National Physical Laboratory

Microwave Frequency Standards Microwave Frequency Standards

Microwave frequency standards are key to dissemination of high accuracy time services to UK science, technology and industry, and to compatibility with international timescales through world-wide time transfer and clock comparison.

Caesium fountain primary frequency standards are used to realise the SI definition of the second. NPL's caesium fountain (NPL-CsF2) is used to contribute to the international timescale UTC and is one of the most accurate in the world. It has also been used as the frequency reference for measurements of the absolute frequencies of optical clock transitions in ingle trapped strontium and ytterbium ions.

A second caesium fountain is being developed to enable the fountains to be validated at the highest accuracy levels and to enable continuous input into international timescales. NPL has also developed a rubidium fountain as a secondary representation of the second. These research activities are supported by work on low phase noise microwave local oscillators.

Compact microwave clocks are being developed for ultimate integration into hand-held and mobile frequency control and communcations equipment for applications within the defence, security and aerospace sectors. Our focus is on the development of 'chip-scale' coherent population trapping clocks based on MEMS on sealed hollow-core-fibre technology.

These research activities also feed into other areas such as atomic magnetometry.

Microwave Frequency Standards research

  • A caesium fountain primary frequency standard is a piece of apparatus that realises the SI definition of the second.
  • Primary frequency standards operate with atoms in states which are insensitive to magnetic fields. There are several techniques that can be used to prepare an atomic sample in the proper internal state.
  • The atomic fountain is given its name because atoms are launched upwards and fall back under gravity, spreading outwards in the horizontal plane.
  • NPL's Rubidium fountain frequency standard is designed to reach the highest frequency stability and accuracy among these types of standards.
  • An efficient way to load the magneto-optical trap in a fountain is to use a beam of slow atoms generated from an additional magneto-optical source of cold atoms.
  • NPL has developed a design of stable microwave oscillator derived from a cryogenic resonator containing a ring of high-purity mono-crystalline sapphire that supports a ‘whispering-gallery’ electromagnetic mode with a Q-factor on the order of one billion.
  • Collisions contribute to the shift of the clock transition frequency. Research at NPL has opened up the possibility of cancelling the collisional shift, leading to an improvement in the performance of caesium fountain primary frequency standards.
  • COMSOL, Matlab and Mathematica sources codes, plus a few extras.
  • Atomic frequency standards, high-stability oscillators and GPS-disciplined oscillators.


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