National Physical Laboratory

Optical Frequency Standards 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.

Optical Frequency Standards research