在物理学里，无限深方形阱（infinite square potential），又称为无限深位势阱（infinite potential well），是一个阱内位势为0，阱外位势为无限大的位势阱。思考一个或多个粒子，永远地束缚于无限深位势阱内，无法逃出。关于这些粒子的量子行为的问题，称为无限深方形阱问题，又称为无限深位势阱问题，盒中粒子问题（particle in a box problem），是一个理论问题。假若，阱内只有一个粒子，则称为单粒子无限深方形阱问题。假若，阱内有两个粒子，则称为双粒子无限深方形阱问题。假若，这两个粒子是完全相同的粒子，则问题又复杂许多，称为双全同粒子无限深方形阱问题。在这里，只讨论单粒子无限深方形阱问题。

In quantum mechanics, the particle in a box model (also known as the infinite potential well or the infinite square well) describes a particle free to move in a small space surrounded by impenetrable barriers. The model is mainly used as a hypothetical example to illustrate the differences between classical and quantum systems. In classical systems, for example a ball trapped inside a large box, the particle can move at any speed within the box and it is no more likely to be found at one position than another. However, when the well becomes very narrow (on the scale of a few nanometers), quantum effects become important. The particle may only occupy certain positive energy levels. Likewise, it can never have zero energy, meaning that the particle can never "sit still". Additionally, it is more likely to be found at certain positions than at others, depending on its energy level. The particle may never be detected at certain positions, known as spatial nodes.