  The realization of correlated multi-photon processes usually depends on the
interaction between nonlinear media and atoms. However, the nonlinearity of
optical materials is generally weak, making it still very challenging to
achieve correlated multi-photon dynamics at the few-photon level. Meanwhile,
giant atoms, with their capability for multi-point coupling, which is a novel
paradigm in quantum optics, mostly focus on the single photon field. In this
work, using the method described in Phys. Rev. Res. 6. 013279 (2024), we reveal
that the ladder-type three-level giant atom spontaneously emits strongly
correlated photon pairs with high efficiency by designing and optimizing the
target function. In addition, by encoding local phases into the optimal
coupling sequence, directional two-photon correlated transfer can be achieved.
This method does not require a nonlinear waveguide and can be realized in the
conventional environment. We show that the photon pairs emitted in both the
bidirectional and the chiral case exhibit strong correlation properties in both
time and space. Such correlated photon pairs have great potential applications
for quantum information processing. For example, numerical results show that
our proposal can realize the two-photon mediated cascaded quantum system.
