  The decay of an unstable system is usually described by an exponential law.
Quantum mechanics predicts strong deviations of the survival probability from
the exponential: indeed, the decay is initially quadratic, while at very large
times it follows a power law, with superimposed oscillations. The latter regime
is particularly elusive and difficult to observe. Here we employ arrays of
single-mode optical waveguides, fabricated by femtosecond laser direct
inscription, to implement quantum systems where a discrete state is coupled and
can decay into a continuum. The optical modes correspond to distinct quantum
states of the photon and the temporal evolution of the quantum system is mapped
into the spatial propagation coordinate. By injecting coherent light states in
the fabricated photonic structures and by measuring light with an unprecedented
dynamic range, we are able to experimentally observe not only the exponential
decay regime, but also the quadratic Zeno region and the power-law decay at
long evolution times.
