Single Flux Quantum-Based Digital Control of Superconducting Qubits in a Multi-Chip Module

• Published in: PRX quantum Vol. 4
• Main Authors: Liu, C. H., Ballard, A., Olaya, D., Schmidt, D. R., Biesecker, J., Lucas, T., Ullom, J., Patel, S., Rafferty, O., Opremcak, A., Dodge, K., Iaia, V., McBroom, T., DuBois, J. L., Hopkins, P. F., Benz, S. P., Plourde, B. L. T., McDermott, R.
• Format: Journal Article
• Language: English
• Published: United States American Physical Society (APS) 24-07-2023
• Subjects: CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; quantum circuits; quantum control; superconducting devices; superconducting qubits
• ISSN: 2691-3399; 2691-3399

Single flux quantum (SFQ) digital logic has been proposed for the scalable control of next-generation superconducting-qubit arrays. In the initial implementation, SFQ-based gate fidelity was limited by quasiparticle (QP) poisoning induced by the dissipative on-chip SFQ driver circuit. In this work, we introduce a multichip-module architecture to suppress phonon-mediated QP poisoning. Here, the SFQ elements and qubits are fabricated on separate chips that are joined with In-bump bonds. We use interleaved randomized benchmarking to characterize the fidelity of SFQ-based gates and we demonstrate an error per Clifford gate of 1.2(1)%, an order-of-magnitude reduction over the gate error achieved in the initial realization of SFQ-based qubit control. We use purity benchmarking to quantify the contribution of incoherent error at 0.96(2)%; we attribute this error to photon-mediated QP poisoning mediated by the resonant millimeter-wave antenna modes of the qubit and SFQ-qubit coupler. We anticipate that a straightforward redesign of the SFQ driver circuit to limit the bandwidth of the SFQ pulses will eliminate this source of infidelity, allowing SFQ-based gates with error approaching approximate known theoretical limits, of order 0.1% for resonant sequences and 0.01% for more complex pulse sequences involving variable pulse-to-pulse separation.