National Physical Laboratory

About: National Physical Laboratory is a facility organization based out in London, United Kingdom. It is known for research contribution in the topics: Dielectric & Thin film. The organization has 7615 authors who have published 13327 publications receiving 319381 citations.

Hertz-level measurement of the optical clock frequency in a single 88Sr+ ion.

Abstract: The frequency of the 5s 2S1/2–4d 2D5/2 electric quadrupole clock transition in a single, trapped, laser-cooled 88Sr+ ion has been measured by using an optical frequency comb referenced to a cesium fountain primary frequency standard. The frequency of the transition is measured as 444,779,044,095,484.6 (1.5) hertz, with a fractional uncertainty within a factor of 3 of that of the cesium standard. Improvements required to obtain a cesium-limited frequency measurement are described and are expected to lead to a 88Sr+ optical clock with stability and reproducibility exceeding that of the primary cesium standard.

The effect of initial conditions on the development of a free shear layer

Abstract: The distance between the separation point and the final approach to a fully developed turbulent mixing layer is found to be of the order of a thousand times the momentum-deficit thickness of the initial boundary layer, whether the latter be laminar or turbulent. There are correspondingly large shifts in the virtual origin of the mixing layer, resulting in spurious Reynolds-number effects which cause considerable difficulties in tests of model jets or blunt-based bodies, and which are probably responsible for the disagreements over the influence of Mach number on the development of free shear layers. These effects are explained.

Displacement Measurement from Double-exposure Laser Photographs

Abstract: Surface displacement can be measured by recording a double-exposure photograph of the laser-illuminated object, followed by optical processing of the recorded speckle-pattern image. The analysis can either be on a point-by-point basis, or by a spatial filtering technique which resolves the motion in any desired direction orthogonal to the line of sight. The limits within which the technique may be used to measure lateral translations and rotations of the surface are examined theoretically and experimentally, and the effect of lens aberrations and surface scattering properties are discussed. Surface tilt may also be measured by recording a defocused double exposure image and analysing its optical transform. The photographic techniques described can be extended to measuring surface vibration, by recording a single ‘time-averaged’ exposure and examining the modified optical transform fringe pattern.