Spectral Quantum Tomography
We introduce spectral quantum tomography, a simple method to extract the
eigenvalues of a noisy few-qubit gate, represented by a trace-preserving
superoperator, in a SPAM-resistant fashion, using low resources in terms of
gate sequence length. The eigenvalues provide detailed gate information,
supplementary to known gate-quality measures such as the gate fidelity, and can
be used as a gate diagnostic tool. We apply our method to one- and two-qubit
gates on two different superconducting systems available in the cloud, namely
the QuTech Quantum Infinity and the IBM Quantum Experience. We discuss how
cross-talk, leakage and non-Markovian errors affect the eigenvalue data.
Jonas Helsen Francesco Battistel Barbara M. Terhal