Quantum Nondemolition Measurements of Harmonic Oscillators

This is the authors' abstract. We don't add key points for this paper.

The complex amplitude X₁+iX₂≡(x+ipmω)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>~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.