As superconducting-qubit architectures progress to more complex layouts, frequency crowding in qubit arrays increasingly hinders computing performance. Frequency-tunable qubits can assuage this problem, but are subject to the magnetic-flux noise that is ubiquitous in such circuitry. The authors exploit asymmetric Josephson junctions in transmon qubits to reduce sensitivity to flux noise, while preserving sufficient tunability to avoid frequency collisions. In their most asymmetric qubit, dephasing due to flux noise is reduced to the point where its rate is independent of the flux bias.
Generating high-fidelity, tunable entanglement between qubits is crucial for realizing gate-based quantum computation. In superconducting circuits, tunable interactions are often implemented using flux-tunable qubits or coupling elements, adding control complexity and noise sources. Here, we realize a tunable
ZZ interaction between two transmon qubits with fixed frequencies and fixed coupling, induced by driving both transmons off-resonantly. We show tunable coupling over one order of magnitude larger than the static coupling, and change the sign of the interaction, enabling cancellation of the idle coupling. Further, this interaction is amenable to large quantum processors: the drive frequency can be flexibly chosen to avoid spurious transitions, and because both transmons are driven, it is resilient to microwave crosstalk. We apply this interaction to implement a controlled phase (CZ) gate with a gate fidelity of
99.43(1)% as measured by cycle benchmarking, and we find the fidelity is limited by incoherent errors.