Scientists have developed world’s sharpest laser that can travel for 2 million miles (3.3 million kilometers) without faltering.
Researchers from the Physikalisch-Technische Bundesanstalt (PTB) and JILA, a joint institute of the National Institute of Standards and Technology (NIST) and the University of Colorado, Boulder, have established world’s sharpest laser that establishes a new world record with a linewidth of only 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. This new laser will enable us to decisively improve the quality of our clocks”, PTB physicist Thomas Legero explains.
In addition to the new laser’s extremely small linewidth, Legero and his colleagues found out by means of measurements that the emitted laser light’s frequency was more precise than what had ever been achieved before.
Although the light wave oscillates approx. 200 trillion times per second, it only gets out of sync after 11 seconds. By then, the perfect wave train emitted has already attained a length of approx. 3.3 million kilometers. This length corresponds to nearly ten times the distance between the Earth and the moon.
Since there was no other comparably precise laser in the world, the scientists working on this collaboration had to set up two such laser systems straight off. Only by comparing these two lasers was it possible to prove the outstanding properties of the emitted light.
The core piece of each of the lasers is a 21-cm long Fabry-Pérot silicon resonator. The resonator consists of two highly reflecting mirrors which are located opposite each other and are kept at a fixed distance by means of a double cone. Similar to an organ pipe, the resonator length determines the frequency of the wave which begins to oscillate, i.e., the light wave inside the resonator. Special stabilization electronics ensure that the light frequency of the laser constantly follows the natural frequency of the resonator. The laser’s frequency stability – and thus its linewidth – then depends only on the length stability of the Fabry-Pérot resonator.
The scientists at PTB had to isolate the resonator nearly perfectly from all environmental influences which might change its length. Among these influences are temperature and pressure variations, but also external mechanical perturbations due to seismic waves or sound. They have attained such perfection in doing so that the only influence left was the thermal motion of the atoms in the resonator. This “thermal noise” corresponds to the Brownian motion in all materials at a finite temperature, and it represents a fundamental limit to the length stability of a solid. Its extent depends on the materials used to build the resonator as well as on the resonator’s temperature.
For this reason, the scientists of this collaboration manufactured the resonator from single-crystal silicon which was cooled down to a temperature of -150 °C. The thermal noise of the silicon body is so low that the length fluctuations observed only originate from the thermal noise of the dielectric SiO2/Ta2O5 mirror layers. Although the mirror layers are only a few micrometers thick, they dominate the resonator’s length stability. In total, the resonator length, however, only fluctuates in the range of 10 attometers. This length corresponds to no more than a ten-millionth of the diameter of a hydrogen atom. The resulting frequency variations of the laser therefore amount to less than 4 × 10–17 of the laser frequency.
The new lasers are now being used both at PTB and at JILA in Boulder to further improve the quality of optical atomic clocks and to carry out new precision measurements on ultracold atoms. At PTB, the ultrastable light from these lasers is already being distributed via optical waveguides and is then used by the optical clocks in Braunschweig.