# Why is the angular momentum of a 3D kicked rotor non-negative?

We know that for a 2D kicked rotor the angular momentum quantum number can be any integer from minus infinity to infinity. However, for a 3D kicked rotor this is not the case: it can only be positive or zero. I'm wondering why that is. Thanks!

• In the first case you're talking about $L_z$, while in the second case you're talking about $L^2$, two very different things. In all cases, $L_z$ can be positive or negative, while $L^2$ is nonnegative. – knzhou Jul 7 at 13:43
• I'm voting to close this question as off-topic because it lacks sufficient previous research. – stafusa Jul 7 at 16:32