Control of Qubit Isolation in a Multi-Qubit System Using a Tunable Coupler

Qubit entanglement is one of the key ingredients for realizing quantum computation. Such entanglement is achieved through the coupling between qubits. However, single qubit operation remains important for characterizing the quality of qubits, microwave pulses, and other factors, serving as an important indicator even in a multi-qubit system. On the other hand, qubit-to-qubit coupling can significantly influence single-qubit operations, resulting in lower single-qubit fidelity.

Here, we introduce a tunable coupler between two physical qubits to form a three-body multilevel system, which can control the effective coupling between the two qubits. Thus, the coupling can be turned off by tuning the coupler frequency, creating isolated single qubits in a multi-qubit system. The qubit isolation is assessed by comparing gate fidelity in individual and simultaneous Randomized Benchmarking. Fidelity is degraded in simultaneous benchmarking if residual couplings are present. We further investigate the relationship between coupling strength and fidelity degradation, comparing results with theoretical modeling.