Past exam of the chemistry A course of the University of Cambridge / 2021 / Paper 1 / A1 / a / v

The squared angular momentum and one chosen component can have simultaneously sharp values. All three components cannot have simultaneously sharp values in a nonzero-$j$ state. The possible measured component values are $m\hslash$, while the squared magnitude is $j(j+1){\hslash }^2$. This concerns the quantum state, rather than imperfect experimental resolution. The Robertson uncertainty relation, for example,quantifies the restriction; its right-hand side may vanish in some states, without making all three components simultaneously sharp.