Light forces in high-Q cavities

Freezing particles in very small regions in space is the prerequisite for any investigation (like spectroscopy) or manipulation (like quantum computation). As photons carry energy and momentum, absorption, emission, and coherent scattering of light creates forces on the involved particles. For light enclosed between two highly reflective mirrors, i.e. an optical resonator, these forces are strongly enhanced. They can be tailored to control the quantum motion of the particles. This allows to trap atoms and molecules and cool them to temperatures very close to the absolute zero.

Schrödinger cats of light

Cats of Light

In an optical microresonator built by supermirrors (99.999% of the light is reflected) a single atom can be trapped by a single photon. At the same time the atomic motion can be monitored with a precision better than the optical wavelength. In the quantum world the cavity photon gets entangled with the atom. This facilitates genuine quantum light sources like single photon guns or traveling Schrödinger cats of light. As two atoms within one resonator see the same photons, this entanglement allows to implement quantum gates between Q-bits stored in atoms.

[1] K. M. Gheri, H. Ritsch, Single-atom quantum gate for light, Phys. Rev. A 56, 3187 (1997).

[1] P. Domokos, H. Ritsch, Mechanical effects of light in optical resonators, Journal of the Optical Society of America B 20, 1098 (2003).

[1] H. Ritsch, G. J. Milburn, T. C. Ralph, Deterministic generation of tailored-optical-coherent-state superpositions, Phys. Rev. A 70, 33804(2004).

Turbo laser cooling with a cavity

Turbo Cooling

Cavities allow for very efficient laser cooling without spontaneous emission. This is facilitated by the mirror induced accelerated decay, which carries away energy. Fortunately this method works for molecules as well, where no alternative laser cooling scheme exists. This opens a road to study quantum physics of ultracold molecules at low densities. A cavity cooler bears also great potential as a loading device for a continuously operating atom laser based on Bose Einstein condensation.

[1] P. Horak et. al., Phys. Rev. Lett. 79, 4974 (1997).

Crystallization of atoms bound by light

Crystallization of atoms

If a polarizable particle like an atom or a molecule is illuminated by laser light, it will scatter the laser light with a characteristic pattern in many directions, acting like a tiny mirror. The light scattered by different particles close to each other will interfere and generate an interference pattern in space. Within a resonator this light field pattern can be so strongly enhanced by superradiance, that it pushes the atoms to the intensity maxima of the pattern. As the scattering itself is maximum at these points, one gets a further enhancement of the scattering and the atoms tend to form a regular crystal-like structure with lattice constant of the size of a wavelength.

[1] P. Domokos and H. Ritsch, Phys. Rev. Lett. 89, 253003 (2002).

[1] Stefano Zippilli, Giovanna Morigi, and Helmut Ritsch, Phys. Rev. Lett. 93, 123002 (2004).

The smallest possible laser

Atom Laser

A single inverted atom in an optical microresonator forms the smallest possible laser system as a coherent source of light. The laser light generated by the atom through stimulated emission exhibits strong forces which in turn provide for self-trapping and cooling of the atom. Due to increased gain, this turbo fridge works even better when more atoms are involved. The atoms arrange in a periodic pattern and cool themselves to very low temperatures within very short times. Output coupling of ultracold atoms and simultaneously replenishing hot atoms generates a continuous beam of coherent atoms. This opens the way to combine optical lasing and atom lasing in a single device.

[1] T. Salzburger and H. Ritsch, Phys. Rev. Lett. 93, 063002(2004).