  Although interference is a classical-wave phenomenon, the superposition
principle, which underlies interference of individual particles, is at the
heart of quantum physics. An interaction-free measurements (IFM) harnesses the
wave-particle duality of single photons to sense the presence of an object via
the modification of the interference pattern, which can be accomplished even if
the photon and the object haven't interacted with each other. By using the
quantum Zeno effect, the efficiency of an IFM can be made arbitrarily close to
unity. Here we report an on-chip realization of the IFM based on silicon
photonics. We exploit the inherent advantages of the lithographically written
waveguides: excellent interferometric phase stability and mode matching, and
obtain multipath interference with visibility above 98%. We achieved a
normalized IFM efficiency up to 68.2%, which exceeds the 50% limit of the
original IFM proposal.
