Optical grid (quantum optics)

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Schematic two-dimensional representation of the potential curve (gray) of an optical lattice with a random distribution of atoms (red)

An optical grating (English technical term optical lattice ) referred to in the quantum optics , a spatially periodic pattern of laser radiation , in which atoms or molecules can be trapped.

functionality

The structure of the optical grating is created by the interference of laser light. With a suitable choice of the laser parameters, a standing wave occurs which, due to the Stark shift, causes a periodic potential for atoms . The underlying principle is identical to that of the optical tweezers : the laser light induces an electrical dipole moment in each of the atoms , the interaction of which with the light results in a force on the atom. Depending on the sign of the detuning of the laser light with respect to the atomic transition frequency , the atoms are drawn into the nodes (intensity minima) or bellies (intensity maxima) of the standing wave. The exact geometry of the generated potential depends on the arrangement of the laser beams and the resulting complexity of the interference pattern.

Band structure

The periodic potential changes the dispersion relation for the motion of the atoms according to Bloch's theorem . The result is a band structure analogous to the band structure of electrons in crystals. With the geometry of the interference pattern, this band structure can in principle also be tailored. In contrast to solid-state systems, it is also possible in optical lattices to change the potential depth and thus the band structure dynamically (i.e. while the atoms are in it).

Consequences

If there is a sufficiently strong interaction between the atoms, the band structure allows the formation of dark (hole-like) and light (particle-like) solitons , since the interaction can, under certain circumstances, precisely compensate for the dispersion. An external force, e.g. B. gravity, the atoms in the optical lattice react with Bloch oscillations , which can be measured extremely precisely in these systems.

observation

Most of the time the atoms are not observed in the optical lattice, but after switching off the light potential and a certain flight time. The absorption of a laser beam that illuminates the atoms is registered on a CCD camera . The methodology is comparable to the detection of Bose-Einstein condensates . In this way one can generally measure the quasi- momentum distribution, but not directly the spatial distribution of the atoms.

In particular, it is difficult to observe individual grid locations , since in extreme cases they are only half a light wavelength apart. Therefore, one has to struggle with the diffraction limitation of the optical resolution in the optical observation of individual grid locations . In 2008, however, several research groups succeeded in imaging individual lattice sites in an optical lattice and - partly in real time and with a detection sensitivity sufficient to detect individual atoms - to track their movement. In addition, a method has been developed with the scanning electron microscopy is used and individual atoms by ionization with an electron beam can prove that extends much sharper focus can.

application

A two-dimensional optical lattice with one atom in each well

If the depressions of a three-dimensional optical lattice are filled with one atom each, it has many properties of crystals . Such optical gratings have the advantage over the systems known from solid state physics that their parameters can be easily changed by the laser light used. They can therefore be used as model systems for problems from solid state physics and are considered promising candidates for the realization of a quantum computer .

In addition to trapped ions, atoms in optical lattices are promising candidates for the realization of even more precise atomic clocks , so-called lattice clocks.

Individual evidence

  1. ↑ T. Gericke et al .: High-resolution scanning electron microscopy of an ultracold quantum gas . In: Nature physics . 2008, doi : 10.1038 / nphys1102 ( Ulm University [PDF; 669 kB ]).

literature

  • Oliver Morsch, Markus Oberthaler: Dynamics of Bose-Einstein condensates in optical lattices . In: Reviews of Modern Physics . tape 78 , no. 1 , February 27, 2006, p. 179-215 , doi : 10.1103 / RevModPhys.78.179 .
  • Immanuel Bloch: Ultracold quantum gases in optical lattices . In: Nature Physics . tape 1 , no. 1 , 2005, p. 23-30 , doi : 10.1038 / nphys138 .
  • H.-J. Briegel, T. Calarco, D. Jaksch, JI Cirac, P. Zoller: Quantum computing with neutral atoms . In: Journal of Modern Optics . tape 47 , no. 2–3 , 2000, pp. 415-451 , doi : 10.1080 / 09500340008244052 , arxiv : quant-ph / 9904010 .

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