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Non-Hermitian systems based on anti-parity-time (PT) symmetry reveal rich physics beyond the Hermitian regime. Here, the anti-PT-symmetric photonic lattice is effectively achieved in a four-level inverted Y-type 85Rb atomic vapor. Such instantaneously reconfigurable anti-PT-symmetric photonic lattice possessing susceptibility of χ (x) = − χ ∗ (− x) , is established by a one-dimensional standing-wave coupling field under the assist of a standing-wave driving field. The input probe beam experiences a periodic refractive index modulation when traveling through the photonic lattice, the evolution of output diffraction patterns with symmetric, asymmetric, and lopsided intensity distribution is obtained by adjusting the relevant parameters of constructed non-Hermitian system. Moreover, the anti-PT symmetry in two-dimensional case of both square and hexagonal lattices are correspondingly realized. Our work promotes the development of non-Hermitian devices, and also has some important applications in quantum simulation. • Providing a theoretical scheme with a four-level inverted Y-type 85Rb atomic vapor for the study of optical anti-PT symmetry. • The transition process from symmetric to lopsided diffractions in nontrivial photonic lattice, especially the asymmetric diffraction process, is theoretically explored. • The lopsided diffraction in this nontrivial photonic lattice highlights the significant role of the standing-wave driving field in transforming a trivial lattice into a nontrivial one. • The anti-PT symmetry are also theoretically obtained in two-dimensional case. [ABSTRACT FROM AUTHOR]