We experimentally demonstrate a simple-design tunable coupler, achieving a continuous tunability for eliminating unwanted qubit interactions. We implement two-qubit iSWAP gate by applyinga fast-flux bias modulation pulse on the coupler to turn on parametric exchange interaction between computational qubits. Aiming to fully investigate error sources on the two-qubit gates, we perform quantum process tomography measurements and numerical simulations as varying static ZZ coupling strength. Our results reveal that the change in the two-qubit gate error is mainly attributed to unwanted high-frequency oscillation error terms, while the dynamic ZZ coupling parasitising in two-qubit gate operation may also contribute to the dependency of the gate fidelity. This approach, which has not yet been previously explored, provides a guiding principle to improve gate fidelity of parametric iSWAP gate by the elimination of unwanted qubit interactions. This controllable interaction, together with the parametric architecture by using modulation techniques, is desirable for crosstalk free multiqubit quantum circuits and quantum simulation applications.
Controllable interaction between superconducting qubits is desirable for large-scale quantum computation and simulation. Here, based on a theoretical proposal by Yan et al. [Phys. Rev.Appl. 10, 054061 (2018)] we experimentally demonstrate a simply-designed and flux-controlled tunable coupler with continuous tunability by adjusting the coupler frequency, which can completely turn off adjacent superconducting qubit coupling. Utilizing the tunable interaction between two qubits via the coupler, we implement a controlled-phase (CZ) gate by tuning one qubit frequency into and out of the usual operating point while dynamically keeping the qubit-qubit coupling off. This scheme not only efficiently suppresses the leakage out of the computational subspace but also allows for the acquired two-qubit phase being geometric at the operating point only where the coupling is on. We achieve an average CZ gate fidelity of 98.3%, which is dominantly limited by qubit decoherence. The demonstrated tunable coupler provides a desirable tool to suppress adjacent qubit coupling and is suitable for large-scale quantum computation and simulation.
Faithfully transferring quantum state is essential for quantum information processing. Here, we demonstrate a fast (in 84~ns) and high-fidelity (99.2%) quantum state transfer in achain of four superconducting qubits with nearest-neighbor coupling. This transfer relies on full control of the effective couplings between neighboring qubits, which is realized only by parametrically modulating the qubits without increasing circuit complexity. Once the couplings between qubits fulfill specific ratio, a perfect quantum state transfer can be achieved in a single step, therefore robust to noise and accumulation of experimental errors. This quantum state transfer can be extended to a larger qubit chain and thus adds a desirable tool for future quantum information processing. The demonstrated flexibility of the coupling tunability is suitable for quantum simulation of many-body physics which requires different configurations of qubit couplings.