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.

Bandwidth characteristics and comparisons of surface texture measuring instruments

Abstract: In this review we will discuss many of the problems that are encountered when designing and carrying out comparisons of surface texture measuring instruments. Previous comparisons are discussed to highlight some of the key issues. The limitations of stylus and optical instruments are identified with a focus on the spatial bandwidths in which they operate. Guidance is given on how to design comparisons to avoid variations in the results that are due to the operating principles and bandwidth limitations of the instruments involved. Methods for matching the bandwidths of different instruments are presented and some examples are given that highlight potential problems. The software aspects of instrument comparisons are also discussed. Finally, some advice is given on how to compare profile and areal surface texture measurements.

Agreement between two 88 Sr + optical clocks to 4 parts in 10 17

Abstract: The frequencies of two nominally identical ${}^{88}{\mathrm{Sr}}^{+}$ trapped single ion optical clocks, based on the 674 nm 5$s$ ${}^{2}$${S}_{1/2}$--4$d$ ${}^{2}$${D}_{5/2}$ electric quadrupole clock transition, have been compared over a period of nine months. The frequencies of the two clocks were found to agree within a total uncertainty of 4 \ifmmode\times\else\texttimes\fi{} 10${}^{\ensuremath{-}17}$, demonstrating that the individual ${}^{88}{\mathrm{Sr}}^{+}$ optical clocks are reproducible at the 3 \ifmmode\times\else\texttimes\fi{} 10${}^{\ensuremath{-}17}$ level. The absolute frequency of the clock transition was measured to be $f$ = 444 779 044 095 486.71(24) Hz using an optical frequency comb referenced to a cesium fountain primary frequency standard. The standard uncertainty of 0.24 Hz (5.3 \ifmmode\times\else\texttimes\fi{} 10${}^{\ensuremath{-}16}$ of the optical frequency) is dominated by measurement statistics and cesium fountain systematics and is around four times lower than previously published.

CHIRRUP: a practical algorithm for unsourced multiple access

Abstract: Unsourced multiple access abstracts grantless simultaneous communication of a large number of devices (messages) each of which transmits (is transmitted) infrequently. It provides a model for machine-to-machine communication in the Internet of Things (IoT), including the special case of radio-frequency identification (RFID), as well as neighbor discovery in ad hoc wireless networks. This paper presents a fast algorithm for unsourced multiple access that scales to $2^{100}$ devices (arbitrary $100$ bit messages). The primary building block is multiuser detection of binary chirps which are simply codewords in the second order Reed Muller code. The chirp detection algorithm originally presented by Howard et al. is enhanced and integrated into a peeling decoder designed for a patching and slotting framework. In terms of both energy per bit and number of transmitted messages, the proposed algorithm is within a factor of $2$ of state of the art approaches. A significant advantage of our algorithm is its computational efficiency. We prove that the worst-case complexity of the basic chirp reconstruction algorithm is $\mathcal{O}[nK(\log_2 n + K)]$, where $n$ is the codeword length and $K$ is the number of active users, and we report computing times for our algorithm. Our performance and computing time results represent a benchmark against which other practical algorithms can be measured.