Optimized control of superconducting qubits

To enhance the capabilities of today’s quantum processors not only the coherence of qubits, but also the preparation of complex quantum states via optimized control signals needs further improvement. For quantum processors based on superconducting qubits it is of particular importance to avoid leakage out of the computational subspace, which is facilitated by the low anharmonicity of transmon-type qubits. This can be achieved by optimizing the shape of the control pulses utilizing closed-loop optimization methods to tailor the microwave pulses towards short and high-fidelity gates. Using a piecewise-constant pulse parameterization we demonstrate single-qubit pulses as short as 4ns with low leakage out of the computational subspace. For the rapid calibration and optimization of single-qubit operations we employ a ‘restless’ measurement protocol that uses the outcome of a projective measurement as the initial state of the next operation. Along with an efficient analysis that compensates for distortions in the signal due to SPAM errors this allows for data collection at a high rate of the order of 100 kHz, which is not limited by the decay time of the qubits.

Another important challenge for scalable quantum computing is the required large amount of control lines. We, therefore, explore architectures that contain 'hidden' qubits. These are not directly addressable, but only via an adjacent control qubit. We discuss the impact of such restricted control capabilities on the performance of specific qubit coupling networks and experimentally demonstrate full control and measurement capabilities of the hidden qubit.