Device‐Independent Dimension Leakage Null Test on Qubits at Low Operational Cost

A null test of the two‐level space of a qubit is constructed, which is both device independent and needs a small number of different experiments. Its feasibility is demonstrated on IBM Quantum, with most qubits failing the test by more than ten standard deviations. The robustness of the test against common technical imperfections, like decoherence and phase shifts, and supposedly negligible leakage, indicates that the origin of deviations is beyond known effects. Testing the two‐level space of a qubit is critical for the future of quantum computation. A simple setup allows to verify the space by a device independent null witness at extreme precision. Applied to IBM Quantum, most qubits fail the test by more than ten standard deviations, which cannot be explained by a standard model of leakage.