Toward an optical evidence of quantum interference between macroscopically distinct states

Abstract

Thus far, all proposals for an optical evidence of macroscopic quantum interference have been frustrated by the low detector efficiencies. Two innovations permit such an evidence with currently available equipment. We refer to the generation scheme of entangled states by parametric amplification proposed by S. Song, C. M. Caves, and B. Yurke [Phys. Rev. A 41, 5261 (1990)]. Setting a low gain for the parametric amplifier introduces a drastic cutoff in the generated photon-number probabilities that compensates for the broadening of the detection probability in the first detector. Furthermore, we supplement the final homodyne detector with a degenerate type-I parametric amplifier which brings nearer the two previously distant states of the superposition. The availability of optical superposition states with large separation suggests a practical way of transforming them into entangled coherent states associated with different paths, thus providing a new convenient source for a ``which path'' experiment.