  Nonlocality is the defining feature of quantum entanglement. Entangled states
with multiple particles are of crucial importance in fundamental tests of
quantum physics as well as in many quantum information tasks. One of the
archetypal multipartite quantum states, Greenberger-Horne-Zeilinger (GHZ)
state, allows one to observe the striking conflict of quantum physics to local
realism in the so-called all-versus-nothing way. This is profoundly different
from Bell's theorem for two particles, which relies on statistical predictions.
Here, we demonstrate an integrated photonic chip capable of generating and
manipulating the four-photon GHZ state. We perform a complete characterization
of the four-photon GHZ state using quantum state tomography and obtain a state
fidelity of 0.729(6). We further use the all-versus-nothing test and the Mermin
inequalities to witness the quantum nonlocality of GHZ entanglement. Our work
paves the way to perform fundamental tests of quantum physics with complex
integrated quantum devices.
