteristic motional frequencies of the ions in the trap), resulting in logic speeds of the order of 10 kilohertz. There have been numerous proposals of methods for performing gates faster than this natural 'speed limit' of the trap. Here we implement one such method, which uses amplitude-shaped laser pulses to drive the motion of the ions along trajectories designed so that the gate operation is insensitive to the optical phase of the pulses. This enables fast (megahertz-rate) quantum logic that is robust to fluctuations in the optical phase, which would otherwise be an important source of experimental error. We demonstrate entanglement generation for gate times as short as 480 nanoseconds-less than a single oscillation period of an ion in the trap and eight orders of magnitude shorter than the memory coherence time measured in similar calcium-43 hyperfine qubits. The power of the method is most evident at intermediate timescales, at which it yields a gate error more than ten times lower than can be attained using conventional techniques; for example, we achieve a 1.6-microsecond-duration gate with a fidelity of 99.8 per cent. Faster and higher-fidelity gates are possible at the cost of greater laser intensity. The method requires only a single amplitude-shaped pulse and one pair of beams derived from a continuous-wave laser. It offers the prospect of combining the unrivalled coherence properties, operation fidelities and optical connectivity of trapped-ion qubits with the submicrosecond logic speeds that are usually associated with solid-state devices.

Functional role of arginine 302 within the lactose permease of Escherichia coli.

Within the lactose permease, an arginine residue is found on a transmembrane segment at position 302. Based upon the effects of mutations at or in the vicinity of Arg-302, this residue has been implicated to be involved with H+ and/or sugar recognition. To further elucidate the role of this residue, we have substituted Arg-302 with serine, histidine, and leucine via site-directed mutagenesis. All three of these substitutions result in an impaired ability to transport galactosides as evidenced by their poor growth on minimal plates supplemented with lactose or melibiose. Furthermore, in vitro transport assays revealed substantial alterations in the kinetic constants for downhill lactose transport. The wild-type strain exhibited a Km for lactose transport of 0.30 mM and a Vmax of 267 nmol of lactose/min.mg of protein. The Ser-302, His-302, and Leu-302 were observed to have Km values of 0.18, 2.3, and 2.8 mM, and Vmax values of 11.6, 56.4, and 22.0 nmol of lactose/min.mg of protein, respectively. In uphill transport assays, all three mutants were unable to accumulate beta-methyl-D-thiogalactoside. However, both the Ser-302 and His-302 mutants were able to accumulate lactose against a concentration gradient. During H+ transport assays, all three mutants were shown to transport H+ in conjunction with thiodigalactoside. In addition, the Ser-302 and His-302 strains exhibited small alkalinizations upon the addition of lactose. However, for the Leu-302 mutant, the addition of lactose did not result in a significant level of H+ transport. Finally, experiments were conducted which were aimed at measuring the ability of the mutant permeases to catalyze an H+ leak. In this regard, a comparison was made between the wild-type and mutant strains concerning their steady state pH gradient and their rates of H+ influx following oxygen pulses. The results of these experiments suggest that mutations at position 302 cause a sugar-dependent H+ leak.