Atoms can absorb and re-emit light - it's an everyday phenomenon. In most cases, though, atoms emit light particles in all possible directions - so recapturing such photons is quite difficult. Scientists have developed "quantum ping pong": using a special lens, two atoms can bounce a photon back and forth with high precision.
Now, a research team from the Technical University of Vienna in Vienna, Austria, has theoretically demonstrated that, using a special lens, it is possible to ensure that a single photon emitted by one atom is reabsorbed by a second atom. However, the second atom not only absorbs the photon, it returns it directly to the first atom. In this way, atoms can pass photons to each other accurately again and again - just like playing ping pong.
"If an atom emits a photon somewhere in free space, the direction of its emission is completely random. This makes it practically impossible to get this photon captured again by another distant atom," said Professor Stefan Rotter from the Institute of Theoretical Physics at TU Vienna. "The photon travels as a wave, which means no one can say exactly which direction it travels. So whether the light particle is reabsorbed by a second atom is purely a matter of chance."
The situation would be different if the experiment were conducted not in free space but in a closed environment. A similar situation occurs with the so-called "whispering corridor" in acoustics: if two people are in an oval room and stand exactly at the focus of the oval, they can hear each other clearly even if they just whisper. After the sound waves are reflected off the oval wall, they meet again exactly where the second person is standing - so this person can hear the quiet whisper perfectly.
"In principle, you could build something similar for light waves by positioning two atoms at the foci of an ellipse," said Oliver Diekmann, first author of the paper. "But in practice, the two atoms have to be positioned very precisely at these foci."
Therefore, the research team came up with a better strategy based on the fisheye lens concept proposed by James Clerk Maxwell, the founder of classical electrodynamics. Fisheye lenses consist of a spatially varying refractive index. Light travels in a straight line in a uniform medium such as air or water, but in a Maxwell fisheye lens the light travels in a curved direction.
"In this way, it is ensured that all light rays emitted from an atom take a curved path to the edge of the lens, are subsequently reflected, and then take another curved path to the target atom. In this case, the effect is much more efficient than a simple ellipse, and deviations from the ideal position of the atom are less harmful," explains Oliver-Dickmann.
"The light field in this Maxwell fisheye lens consists of many different oscillation modes. This is reminiscent of the different harmonics produced simultaneously when playing a musical instrument," says Stefan Rotter. "We were able to show that the coupling between atoms and these different oscillation modes can be tuned in such a way that photons are almost certainly transferred from one atom to another - quite unlike what is the case in free space."
Once an atom absorbs a photon, it remains in a higher energy state until it emits the photon again a short time later. Then the game starts over: the two atoms swap roles, and the photon returns from the receiving atom to the original sending atom, and so on.
So far, this effect has been demonstrated in theory, but with today's technology it can be tested in practice. "In practice, it is possible to use not only two atoms, but also two groups of atoms, thereby further increasing the efficiency. This concept could be an interesting starting point for quantum control systems to study the effects of extremely intense light-matter interactions," says Stefan Rotel.
Compiled source: ScitechDaily