Zong-Quan Zhou, Susana F. Huelga, Chuan-Feng Li, Guang-Can Guo
The quantum mechanical description of the world leads to predictions that defy our daily experience when dealing with macroscopic objects. Schr\"{o}dinger's famous cat illustrates most graphically the paradoxical situations that one encounters when extrapolating the quantum rules to systems possessing macroscopically distinguishable states. Leggett and Garg formulated this question qualitatively by deriving a series of inequalities based upon the premises of macroscopic realism which have been recently tested within several microscopic systems and micrometer-sized superconducting circuits. Here we present an experimental violation of Legget-Garg inequalities (LGI) with millimeter-sized crystals whose state can be thought of as a cat of cats. Through the reversible transfer of polarization state of single photons and the collective excitation states of crystals, we prepared superposition states of two macroscopic crystals separated by a distance in the mm range. The phase relation between the states in the superposition is governed by the dynamics of a large number of individual atoms and can be controlled with a frequency detuned and polarization dependent atomic frequency comb. The quantum character of the considered global state is therefore governed by a macroscopic quantity, which allows for an additional level of complexity in the system. The observed violation of a LGI confirms the persistence of quantum coherence effects within macroscopic objects which can be seen with a naked eye.
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http://arxiv.org/abs/1209.2176
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