Understanding the mechanisms of action of membrane proteins requires the elucidation of their structures to high resolution. The critical step in accomplishing this by x-ray crystallography is the ...routine availability of well-ordered three-dimensional crystals. We have devised a novel, rational approach to meet this goal using quasisolid lipidic cubic phases. This membrane system, consisting of lipid, water, and protein in appropriate proportions, forms a structured, transparent, and complex three-dimensional lipidic array, which is pervaded by an intercommunicating aqueous channel system. Such matrices provide nucleation sites (``seeding'') and support growth by lateral diffusion of protein molecules in the membrane (``feeding''). Bacteriorhodopsin crystals were obtained from bicontinuous cubic phases, but not from micellar systems, implying a critical role of the continuity of the diffusion space (the bilayer) on crystal growth. Hexagonal bacteriorhodopsin crystals diffracted to 3.7 angstrom resolution, with a space group P63, and unit cell dimensions of a = b = 62 angstrom, c = 108 angstrom; α = β = 90 degrees and γ = 120 degrees.
We perform Ramsey spectroscopy on the ground state of ultracold 87Rb atoms magnetically trapped on a chip in the Knudsen regime. Field inhomogeneities over the sample should limit the 1/e contrast ...decay time to about 3 s, while decay times of 58 ± 12 s are actually observed. We explain this surprising result by a spin self-rephasing mechanism induced by the identical spin rotation effect originating from particle indistinguishability. We propose a theory of this synchronization mechanism and obtain good agreement with the experimental observations. The effect is general and may appear in other physical systems.
Porins form transmembrane pores in the outer membrane of Gram-negative bacteria with matrix porin OmpF and osmoporin OmpC from
Escherichia coli being differentially expressed depending on ...environmental conditions. The three-dimensional structure of OmpC has been determined to 2.0 Å resolution by X-ray crystallography. As expected from the high sequence similarity, OmpC adopts the OmpF-like 16-stranded hollow β-barrel fold with three β-barrels associated to form a tight trimer. Unlike in OmpF, the extracellular loops form a continuous wall at the perimeter of the vestibule common to the three pores, due to a 14-residues insertion in loop L4. The pore constriction and the periplasmic outlet are very similar to OmpF with 74% of the pore lining residues being conserved. Overall, only few ionizable residues are exchanged at the pore lining. The OmpC structure suggests that not pore size, but electrostatic pore potential and particular atomic details of the pore linings are the critical parameters that physiologically distinguish OmpC from OmpF.
Progress in realizing the SI second had multiple technological impacts and enabled further constraint of theoretical models in fundamental physics. Caesium microwave fountains, realizing best the ...second according to its current definition with a relative uncertainty of 2-4 × 10(-16), have already been overtaken by atomic clocks referenced to an optical transition, which are both more stable and more accurate. Here we present an important step in the direction of a possible new definition of the second. Our system of five clocks connects with an unprecedented consistency the optical and the microwave worlds. For the first time, two state-of-the-art strontium optical lattice clocks are proven to agree within their accuracy budget, with a total uncertainty of 1.5 × 10(-16). Their comparison with three independent caesium fountains shows a degree of accuracy now only limited by the best realizations of the microwave-defined second, at the level of 3.1 × 10(-16).
Lipidic cubic phases provide a continuous three-dimensional bilayer matrix that facilitates nucleation and growth of bacteriorhodopsin microcrystals. The crystals diffract x-rays isotropically to 2.0 ...angstroms. The structure of this light-driven proton pump was solved at a resolution of 2.5 angstroms by molecular replacement, using previous results from electron crystallographic studies as a model. The earlier structure was generally confirmed, but several differences were found, including loop conformations and side chain residues. Eight water molecules are now identified experimentally in the proton pathway. These findings reveal the constituents of the proton translocation pathway in the ground state.
