Experimental generation of Schrodinger cats via quantum measurement

Recently, S. Song et al. (1989) showed that the superposition of macroscopic quantum states, commonly referred to as Schrodinger cats, can be generated by quantum measurement. The apparatus and method they proposed are described. Two light beams, A and B, with nonclassical phase-quadrature correlations are first generated by a backaction evasion apparatus. Beam A is then detected by a photon counter, D, while beam B goes through a parametric amplifier and comes out as beam B'. S. Song et al. show that at B' one has a Schrodinger cat whose state is conditioned on the photon number n_R counted by D. B. Yurke and D. Stoler (1986) showed that for the quantum signature of the interference fringes to exist, one can tolerate a loss of no more than one photon in beam A. Thus, to generate a cat with even n_R = 3, one needs a detector capable of distinguishing three photons from two or four photons. This requires not only that the detector quantum efficiency be larger than 70% but also that the detector multiplication and dark-current noise be low. A photon counter satisfying all three criteria does not yet exist. It is proposed that the multiplication noise be reduced by subdividing A into many beams with 50-50 beam splitters. An example of seven beam splitters and eight detectors is shown and discussed.