Optical Frequency Standards & Metrology
New types of atomic clock operating at optical rather than microwave frequencies promise significant advances in both science and technology.
All other things being equal, the stability of an atomic clock is proportional to its operating frequency. Visible light has a frequency roughly 100 000 times higher than that of microwaves. This means that clocks based on narrow absorptions at optical, rather than microwave, frequencies should be much more stable.
Optical clocks have many potential applications. These range from improved satellite navigation systems and better tracking of deep space probes to sensitive tests of general relativity and measurements of fundamental physical constants. In future they could even lead to the SI unit of time, the second, being redefined.
The heart of an optical clock is a highly stable reference frequency provided by a narrow optical absorption in an atom or ion. At NPL we are developing clocks based on transitions in single trapped ions and neutral atoms confined in an optical lattice.
- 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.
- NPL offers a routine service for the verification of interferometer system accuracy.
- Information about NPL's beat frequency monitoring systems, including hardware and software.
- Find out more about NPL's iodine-stabilised HeNe reference lasers.
- The NPL low drift Fabry-Perot etalon has been designed to provide the best possible frequency stability.
- NPL has many years of experience with laser stabilisation as part of its programme of research into optical frequency (wavelength) standards for use in interferometry.
- Information about NPL's Optical Frequency metrology and standards training services.
- The SI unit of length is the metre. Since 1983 the metre has been defined as “the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.”
- The most accurate optical frequency measurements are limited by the caesium primary standard itself, raising the prospect of an optical redefinition of the second in the future.