UTC(OP), the French national realization of the international coordinated universal time, was redesigned and rebuilt. The first step was the implementation in October 2012 of a new algorithm based on ...a H-maser and on atomic fountain data. Thanks to the new implementation, the stability of UTC(OP) was dramatically improved and UTC(OP) competes with the best time scales available today. Then the hardware generation and distribution of the UTC(OP) physical signals were replaced. Part of the new hardware is composed of commercial devices, but the key elements were specifically developed. One of them is a special switch that allows the UTC(OP) signals to be derived from one of two time scales, based on two different H-masers, which are generated simultaneously. This insures the continuity of the UTC(OP) signal even when a change of the reference H-maser is required. With the new hardware implementation, UTC(OP) is made available through three coherent signals: 100 MHz, 10 MHz and 1 PPS. For more than 3 years, UTC(OP) remained well below 10 ns close to UTC, with a difference even less than 5 ns if we except a short period around MJD 56650.
We propose a novel and robust technique to realize a beam splitter for trapped Bose-Einstein condensates (BECs). The scheme relies on the possibility of producing different potentials simultaneously ...for two internal atomic states. The atoms are coherently transferred, via a Rabi coupling between the two long-lived internal states, from a single well potential to a double-well. We present numerical simulations supporting our proposal and confirming excellent efficiency and fidelity of the transfer process with realistic numbers for a BEC of 87Rb. We discuss the experimental implementation by suggesting state-selective microwave (MW) potentials as an ideal tool to be exploited for magnetically trapped atoms. The working principles of this technique are tested on our atom chip device which features an integrated coplanar MW guide. In particular, the first realization of a double-well potential by using a MW dressing field is reported. Experimental results are presented together with numerical simulations, showing good agreement. Simultaneous and independent control on the external potentials is also demonstrated in the two Rubidium clock states. The transfer between the two states, featuring respectively a single and a double-well, is characterized and it is used to measure the energy spectrum of the atoms in the double-well. Our results show that the spatial overlap between the two states is crucial to ensure the functioning of the beamsplitter. Even though this condition could not be achieved in our current setup, the proposed technique can be realized with current state-of-the-art devices being particularly well suited for atom chip experiments. We anticipate applications in quantum enhanced interferometry.
Optically trapped ensembles are of crucial importance for frequency measurements and quantum memories but generally suffer from strong dephasing due to inhomogeneous density and light shifts. We ...demonstrate a drastic increase of the coherence time to 21 s on the magnetic field insensitive clock transition of (87)Rb by applying the recently discovered spin self-rephasing C. Deutsch et al., Phys. Rev. Lett. 105, 020401 (2010). This result confirms the general nature of this new mechanism and thus shows its applicability in atom clocks and quantum memories. A systematic investigation of all relevant frequency shifts and noise contributions yields a stability of 2.4×10(-11)τ(-1/2), where τ is the integration time in seconds. Based on a set of technical improvements, the presented frequency standard is predicted to rival the stability of microwave fountain clocks in a potentially much more compact setup.
We study experimentally the line of a single quantized vortex in a rotating prolate Bose-Einstein condensate confined by a harmonic potential. In agreement with predictions, we find that the vortex ...line is in most cases curved at the ends. We monitor the vortex line leaving the condensate. Its length is measured as a function of time and temperature. For a low temperature, the survival time can be as large as 10 sec. The length of the line and its deviation from the center of the trap are related to the angular momentum per particle along the condensate axis.
We investigate the hyperfine transition of magnetically trapped non-condensed atoms. The two principal frequency shifts, the second order Zeeman effect and the mean field interaction are considered. ...Analytic models of the mean frequency and its trap induced spread are developed. Comparisons with existing experiments evaluate the role of the atoms’ oscillatory motion. The analytic model proves to be equivalent to existing Monte Carlo simulations. The formulae provide a simple tool for optimising the design of a new experiment. Applied to the two-photon transition |
F
=1,
m
F
=−1〉→|
F
=2,
m
F
=1〉 in
87
Rb and the conditions of a typical atom chip experiment, a line spread as small as 11 mHz is predicted giving a quality factor of 10
12
. The system is promising for application in precision instruments such as compact atomic clocks.
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