Quantum process tomography of a universal entangling gate implemented with Josephson phase qubits

Quantum gates must perform reliably when operating on standard input basis states and on complex superpositions thereof. Experiments using superconducting qubits have validated truth tables for particular implementations of, for example, the controlled-NOT gate(1,2), but have not fully characterized gate operation for arbitrary superpositions of input states. Here we demonstrate the use of quantum process tomography(3,4) (QPT) to fully characterize the performance of a universal entangling gate between two superconducting qubits. Process tomography permits complete gate analysis, but requires precise preparation of arbitrary input states, control over the subsequent qubit interaction and ideally simultaneous single-shot measurement of output states. In recent work, it has been proposed to use QPT to probe noise properties' and time dynamics(6) of qubit systems and to apply techniques from control theory to create scalable qubit benchmarking protocols(7,8). We use QPT to measure the fidelity and noise properties' of an entangling gate. In addition to demonstrating a promising fidelity, our entangling gate has an on-to-off ratio of 300, a level of adjustable coupling that will become a requirement for future high-fidelity devices. This is the first solid-state demonstration of QPT in a two-qubit system, as QPT has previously been demonstrated only with single solid-state qubits(9-11).