  When overlapping in an optical medium with nonlinear susceptibility, light
waves can interact with each other, changing their phases, wavelengths, shapes,
and so on. Such nonlinear effects, discovered over a half century ago, have
given rise to a breadth of important applications. Applying to
quantum-mechanical signals, however, they face fundamental challenges arising
from the multimode nature of the interacting electromagnetic fields, such as
phase noises and Raman scattering. Quantum Zeno blockade allows strong
interaction of light waves without them physically overlapping, thus providing
a viable solution for those challenges, as indicated in recent bulk-optics
experiments. Here, we report on the observation of quantum Zeno blockade on
chip, where a light wave is modulated by another in a distinct
"interaction-free" manner. For quantum applications, we also verify its
operations on a single-photon level. Our results promise a scalable platform
for overcoming several grand challenges faced by nonlinear optics and quantum
information processing, enabling, e.g., manipulation and interaction of quantum
signals without decoherence.
