Strontium filter stabilises light for atomic clocks

It is challenging to obtain the required ultra-precise laser light frequency - there will always be a little ‘noise’. Now, researchers from the Niels Bohr Institute, University of Copenhagen, have developed a method that reduces the noise so that it is up to 100 times quieter. The results are published in the scientific journal Physical Review Letters.

The atoms in the atomic clock are made up of strontium gas, kept in a vacuum chamber. Using magnetic fields and precise beams of blue laser light, the atoms are cooled down to near absolute zero, -273°C, where they are maintained.

The electrons are located in certain orbits around the nucleus and each orbit has one energy level. By flashing the strontium atoms with red laser light, the electrons take on a higher energy level and jump from one orbit to the next, but they immediately jump right back to their normal orbit. When you then shine the light on the strontium atoms, the electrons keep jumping back and forth in a classical sense, and this constitutes the ‘pendulum’ in the atomic clock.

An atomic clock is now so precise that it only loses one second every 300m years, but scientists at the NBI are working to make it even more precise. This has great potential, especially for navigation and space based optical technology, for exploration of the universe. The problem with making clocks more precise is controlling the laser light, so that the light has exactly the right wavelength and makes the electrons oscillate very precisely and very accurately.

“The laser light is stabilised, but it fluctuates a litttle, and creates ‘noise’. Since there are several wavelengths at the same time due to the noise, we send the light via a mirror to a ‘resonator’, which is two mirrors joined together so that it allows some waves to pass, while the rest disappear. So it is a sorting mechanism so that the laser light wavelengths become more precise. So, everyone should be happy, but the mirrors fluctuate slightly – simply because the atoms in the mirror vibrate and this puts some limitations on the stability, that we could not get rid of. So we said – why don’t we try to change our mindset and turn the whole thing upside down,” explained Jan Thomsen, Associate Professor and Head of the Ultra Cold Atoms Research Group, Niels Bohr Institute.

And so they turned it all upside down. Instead of trying to further stabilise the mirrors, they decided to completely ignore the vibrations. They decided to put ‘something’ between the laser light and the resonator’s two mirrors. This ‘something’ would act as a filter.

The filter consisted of a vacuum chamber with ultra cold strontium atoms between the two mirrors. Strontium is a very ‘demanding’ atom, which must have a very specific wavelength in order to react with the light. The light is now sent back and forth between the two mirrors and even though the two mirrors vibrate a little due to the temperature in the room, the light does not care, because it is primarily the cold atoms that sort the wavelengths.

“The method is simple, but effective and the result is that the laser beam is much more precise and stable and the noise is reduced by up to 100 times. So we have developed a technique that can create an ultra-precise laser beam using a quantum frequency filter,” explains Jan Thomsen, who points out that the technique could be used to make atomic clocks more precise than until now and in a much simpler way than before.