We describe a method for generating superpositions of classically distinguishable quantum states in an optical-frequency mode. The method uses the optical back-action-evading apparatus demonstrated recently by La Porta, Slusher, and Yurke [Phys. Rev. Lett. 62, 28 (1989)].
A full comprehension of the fuzzy boundary between the classical and the quantum world is still lacking. A deeper understanding of the problem can be obtained form the study of the so-called "Schrödinger cat states", i.e. the paradoxical states arising when the superposition principle of quantum mechanics is extended to the macroscopic world. Quantum optics is, up to now, the most studied field for the experimental realization of these states. A survey of all the proposals made for generating and detecting linear superpositions of classically distinguishable states in quantum optical systems is presented. Then, one of the most recent proposals [P. Tombesi and D. Vitali, Phys. Rev. Lett.77, 411 (1996)] is presented in detail and its experimental feasibility discussed.
We analyze a quantum nondemolition measurement scheme similar to that advocated by Song, Caves, and Yurke [Phys. Rev. A 41, 5261 (1990)] for generating superpositions of macroscopically distinct quantum states, but with the parametric amplifier and the back-action evader interchanged. We determine the asymptotic behavior of the output and obtain the conditions under which a superposition of two coherent states is approached. By presqueezing the signal port input, we find that gain requirements of the back-action evader are much less demanding.