Pairwise Perturbative Ansatz for Quantum Process Tomography Part 1: Theory

As candidate quantum processors increase both in size and fidelity, so too does the need for robust verification and validation of their operation. Full characterization of these devices would be highly desirable; however, standard quantum process tomography scales exponentially with the number of qubits. Even for small scale systems, the experimental resource requirements make full tomography very challenging in practice. To circumvent this, we present an ansatz to describe an arbitrary quantum process on a multi-qubit system that only requires characterization of two-qubit processes, such that the number of measurements scales only quadratically with the number of qubits. Our Pairwise Perturbative Ansatz (PAPA) builds a description of the multi-qubit process from tomographic reconstruction of the reduced two-qubit processes on all pairs of qubits in the system. In part 1 of this talk we outline the PAPA approach to multi-qubit process tomography, and show through theoretical simulation how it can be used for excellent characterization of multi-qubit quantum processes.