Finding operating conditions for a QPU based on tunable transmons and couplers

A system of flux-tunable transmons connected with flux-tunable couplers is one of the promising superconducting circuit architectures currently being pursued to realize a quantum processor [1, 2, 3]. The frequency tunability of couplers via flux lines offers the flexibility to suppress effective inter-qubit coupling during single-qubit gates and activate coupling during two-qubit gates, thereby maximizing the fidelities of both operations [4, 5]. In these systems, finding the optimal operational working point—the required simultaneous flux voltage offsets applied to transmons and couplers—is crucial to optimize single- and two-qubit gate fidelities across the entire chip. This optimization problem is inherently non-local, and crosstalk among different flux lines further complicates the bring-up process. In this talk, we present efficient protocols to find the operating conditions for such a multi-qubit QPU, discuss their implementation, and share the obtained results.

References:

[1] F. Yan et al. Phys. Rev. Appl. 10, 054062 (2018).

[2] M. Field et al. Phys. Rev. Appl. 21, 054063 (2024).

[3] F. Marxer et al. PRX Quantum 4, 010314 (2023).

[4] J. Stehlik et al. Phys. Rev. Lett. 127, 080505 (2021).

[5] X. Li et al. Phys. Rev. Appl. 14, 024070 (2020).