Effective reflection of light for quantum technology

NBI researchers have developed a new method where they create a very strong interaction between light and atoms, which means that the light can be controlled and reflected on a glass fibre. The results are published in the scientific journal, Physical Review Letters.

The experiments are carried out in a glass chamber with an ultra thin optical glass fibre stretched across it. The optical glass fibre has a diameter of 500nm – that is 1000 times smaller than the diameter of a strand of hair. In the glass chamber there is also a gas of caesium atoms, which is cooled down to 50 micro-degrees Kelvin, which is almost absolute zero at -273°C. Due to the ultracold temperature, the caesium atoms are almost motionless and they are held close to the surface of the glass fibre. Using laser light, the researchers can push the individual atoms a bit so that they are evenly spaced along the surface of the glass fibre.

Mirror effect

“We now send laser light through the glass fibre. The light has a particular wavelength and when the fibre is thinner than the wavelength of the light, the light moves along the surface of the fibre, where the atoms sit in a row. When the light hits the first atom, a strong interaction is created between the light and the atom and the atom moves with the light wave. With the atom’s precise distance, which matches the wavelength of the light, you get a backwards reflection of the light,” explains Jürgen Appel, associate professor in the research group Quantop at the Niels Bohr Institute at the University of Copenhagen.

He explains that it is this backward reflection that is so important. When the light hits the next atom in the row the same thing happens – and the next, and the next. For every time the light hits an atom, a small part of the light is reflected and sent backwards.

“We have managed to divert more than 10% of the light. With only 1000 atoms, an interaction is created that is just as strong as a glass plate with billions of atoms. We have created a mirror that effectively reflects light and we can even turn it on and off,” says Jürgen Appel.

Using lasers, the physicists have arranged the atoms along the glass fibre with distances that precisely match the wavelength of the light transmitted through the glass fibre. With only 1000 atoms, they have thus achieved a mirror effect that reflects and sends more than 10 percent of the light back.

The result of the experiment is a step on the road to controlling light for future quantum technology. Such an on/off mirror based on just a few atoms can be used to improve the interaction between individual atoms and the fibre-guided light and it can be developed to improve entangled quantum states in connection with captured atomic systems for future quantum technology. The research was conducted in the group’s quantum optics laboratories at the Niels Bohr Institute in Copenhagen.