Excitation of a symmetry broken state in an optical lattice of rubidium atoms

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We propose to prepare a stationary quantum state with dx2-y2-geometry in a twodimensional optical square lattice of rubidium atoms. This state, characterized by alternating rotational flux in each unit cell, is excited by a simple stimulated Raman scattering process, which does not require internal atomic structure apart from a non-zero polarizability, thus permitting a straight forward experimental implementation. The proposed Raman-technique exploits the interference between two appropriate light fields with slightly different frequencies, in order to produce a two-dimensional square array of microscopic rotors, each applying angular momentum to a single unit cell of the lattice with alternating rotational direction for adjacent cells. In the framework of the Bose-Hubbard model, this Raman-coupling gives rise to a two-body ring exchange interaction, which should lead to novel quantum phases near the Mott-insulator phase. We will experimentally investigate the phase diagram of the system by studying the interference capability of the atoms in presence of the ring exchange interaction. Our findings will be confronted with calculations by condensed matter theorists at Utrecht University.