In circuit-based quantum computing the available gate set typically consists of single-qubit gates acting on each individual qubit and at least one entangling gate between pairs of qubits. In certain physical architectures, however, some qubits may be “hidden” and lacking direct addressability through dedicated control and readout lines, for instance, because of limited on-chip routing capabilities, or because the number of control lines becomes a limiting factor for many-qubit systems. In this case, no single-qubit operations can be applied to the hidden qubits and their state cannot be measured directly. Instead, they may be controlled and read out only via single-qubit operations on connected “control” qubits and a suitable set of two-qubit gates. We first discuss the impact of such restricted control capabilities on the performance of specific qubit coupling networks. We then experimentally demonstrate full control and measurement capabilities in a superconducting two-qubit device with local single-qubit control and iswap and controlled-phase two-qubit interactions enabled by a tunable coupler. We further introduce an iterative tune-up process required to completely characterize the gate set used for quantum process tomography and evaluate the resulting gate fidelities.

中文翻译：

---

隐藏量子位的表征和断层扫描

在基于电路的量子计算中，可用的门组通常由作用于每个单独量子位的单量子位门和至少一个量子位对之间的纠缠门组成。然而，在某些物理架构中，一些量子位可能被“隐藏”，并且缺乏通过专用控制和读出线的直接寻址能力，例如，由于片上路由能力有限，或者因为控制线的数量成为许多人的限制因素-量子位系统。在这种情况下，不能对隐藏的量子位应用单量子位操作，也不能直接测量它们的状态。相反，它们只能通过对连接的“控制”量子位和一组合适的双量子位门进行单量子位操作来控制和读出。我们首先讨论这种受限控制能力对特定量子比特耦合网络性能的影响。然后，我们在具有本地单量子位控制的超导双量子位设备中通过实验证明了完全控制和测量能力由可调耦合器实现的交换和受控相位双量子位交互。我们进一步介绍了完全表征用于量子过程断层扫描的门组并评估产生的门保真度所需的迭代调整过程。