  We present a precise certification test of the dimension of a qubit system on
the public IBM quantum computer, using the determinant dimension witness and
with a minimal number of independent parameters. We achieve it by mapping the
Bloch sphere $\pi/2$-rotation axis angle on the nonplanar so-called Viviani
curve. During the implementation of the rotation by single qubit gates on IBM
devices, we found the majority of qubits passing the test, although some
specific qubits failed by more than ten standard deviations. The nature of
those deviations has no simple explanation, as the test is robust against
common non-idealities.
