  Schroedinger's famous thought experiment involves a (macroscopic) cat whose
quantum state becomes entangled with that of a (microscopic) decaying nucleus.
The creation of such micro-macro entanglement is currently being pursued in
several fields, including atomic ensembles, superconducting circuits,
electro-mechanical and opto-mechanical systems. For purely optical systems,
there have been several proposals to create micro-macro entanglement by greatly
amplifying one half of an initial microscopic entangled state of light, but
experimental attempts have so far been inconclusive. Here we experimentally
demonstrate micro-macro entanglement of light. The macro system involves over a
hundred million photons, while the micro system is at the single-photon level.
We show that microscopic differences (in field quadrature measurements) on one
side are correlated with macroscopic differences (in the photon number
variance) on the other side. On the other hand, we demonstrate entanglement by
bringing the macroscopic state back to the single-photon level and performing
full quantum state tomography of the final state. Our results show that it is
possible to create and demonstrate micro-macro entanglement for unexpectedly
large photon numbers. Schroedinger's thought experiment was originally intended
to convey the absurdity of applying quantum mechanics to macroscopic objects.
Today many quantum physicists believe that quantum principles in fact apply on
all scales. By combining the present approach with other (e.g. mechanical)
systems, or by applying its basic ideas in different contexts, it may be
possible to bring quantum effects ever closer to our everyday experience.
