  Higher-order topological insulator, as a newly found non-trivial material and
structure, possesses a topological phase beyond the bulk-boundary
correspondence. Here, we present an experimental observation of photonic
higher-order topological crystalline insulator and its topological protection
to quantum superposition and entanglement in a two-dimensional lattice. By
freely writing the insulator structure with femtosecond laser and directly
measuring evolution dynamics with single-photon imaging techniques, we are able
to observe the distinct features of the topological corner states in C_4 and
C_2 photonic lattice symmetry. Especially, we propose and experimentally
identify the topological corner states by exciting the photonic lattice with
single-photon superposition state, and we examine the protection impact of
topology on quantum entanglement for entangled photon states. The single-photon
dynamics and the protected entanglement reveal an intrinsic topological
protection mechanism isolating multi-partite quantum states from
diffusion-induced decoherence. The higher-order topological crystalline
insulator, built-in superposition state generation, heralded single-photon
imaging and quantum entanglement demonstrated here link topology, material, and
quantum physics, opening the door to wide investigations of higher-order
topology and applications of topological enhancement in genuine quantum regime.
