  As one of the most intriguing intrinsic properties of quantum world, quantum
superposition provokes great interests in its own generation. Oszmaniec [Phys.
Rev. Lett. 116, 110403 (2016)] have proven that though a universal quantum
machine that creates superposition of arbitrary two unknown states is
physically impossible, a probabilistic protocol exists in the case of two input
states have nonzero overlaps with the referential state. Here we report a
heralded quantum machine realizing superposition of arbitrary two unknown
photonic qubits as long as they have nonzero overlaps with the horizontal
polarization state $|H\rangle$. A total of 11 different qubit pairs are chosen
to test this protocol by comparing the reconstructed output state with
theoretical expected superposition of input states. We obtain the average
fidelity as high as 0.99, which shows the excellent reliability of our
realization. This realization not only deepens our understanding of quantum
superposition but also has significant applications in quantum information and
quantum computation, e.g., generating non-classical states in the context of
quantum optics and realizing information compression by coherent superposition
of results of independent runs of subroutines in a quantum computation.
