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Physical Review A

Quantum superpositions generated by quantum nondemolition measurements

Bernard Yurke, Wolfgang P. Schleich, Daniel F. Walls

Published Aug 1, 1990• 151 citations
Publisher

Abstract

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.

DOI
10.1103/physreva.42.1703

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Quantum Electronics and Laser Science Conference•Jun 2, 1996•0 citations
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.
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.
Summary form only given. Quantum nondemolition (QND) measurements seek to measure a quantum mechanical observable precisely without disturbing the observable itself and its time evolution. They are of great importance in precision metrology and information processing at the quantum level. In this work, a new QND scheme for optical continuous-wave measurements of quadrature observables using a degenerate, type-I phasematched, dual port, optical parametric amplifier (OPA), is implemented. We employ a low-loss monolithic MgO:LiNbO/sub 3/ ring resonator.