Barbara Maria Terhal

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.

I address the question of how a quantum computer can be used to simulate experiments on quantum systems in thermal equilibrium. I present two approaches for the preparation of the equilibrium state on a quantum computer. For both approaches, one can show that the output state of the algorithm, after long enough time, is the desired equilibrium.