  Due to their ability to confine light, optical resonators are of great
importance to science and technology, yet their performances are often limited
by out-of-plane scattering losses from inevitable fabrication imperfections.
Here, we theoretically propose and experimentally demonstrate a class of guided
resonances in photonic crystal slabs, where out-of-plane scattering losses are
strongly suppressed due to their topological nature. Specifically, these
resonances arise when multiple bound states in the continuum - each carrying a
topological charge - merge in the momentum space and enhance the quality
factors of all resonances nearby. We experimentally achieve quality factors as
high as $4.9\times 10^5$ based on these resonances in the telecommunication
regime, which is 12-times higher than ordinary designs. We further show this
enhancement is robust across the samples we fabricated.Our work paves the way
for future explorations of topological photonics in systems with open boundary
condition and their applications in improving optoelectronic devices in
photonic integrated circuits.
