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( Optical lattice)
An optical lattice is formed by the interference of counterpropagating laser beams, which creates a periodic (in space) intensity pattern. The resulting periodic potential can then be used to trap neutral atoms via the Stark shift. Atoms are cooled and congregated in the potential minima. The resulting system of trapped atoms resembles a crystal in the sense that the atoms are in a periodical potential.[1] Because of quantum tunneling, atoms can move in the optical lattice even if the well depth of the lattice is higher than the energy of the atoms, which is similar to the electrons in a conductor. However, there will be a superfluid–Mott insulator transition[2] if the interaction energy between the atoms becomes larger than the hopping energy when the well depth is very large. In the Mott insulator phase, atoms will be trapped in the potential minima and cannot move freely, which is similar to the electrons in an insulator. Atoms in an optical lattice provide an ideal quantum system where all parameters can be controlled. Thus they can be used to study effects that are difficult to observe in real crystals. They are also promising candidates for quantum information processing.[3] There are two important parameters of an optical lattice the well depth and the periodicity. The well depth of the optical lattice can be tuned in real time by changing the power of the laser, which is normally controlled by an AOM (acousto-optic modulator). The periodicity of the optical lattice can be tuned by changing the wavelength of the laser or by changing the relative angle between the two laser beams. The real-time control of the periodicity of the lattice is still a challenging task. Because the wavelength of the laser cannot be varied over a large range in real time, the periodicity of the lattice is normally controlled by the relative angle between the laser beams.[4] However, it is difficult to keep the lattice stable while changing the relative angles, since the interference is sensitive to the relative phase between the laser beams. Recently, a novel method of real-time control of the lattice periodicity was demonstrated,[5] in which the center fringe moved less than 2.7&_160;µm while the lattice periodicity was changed from 0.96 to 11.2&_160;µm. Whether this method can keep atoms (or other particles) trapped while changing the lattice periodicity remains to be tested experimentally. Such accordion lattices are useful for controlling untracold atoms in optical lattices, where small spacing is essential for quantum tunneling, and large spacing enables single-site manipulation and spatially resolved detection. Besides trapping cold atoms, optical lattices have been widely used in creating gratings and photonic crystals. They are also useful for sorting microscopic particles,[6] and may be useful for assembling cell arrays.
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