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Narrower Lasers Could Yield Better Atomic Clocks

Photonics.com
Mar 2015
COPENHAGEN, Denmark, March 10, 2015 — Atomic clocks lose 1 second every 300 million years, but that level of accuracy isn’t good enough for researchers at the Niels Bohr Institute.

For even better precision, they’ve devised a new way to narrow the linewidth of the red lasers that excite the electrons that function as their clock’s pendulum.

Typical resonators used to stabilize lasers still produce some noise because of the vibrations of the atoms that make up their mirrors.

Bjarke Takashi Rojle Christensen
Doctoral student Bjarke Takashi Rojle Christensen works in the quantum optics laboratory at the Niels Bohr Institute. Courtesy of Ola Jakup Joensen/NBI.


Instead of trying to further stabilize the mirrors, the researchers decided to put a new medium between the mirrors. They chose ultracold strontium because it is a very “demanding” element requiring a very specific wavelength in order to react with the light.

“This makes the system sensitive to velocity-dependent multiphoton scattering events (Dopplerons) that affect the cavity field transmission and phase,” the researcher wrote in Physical Review Letters (doi: 10.1103/PhysRevLett.114.093002).

The approach could lead to sub-100-mHz laser stabilization, as well as superradiant laser sources, the researchers said.

Laser resonator augmented with ultracold atoms
Laser light is sent through a resonator consisting of two mirrors to filter out undesired wavelengths. A vacuum chamber containing ultracold atoms is placed in between the two mirrors and acts as a further frequency filter. As a result, the laser can be stabilized much better than with just the empty resonator. Courtesy of Bjarke Takashi Rojle Christensen/Niels Bohr Institute.


“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,” said professor Dr. Jan Thomsen. “So we have developed a technique that can create an ultraprecise laser beam using a quantum frequency filter.”

The atomic clock itself is also made up of strontium cooled to near absolute zero with a blue laser. The rate at which the clock ticks is determined by how quickly electrons excited by the red laser jump from one orbit to another and then relax back down.

A more precise atomic clock could have applications in navigation and space-based optical technology, the researchers said.

For more information, visit www.ku.dk.


GLOSSARY
laser cooling
A process and method by which manipulation and orientation of a given number of directed laser beams decreases the motion of a group of atoms or molecules such that their internal thermodynamic temperatures reach near absolute zero.  The !997 Nobel Prize in Physics was awarded to Steven Chu, Claude Cohen-Tannoudji and William D. Phillips for the development of methods to cool and trap atoms with laser light. 
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