Two-qubit tomography in the presence of stray couplings*

Tomography is an indispensable part of quantum computation as one needs to verify or determine the state of a quantum system after certain evolution. Existing tomographic protocols are based on determining various correlators assuming non-interacting qubits. However, in realistic systems, qubits often develop some form of unavoidable stray coupling. These couplings lead to incorrect correlator measurements resulting in unfaithful reproduction of the true quantum state. We have developed a protocol that can correct for these errors by computing the evolution of the system in software and is able to correctly determine the quantum state. We demonstrate the performance of our scheme on a system of two transmon qubits with always-on ZZ coupling. This technique is general enough to allow an extension to larger systems with different types of stray coupling and even allows the use of non-π/2 pulses for pre-rotations during tomography.