Atomic Clocks Get Boost From The Most Stable Laser In The World
September 15, 2012

Scientists Create Most Stable Laser In The World

April Flowers for -- Your Universe Online

An international team of scientists has developed a laser with a frequency stability previously unequalled.

This is the result of a research cooperation of the Physikalisch-Technische Bundesanstalt (PTB) within the scope of the Excellence Cluster QUEST (Centre for Quantum Engineering and Space-Time Research) with colleagues from the National Institute of Standards and Technology/JILA.

Their development, published in the journal Nature Photonics, is important for optical spectroscopy with highest resolution. Moreover, an even more stable interrogation laser is now available for use in optical atomic clocks.

Laser sources that radiate light with an extremely constant frequency are needed for optical atomic clocks. Commercially available laser systems are not suited to this purpose without modifications. However, this can be achieved by stabilizing them, for example, with the aid of optical resonators. Optical resonators are composed of two highly reflecting mirrors, which are kept at a fixed distance by means of a spacer. The resonator length determines the frequency with which light can begin to oscillate in the resonator, making this length a decisive aspect of the resonator. Consequently, a resonator with high length stability is required for a stable laser; the distance between the mirrors must be kept as constant as possible.

The technical development of modern resonator-stabilized laser systems is at such an extent that their stability is only limited by the thermal noise of the resonators. Similar to the Brownian motion of molecules (the seemingly random motion of particles suspended in a fluid as they are bombarded by other molecules), the atoms in the resonator are constantly moving and are, thus, limiting its length stability.

Until now, resonators have been made of glass because of its disordered and "soft" material structure which shows particularly strong movements. For the new resonator, the research group has used single-crystal silicon, a particularly "stiff" and thus low-noise material. Cooled down to a temperature of 124 Kelvin (-149 degrees Celsius), silicon is characterized by an extremely small thermal expansion, and the remaining thermal noise is also reduced. To operate the resonator at this temperature, the researchers had to design a suitable low-vibration cryostat. Comparison measurements with two glass resonators allowed the scientists to demonstrate a previously unequalled frequency stability of 1 • 10-16 for the laser stabilized to the silicon resonator.

This development removes an important obstacle in the development of more precise atomic clocks, since the stability of the lasers used is a critical point. The pendulum is a narrow optical absorption line in an atom or ion, whose transition frequency is read out by a laser. The linewidth of these transitions typically amounts to a few millihertz, a value which could not be reached by glass resonators due to their limited length stability.

Reaching this value is now possible with the silicon resonator stabilized laser that reaches a linewidth of less than 40 mHz. This opens a new dimension in the development of optical atomic clocks.

This work could also benefit optical precision spectroscopy, another focal point of research of the Excellence Cluster QUEST.

"For the future, there is still room to improve the optical mirrors whose thermal noise limits the achievable stability", explains PTB physicist Christian Hagemann. Therefore, the researchers will in future go down to even lower temperatures and use novel highly reflecting structures to improve the frequency stability by another order of magnitude.