We introduce a new entangling gate between two fixed-frequency qubits statically coupled via a microwave resonator bus which combines the following desirable qualities: all-microwave control, appreciable qubit separation for reduction of crosstalk and leakage errors and the ability to function as a two-qubit conditional-phase gate. A fixed, always-on interaction is explicitly designed between higher energy (non-computational) states of two transmon qubits, and then a conditional-phase gate is 'activated' on the otherwise unperturbed qubit subspace via a microwave drive. We implement this microwave-activated conditional-phase gate with a fidelity from quantum process tomography of ∼ 87%.
High-fidelity two-qubit gates are crucial for the scalability of superconducting quantum processors. While quantum information processing is typically based on qubits, qutrits (or qudits) provide a larger state space for quantum information storage and processing. In this study, we analyze a high-fidelity two-qutrit gate using microwave pulses on fixed-frequency superconducting transmon qutrits and show that a high-fidelity imaginary swap-like (
iswap-like) gate can be achieved. Perturbation theory is employed to derive an effective interaction Hamiltonian, allowing us to estimate the gate time for the two-qutrit system. Moreover, by employing a microwave-activation scheme, we can realize a high-fidelity
iswap-like gate on different excited states of the two qutrits. Our numerical results indicate that the proposed scheme can be readily extended to multiqutrit scenarios. Additionally, we investigate the impact of microwave pulse ramp time and qutrit relaxation on gate fidelity. The results demonstrate that, with a longer ramp time and the currently accessible qutrit relaxation time, the gate error remains sufficiently small. This proposed gate scheme enables the implementation of qutrit-qutrit entangling gates, providing a promising approach to realizing superconducting quantum computation.
The control of highly coherent qubits is crucial to building scalable superconducting quantum processors; in particular, the performance of entangling gates relies on the coherence times of individual qubits. Parametrically activated entangling gates between two coupled qubits are realized through rf flux modulation of one qubit. Here the authors discuss modulation amplitudes at which the qubit is first-order insensitive to
1/f flux noise: ``sweet spots'' in the ac flux. By properly filtering the white flux noise from the control electronics, one can realize high-fidelity two-qubit gates at a sweet spot. Recently this protection from flux noise has also been seen experimentally.