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Published March 13, 1978 | Published
Journal Article Open

Quantum Nondemolition Measurements of Harmonic Oscillators


The complex amplitude X1+iX2≡(x+ip / mω)e^(iωt) of a harmonic oscillator is constant in the absence of driving forces. Although the uncertainty principle forbids precise measurements of X1 and X2 simultaneously (ΔX1ΔX2>~ℏ / 2mω), X1 alone can be measured precisely and continuously ("quantum nondemolition measurement"). Examples are given of measuring systems that do this job. Such systems might play a crucial role in gravitational-wave detection and elsewhere.

Additional Information

©1978 The American Physical Society. Received 23 January 1978. For helpful discussion we thank Richard P. Feynman, Karel Juchař, and Amnon Yariv. This work was supported in part by the National Aeronautics and Space Administration under Grant No. NGR 05-002-256 and a grant from their Physics and Chemistry Experiments in Space (PACE) Program, and by the National Science Foundation under Grant No. AST 76-80801 A01.

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