  We study the emergence of macrorealism in a harmonic oscillator subject to
consecutive measurements of a squeezed action. Since the harmonic oscillator
dynamics admits a hidden trajectory formulation, the assumptions of
macrorealism are violated only by the measurement process. We demonstrate a
breakdown of macrorealism in a wide parameter range that is maximized in a
scaling limit of extreme squeezing. A semiclassical analysis shows that
macrorealism is violated even with measurements of classically smooth
observables that do not resolve quantum levels. We propose an experimental test
of macrorealism with entangled photons by demonstrating that local realism in a
composite system implies macrorealism in a subsystem.
