Optical Frequency StandardsNew types of atomic clock operating at optical rather than microwave frequencies promise significant advances in both science and technology and may lead to a future redefinition of the second.
The next generation of atomic clocks will operate at optical rather than microwave frequencies.
All other things being equal, the stability of an atomic clock is proportional to its operating frequency. Visible light has a frequency roughly five orders of magnitude higher than that of microwaves. This means that clocks based on narrow atomic absorptions at optical, rather than microwave, frequencies should be much more stable. They also have the potential to achieve higher accuracy.
The heart of an optical atomic clock is a highly stable reference frequency provided by a narrow optical absorption in an atom or ion. NPL is developing optical frequency standards based on transitions in single trapped ions of strontium and ytterbium, and neutral strontium atoms confined in an optical lattice.
Optical atomic clocks have many potential applications. These range from improved satellite navigation systems and better tracking of deep space probes to sensitive tests of fundamental physical theories. In future they could even lead to the SI unit of time, the second, being redefined.
Although cold trapped ions or atoms provide the most reproducible frequency references, lower accuracy frequency references based on lasers stabilised to absorbers in gas or vapour cells are suitable for many applications.
NPL uses iodine-stabilised helium-neon lasers for the practical implementation of the SI unit of length, the metre. We are also developing compact and portable standards in the 1.5 micrometre region for telecommunications applications.
- 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.