One more step towards a quantum leap in computing

The research is carried out in the underground quantum optics laboratory in the basement of the NBI in Copenhagen, set back from the road so that vibrations from traffic do not affect results. Here, the researchers have designed experiments in which they can perform ultrasensitive trials with quantum optics.

The experiment is carried out in a glass cell with very low pressure, pictured above, in which there is an ultra-thin glass fibre and a gas of caesium atoms.

“We have an ultra-thin glass fibre with a diameter of half a micrometre (a hundred times smaller than a strand of hair). Along this glass fibre we capture caesium atoms. They are cooled down to 100μK 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 fibre,” explains Jürgen Appel, Associate Professor, Quantop Research Group, the Niels Bohr Institute, University of Copenhagen. “When light is transmitted through the glass fibre thread, the light will also move along the surface because the fibre is thinner than wavelength of the light. This creates strong interaction between the light and the atoms sitting securely above the surface of the fibre.”

The Niels Bohr Institute has developed a method whereby the number of atoms on the glass rod can be measured. Two laser beams with different frequencies are sent through the glass fibre. If there were no atoms on the fibre, 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 the atoms’ absorption lines, the number of atoms along the fibre can be measured. The NBI has shown that it can hold 2,500 atoms with an uncertainty of just eight atoms.

“These are fantastic results. 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,” says Appel, explaining that with this method, the NBI can measure and control the atoms so that only 14% are kicked out of the trap and are 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 so-called entangled states of atoms along the fibre. Such an entangled system with strongly interacting atoms and light is of great interest for future quantum computer networks.”