  Heisenberg-Robertson's uncertainty relation expresses a limitation in the
possible preparations of the system by giving a lower bound to the product of
the variances of two observables in terms of their commutator. Notably, it does
not capture the concept of incompatible observables because it can be trivial,
i.e., the lower bound can be null even for two non-compatible observables. Here
we give two stronger uncertainty relations, relating to the sum of variances,
whose lower bound is guaranteed to be nontrivial whenever the two observables
are incompatible on the state of the system.
