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The most stable laser in the world

Especially interesting for
  • developers of optical atomic clocks
  • highest-resolution spectroscopy

A laser with a frequency stability so far 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 American NIST (National Institute of Standards and Technology)/JILA Institue of NIST (National Institute of Standards and Technology) in Boulder/Colorado. Their development is important for optical spectroscopy with highest resolution, e. g. of ultra-cold atoms. But, above all, an even more stable interrogation laser is now available for use in optical atomic clocks.

The size of the new silicon resonator compared to the size of a coin

Optical atomic clocks require laser sources that irradiate light with an extremely constant frequency. For this purpose, commercial laser systems have to be stabilized, by means of, e. g., optical resonators. They are composed of two highly reflecting mirrors which are kept at a fixed distance by means of a spacer. In analogy to an organ pipe, the resonator length determines the oscillation frequency. Consequently, a resonator with a high lengthstability is required for a stable laser, i.e. the distance between the mirrors must be kept as constant as possible.

Modern resonator-stabilized laser systems have meanwhile been technically developed to such an extent that their stability is only limited by the thermal noise of the resonators. Similar to the Brownian motion of molecules, the atoms in the resonator are constantly moving and are, thus, limiting its length stability. Up to now, resonators have been made of glass, whose disordered and “soft” material structure, however, rather shows strong movements. For the new resonator, the research group has used single-crystal silicon, a particularly “stiff” and thus low-noise material. In addition, cooled down to a temperature of 124 K (‒149 °C), silicon exhibits an extremely small thermal expansion; the low temperature contributes to additionally reducing the thermal noise. To operate the resonator at this temperature, the researchers had to develop, first of all, a suitable low-vibration cryostat. Comparison measurements with two glass resonators allowed the scientists to demonstrate a frequency stability so far unequalled of 1 ∙ 10–16 for the laser stabilized to the silicon resonator, which corresponds to twice the stability of the best laser in the world.

The “pendulum” of an optical clock, i. e. The new measuring facility allows numerous different measurands to be determined. Contact Christian Hagemann Department 4.3 Quantum optics and Unit of Length Phone: +49 (0)531 592-4357 E-mail: christian.hagemann@ptb.de Scientific publication Kessler, T.; Hagemann, C.; Grebing, C.; Legero, T.; Sterr, U.; Riehle, F.; Martin, M.J.; Chen, L.; Ye, J.: A sub- 40-mHz-linewidth laser based on a silicon single-crystal optical cavity. Nature Photonics 6 (2012) 687–692 the swinging system of an optical clock, 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. This value could not be reached by glass resonators due to their limited length stability.

The laser to which the silicon resonator is stabilized reaches a linewidth of less than 40 mHz and can, thus, read out significantly narrower atomic lines. This contributes to improving the stability and accuracy of optical atomic clocks – the objective being an order of magnitude each. And also optical precision spectroscopy, another focal point of research of the Excellence Cluster QUEST, can receive decisive impetus.

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

Scientific publication

Kessler, T.; Hagemann, C.; Grebing, C.; Legero, T.; Sterr, U.; Riehle, F.; Martin, M.J.; Chen, L.; Ye, J.: A sub- 40-mHz-linewidth laser based on a silicon single-crystal optical cavity. Nature Photonics 6 (2012) 687–692