Research
NPL has an extensive programme of research into the development of new optical frequency standards for applications to time, frequency and length metrology.
Frequency standards based on narrow optical transitions in single trapped ions or neutral atoms confined in an optical lattice are being developed as the next generation of primary frequency standards. Other optical frequency standards provide high accuracy working standards for particular applications. These include iodine-stabilised helium-neon lasers for use in dimensional metrology and acetylene-stabilised lasers for applications in the telecommunications industry.
Optical 'flywheel' oscillators based on lasers stabilised to high finesse optical cavities are being studied to provide high stability short-term frequency references. These form a critical component of any high accuracy optical frequency standards. In addition this technology is being applied in experiments to measure fundamental physical constants such as the Rydberg constant.
Techniques for optical-microwave frequency intercomparison using femtosecond optical frequency combs are being developed and evaluated. These techniques are used to measure optical frequencies relative to the caesium primary frequency standard and allow optical frequency standards to be operated as optical clocks. The potential of installed fibre networks for remote comparison of optical frequency standards at the highest level of accuracy is also being investigated.
Optical Frequency Standards & Metrology research
- Information about research into trapped ion optical frequency standards.
- At NPL we are working on a variety of frequency standards based on cold atoms. Reducing the temperature of the atoms or ion used for frequency metrology allows for long measurement cycles and greatly reduces velocity-related shifts of the clock transition frequency.
- Whilst cold trapped ions or atoms provide the most reproducible frequency references, the more mature technology relies on Doppler-free spectroscopic frequency references in gases or vapours contained in short cells.
- Optical frequency standards are based on atomic spectral features which are noisy for short measurement periods.
- Octave-spanning optical frequency combs allow the direct comparison of optical and microwave frequency standards.
- Information about research into the comparison of optical frequency standards.
- Advances in optical frequency standards and metrology are opening up new horizons in fundamental physics. In particular, they provide powerful tools for scientists to make highly precise measurements of fundamental constants.
- Time variation of the fundamental constants is a manifestation of the violation of Einstein’s Equivalence Principle required by theories uniting gravitation with the strong and electroweak interactions.
- Stabilised diode laser systems based on saturated absorptions in acetylene have been developed at NPL.
- The strontium ion optical frequency standard is based on the electric quadrupole transition at 674 nm in 88Sr+, which has a natural linewidth of 0.4 Hz.
- The singly charged ytterbium ion is unique among optical frequency standards in that the lowest-lying excited state with an estimated lifetime of around six years.
- At NPL we are designing and implementing an optical lattice clock based on the 1S0 → 3P0 transition at 698 nm in neutral strontium.