  Using only linear interactions and a local parity measurement we show how
entanglement can be detected between two harmonic oscillators. The scheme
generalizes to measure both linear and non-linear functionals of an arbitrary
oscillator state. This leads to many applications including purity tests,
eigenvalue estimation, entropy and distance measures - all without the need for
non-linear interactions or complete state reconstruction. Remarkably,
experimental realization of the proposed scheme is already within the reach of
current technology with linear optics.
