提出了基于探测高能级上原子布居数实现近简并准Λ型四能级原子系统局域化的方案。利用微扰理论求解薛定谔方程得到了基于上能级原子布居的原子位置的条件几率分布表达式。理论分析了原子局域峰的位置及宽度，得到了它们的解析表达式，给出了原子局域峰分裂的条件。研究表明，当系统中的耦合场和探测场满足电磁诱导透明配置条件或高能级向近简并的两个低能态跃迁的几率幅相干相消时，不能实现原子局域化。在电磁诱导透明窗附近，可以实现高精度原子局域化。原子局域峰的宽度取决于耦合场强度和近简并能级之间的间隔。

A scheme for atom localization via measuring population of the upper level in a quasi-Λ-type four-level atomic system is proposed. The function of conditional position probability distribution based on population of the upper level is obtained by solving the Schr?dinger equation using the perturbation theory. The position and width of the atomic location peak are analyzed theoretically and their equations are obtained. It is found that when the coupling field and probing field are in accord with the configuration of electromagnetically induced transparency (EIT) or when the probability amplitudes of the atom transitions from the upper level to the near-degenerate lower levels are destructive interference, atomic location cannot be obtained. But near the EIT window, the atom can be localized precisely. The peak position and its width of atom location are determined by the coupling field intensity and the space between the near-degenerate lower levels.