In order to develop future quantum computer networks, it is necessary to hold a known number of atoms and read them without them disappearing. To do this, researchers from the Niels Bohr Institute at the University of Copenhagen have developed a method with a trap that captures the atoms along an ultra-thin glass fiber, where the atoms can be controlled. The results have been published in the scientific journal Physical Review Letters.

The research is carried out in the quantum optics laboratory in the basement of the institute. The underground laboratory is set back from the road to prevent vibrations from traffic. Here, the researchers have designed experiments in which they can perform ultrasensitive trials with quantum optics.

“We have an ultra-thin glass fiber with a diameter of half a micrometer—a hundred times smaller than a strand of hair,” said Jürgen Appel, Ph.D., associate professor in the research group Quantop at the Niels Bohr Institute. “Along this glass fiber, we capture cesium atoms. They are cooled down to 100 microkelvin using a laser. This is almost absolute zero, which is equivalent to -273°C. This system acts like a trap that holds the atoms on the side of the glass fiber.”

Atoms and Light Linked Together

When light is transmitted through the glass fiber, the light will also move along the surface because the fiber is thinner than the wavelength of the light. This creates a strong interaction between the tightly confined light and the atoms sitting securely above the surface of the fiber.

“We have developed a method where we can measure the number of atoms,” said Appel. “We send two laser beams with different frequencies through the glass fiber. If there were no atoms on the fiber, the speed of light would be the same for both light beams. However, the atoms affect the two frequencies differently, and by measuring the difference in the speed of light for the two light beams on each side of an atom absorption line, you can measure the number of atoms along the fiber. We have shown that we can hold 2,500 atoms with an uncertainty of just eight atoms.”

According to Appel, without this method, you would have to use resonant light (light that the atoms absorb) and then you would scatter photons, which would kick the atoms out of the trap. Appel explains that with this new method, the researchers can measure and control the atoms so that only 14% are kicked out of the trap and lost.

“Our resolution is only limited by the natural quantum noise—the laser light’s own minimal fluctuations—so our method could be used for preparing so-called entangled states of atoms along the fiber,” said Appel. “Such an entangled system with strongly interacting atoms and light is of great interest for future quantum computer networks.”

 For more information, visit quantop.nbi.ku.dk or http://physics.aps.org.