Gate set tomography on more than two qubits*

Quantum tomography is often deemed impractical for “large” quantum information processors of more than 2 qubits. Quantum process tomography and gate set tomography (GST) can be, and have been, used to probe the behavior of gates on 1-2 qubits, but are rarely applied to 3 or more qubits. This stems in large part from the sheer number of parameters that need to be probed in standard gate tomography: O(16^N) for N qubits. We present a framework for intelligently reducing this parameter explosion to poly(N) parameters. Using reduced models that capture physically plausible errors (e.g., few-qubit interactions and Pauli stochastic noise), we show how to perform gate set tomography effectively on larger systems. We present results from applying these methods to processors with 3+ qubits.