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Bulletin of the American Physical Society

Suppressed crosstalk between two-junction superconducting qubits with mode-selective coupling.

A. D. K. Finck, Santino D. Carnevale, Dave Klaus, Christopher Scerbo, J. M. Blair, Thomas McConkey, Cihan Kurter, April Carniol, George Keefe, Muir Kumph, Oliver Dial

Published Mar 15, 2021• 0 citations
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•Jan 1, 2022•0 citations
Fixed-frequency qubits can suffer from always-on interactions that inhibit independent control. Here, we address this issue by experimentally demonstrating a superconducting architecture using qubits that comprise of two capacitively shunted Josephson junctions connected in series. Historically known as tunable coupling qubits (TCQs), such two-junction qubits support two modes with distinct frequencies and spatial symmetries. By selectively coupling only one type of mode and using the other as our computational basis, we greatly suppress crosstalk between the data modes while permitting all-microwave two-qubit gates.
As superconducting quantum processors scale to large qubit counts, unwanted microwave drive crosstalk becomes an important source of coherent errors. Here, we investigate the role of parasitic coupling paths in the suppression of drive crosstalk in a processor with two fixed-frequency transmon qubits and a tunable coupler. To account for the many different coupling paths, we develop a detailed circuit model to capture the dependence of crosstalk on the coupler frequency. We experimentally demonstrate suppression of crosstalk, finding that the suppression does not originate solely from cancellation of the effective qubit-qubit coupling, but also is a result of interference between other coupling paths. Building on these insights, we show that drive crosstalk can be suppressed with the right choice of parasitic capacitances, which otherwise often are treated as unwanted design imperfections. Our results provide practical guidance for low-crosstalk device design of superconducting quantum processors and point toward a broader strategy in which parasitic coupling paths are engineered rather than simply minimized.
Superconducting circuits are well established as a viable candidate for the realisation of quantum computers. Circuits based on the transmon qubit are now ubiquitous, owing to its simple design and reduced control wiring overhead. An issue with transmon qubit-based architectures is the always-on unwanted interactions that impose limits on gate speeds and introduce errors into their operation. In addition, understanding sources of noise and decoherence is essential to the low-error operation quantum computers. This thesis describes the implementation of a multi-mode superconducting qubit in a coaxial circuit QED architecture. Constructed from three superconducting islands, connected via two Josephson junctions, the device possesses two transmon-like modes with orthogonal field symmetries. The unique polarisation of each mode allows for engineering dissipation and coupling in the system, extending functionality beyond the single-mode transmon. Experimental results on the unit-cell of the two-mode coaxial transmon are presented, demonstrating coherent control and simultaneous dispersive readout of the modes of the device. A predictive theory of charge sensitivity in a multi-mode superconducting qubit is presented, and experimental results in agreement of this theory are shown, observing sensitivity to four charge-parity configurations and two independent gate-charge offsets. The utility of a multi-mode qubit as a charge detector in spatially tracking local-charge drift of ≃ 100 µm length scales is also shown, demonstrating the use of these devices as tools in understanding charge noise in superconducting circuits. Finally, a system of a pair of coupled two-mode coaxial transmons is introduced, demonstrating a highly mode-selective coupling architecture. A suppressed quantum crosstalk of 2 kHz between protected modes of the devices is measured, along with equal single qubit gate fidelities when operated both individually and simultaneously. A first characterisation of a microwave activated conditional phase interaction between computational modes driven via ancillary transitions (AT-MAP) is presented. Whilst not shown in this work, this state-dependent two-qubit interaction can be used to generate entanglement. Combined with the low crosstalk demonstrated, this shows the multi-mode qubit architecture is a promising candidate for the construction of larger scale quantum processors with fast gates and low crosstalk-related errors.