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Bulletin of the American Physical Society

Effects of qubit frequency crowding on scalable quantum processors

Jared Hertzberg, Sami Rosenblatt, José Manuel Morgado Chávez, Easwar Magesan, John A. Smolin, Jeng-Bang Yau, Vivekananda P. Adiga, Markus Brink, Eric Zhang, Jason S. Orcutt, Jerry M. Chow

Published Mar 5, 2020• 1 citation
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Abstract Fabrication-friendly superconducting qubits continue to be a leading candidate for scalable quantum computing. Recent developments in tunable couplers have significantly advanced the progress toward practical quantum processors. However, high-performance quantum control, particularly two-qubit gates, depends on the delicate tuning of the coupler spectrum, as misalignment can lead to undesirable phenomena such as frequency crowding, which may cause errors including state leakage. Here, we propose an efficient method for characterizing the coupler spectrum through sideband drivings, obviating the need for additional components in current quantum processors. We demonstrate this technique experimentally by employing both continuous-wave and pulsed measurement protocols, successfully extracting the coupler spectrum. Furthermore, by utilizing the measured coupler spectrum, we calibrate the frequency dependence of the effective coupling strength between two qubits linked by the coupler. The proposed approach offers significant practical benefits, enabling the efficient characterization of the coupler spectrum in existing quantum architectures, thus paving the way for enhanced quantum control and scalability.
One of the major challenges in building fully functional quantum computers based on superconducting circuits is a scalable, high-fidelity two-qubit gate. Microwave-induced gates are appealing, but so far have been restricted to small qubit detunings, leading to frequency crowding and reduced gate speed and qubit addressability, due to crosstalk. The authors present a high-fidelity all-microwave gate based on a Raman transition, which allows for detunings that are large compared to the anharmonicity of the qubits, setting the stage for scalable, resource-efficient quantum processors.
A challenge for scaling up quantum processors using frequency-crowded, weakly anharmonic qubits is to drive individual qubits without causing leakage into non-computational levels of the others, while also minimizing the number of control lines. To address this, we implement single-qubit Wah-Wah control in a circuit QED processor with a single feedline for all transmon qubits, operating at the maximum gate speed achievable given the frequency crowding. Randomized benchmarking and quantum process tomography confirm alternating qubit control with $\leq$1% average error per computational step and decoherence-limited idling of one qubit while driving another with a Wah-Wah pulse train.