Key points are not available for this paper at this time.
The complex amplitude X₁+iX₂ (x+ipm) e^i of a harmonic oscillator is constant in the absence of driving forces. Although the uncertainty principle forbids precise measurements of X₁ and X₂ simultaneously (X₁X₂>~2m), 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.
Thorne et al. (Mon,) studied this question.