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Abstract
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 quantum volume 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.
Highlights
The sizes of engineered quantum systems encountered in state-of-the-art laboratories [1,2,3,4,5] have been steadily increasing, enabled by progress in packaging technology [6] and integration of control electronics [7]
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
We develop a modified quantum process tomography (QPT) method which allows us to characterize the gates in a way that is robust against state preparation and measurement errors
Summary
The sizes of engineered quantum systems encountered in state-of-the-art laboratories [1,2,3,4,5] have been steadily increasing, enabled by progress in packaging technology [6] and integration of control electronics [7] Scaling such systems even further, still raises practical challenges as the amount of required control hardware and signal lines is proportional to the growing number of qubits. A more favorable ratio of control lines per qubit may be achieved by forgoing the direct control lines of a fraction of the qubits and controlling them indirectly by means of their coupling to neighboring qubits We call such qubits hidden, and as we show here, they allow one to reduce the total number of control lines without compromising the computational power of the device. This is especially important for systems with hidden qubits where preparation and measurement of arbitrary states relies heavily on the use of typically more error-prone two-qubit gates, but our technique can in principle be applied to standard network topologies without hidden qubits
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