Atoms Under Control

The scientists report in the journal Nature Physics that they were able to cool single rubidium atoms in every direction of motion using a sophisticated array of lasers in an optical resonator, and keep them there on average for 17 seconds. This is, by far, the longest storage time ever reached in a strongly coupled atom-resonator system.

Trapping, cooling, and storing neutral atoms requires an elaborate process. As a first method we have, now almost classic, laser cooling in a "magneto-optical trap". Atoms are shot from six directions with laser beams whose frequency lies somewhat below their excitation energy. In this way, the particles only absorb light when they move towards a beam - because of the Doppler effect, they are in resonance - and are then slowed down in that direction. We call it cooling, when an individual atom or molecule has more and more motional energy taken from it.

In the experiment at the Max Planck Institute of Quantum Optics, rubidium atoms are prepared in this way and finally, in the electromagnetic field of a laser beam (acting as a "light trap"), led over a distance of 14 millimetres into an optical resonator made of two opposite concave mirrors of the very highest quality.

As soon as the atoms are between the mirrors, the scientists change the geometry of the light trap, reflecting the laser beam back onto itself. Doing this, a standing light wave builds and atoms are held in the trap that is formed by its antinodes. In addition, two lasers, traveling in opposite directions and set at an angle of 45 degrees to the standing wave and 90 degrees to the resonator axis, hit the atoms (see image)

In this special setup, a number of cooling mechanisms are at work. Atom, resonator, and light trap together make up a strongly coupled system in which an excited atom prefers emitting photons in the direction of the resonator axis. In this way, a light field builds up between the two mirrors which is extremely dependent on the position of the atom. That is because the position of the atom determines the strength of its coupling to the resonator as well as the exact frequency of the atomic transition, because the energy levels of the atom shift in the light trap. If the atom moves, the light field adjusts to the new situation, but with a delay which depends on the average storage time of the photons in the resonator. Because of this delay, all the light forces which affect the atom and slow it down, depend on its speed. So, borrowing a term from mechanics, these forces are called friction forces.

Cooling effects particularly appear when the frequency of the resonator is somewhat greater than the frequency of the exciting laser. In this case, the atom emits photons of a higher energy, with preference into the direction of motion. The recoil the atom experiences due to this process slows it down along the axis of the resonator. The absorption of photons occurs preferentially when the atom moves against the laser beam. This leads to a slowing down in the direction of the laser. Both effects are due to the Doppler effect, mentioned above.

Further friction forces act along the direction of the light trap. They are provoked, first of all, because the light field - as mentioned above - reacts with a delay to the movement of the atom. Second of all, the transition energy of the atom is higher in the antinodes of the light trap than in the nodes. The frequency of the atomic transition is the same as that of the exciting laser only at the nodes. A warm atom is more often near a node and is excited there. If it moves away from the node, it gains potential energy, which looses during the transfer into the ground state. This process is repeated periodically and is called, in analogy to the ancient myth, "Sisyphus cooling."

To detect the atom and determine its storage time, the photons that it scatters into the resonator are counted. A few milliseconds after turning on the "light trap" the count rate increases significantly, because an atom enters the space between the mirrors that is initially relatively "hot". Within 100 microseconds the atom is cooled and reaches its final temperature, about six millionths of a Kelvin above absolute zero. It then scatters a very small amount of light, but with a constant rate.

The scientists investigated systematically how the various cooling forces had an effect on the storage times of the atoms. Atoms without cooling were held on average only 2.7 seconds, but a Sisyphus-cooled atom stayed in the resonator for 17 seconds. If one chooses the appropriate frequency for the resonator and the exciting laser, the lifetime of the stored atom increases by a factor four. In this way it could even be possible to hold a single atom for more than one minute in an optical resonator.

Using this trick of combining cooling methods which act along different directions, the researchers were able to prepare an exactly known number of atoms in the centre of an optical resonator. The storage times, on average more than 15 seconds, allow experiments in which the interaction of individual atoms with individual photons can be controlled. This is, for example, a pre-condition for the entanglement, coupling, and teleportation of quantum states between very distant atoms with the help of photons. In this way, the researchers have taken a tangible step toward creating a distributed quantum computer made of a number of strongly coupled atom resonator systems. The trapped atoms store the quantum bits, while the photons they emit carry out the computing operations.