Theoretically, a laser (a device which emits light through optical amplification) has a fixed color, frequency or wavelength. In reality, however, lasers always have a certain linewidth, and researchers have just developed a laser with a linewidth of 10 mHz, establishing a new world record in the process.

The precision of this laser is useful for a number of applications, such as optical atomic clocks, precision spectroscopy, radioastronomy and for testing the theory of relativity. We already use lasers in numerous ways, such as in medicine and information technologies, but the precision of this new laser will allow for more exact experiments.

Most lasers range from a few kHz to a few MHz in width, which is a large range for experiments that require high precision and accuracy of linewidth. Recent research, therefore, has focused on developing lasers with greater frequency stability (or wavelength frequency) and a narrower linewidth.

The new laser, which was developed after 10 years of research at the Physikalisch-Technische Bundesanstalt (PTB) in Germany in collaboration with US researchers from JILA, has a linewidth of 10 mHz (0.01 Hz). "The smaller the linewidth of the laser, the more accurate the measurement of the atom's frequency in an optical clock," said PTB physicist Thomas Legero, adding that the new laser will allow scientists to improve the quality of their optical clocks.

Optical clocks are a type of atomic clock, which are devices that use an electronic transition frequency in the microwave, optical or ultraviolet region of the electromagnetic spectrum as a frequency standard for its timekeeping element. Atomic clocks, therefore, are the most accurate time and frequency standards known, and are used for international time distribution services, controlling the wave frequency of televesion broadcasts, and GPS, among other applications.

The core piece of the lasers is a 21-cm long Fabry-Perot silicon resonator, which consists of two highly reflective mirrors placed opposite each other and kept at a fixed distance by means of a double cone. Similar to organ pipes, the length of the resonator determines the frequency of the wave oscillating inside the resonator. "The laser's frequency stability -- and thus its linewidth -- then depends only on the length stability of the Favry-Perot resonator," according to Science Daily.

The new laser is now being used at both PTB and JILA to further improve the quality of atomic clocks and test new precision measurements on ultracold atoms. "In the future, it is planned to disseminate this light also within a European network," said Legero. "This plan would allow even more precise comparisons between optical clocks in Braunschweig and the clocks of our European colleagues in Paris and London," he added. A similar plan is in place to connect the lasers between JILA and various NIST labs across a fiber network in the U.S.

It is possible that in the future, further lowering of the temperature of the resonator and thus the reduction of thermal noise, combined with novel crystalline mirror layers, might result in even sharper lasers, with a linewidth smaller than 1 mHz.