Abstract
High-fidelity two-qubit gates at scale are a key requirement to realize the full promise of quantum computation and simulation. The advent and use of coupler elements to tunably control two-qubit interactions has improved operational fidelity in many-qubit systems by reducing parasitic coupling and frequency crowding issues. Nonetheless, two-qubit gate errors still limit the capability of near-term quantum applications. The reason, in part, is the existing framework for tunable couplers based on the dispersive approximation does not fully incorporate three-body multi-level dynamics, which is essential for addressing coherent leakage to the coupler and parasitic longitudinal ($ZZ$) interactions during two-qubit gates. Here, we present a systematic approach that goes beyond the dispersive approximation to exploit the engineered level structure of the coupler and optimize its control. Using this approach, we experimentally demonstrate CZ and $ZZ$-free iSWAP gates with two-qubit interaction fidelities of $99.76 \pm 0.07$% and $99.87 \pm 0.23$%, respectively, which are close to their $T_1$ limits.
Highlights
A key challenge for large-scale quantum computation and simulation is the extensible implementation of highfidelity entangling gates [1]
We significantly reduce the nonadiabatic error of a 60-ns-long CZ gate, thereby demonstrating a two-qubit interaction fidelity of 99.76 Æ 0.07% in interleaved randomized benchmarking
Unlike the CZ gate, when performing single-qubit gates, we bias qubit 1 (QB1) and qubit 2 (QB2) in resonance to synchronize their XY axes in the Bloch sphere. This biasing is facilitated by the tunable coupler, which switches off the effective transverse coupling between QB1 and QB2
Summary
A key challenge for large-scale quantum computation and simulation is the extensible implementation of highfidelity entangling gates [1]. The perturbative treatment of tunable couplers disregards the presence of higher levels of the coupler [13] This omission is significant; the higher level of the coupler participates in the multilevel dynamics of two-qubit gates and, thereby, adds a considerable amount of residual two-qubit interactions. We engineer the control and level structure of the coupler by going beyond the dispersive approximation in order to realize high-fidelity two-qubit gates. We implement both longitudinal (CZ) and transversal (iSWAP) two-qubit gates; the availability of both type of gates generally reduces gate overheads of NISQ algorithms [3,24]. We successfully suppress the residual ZZ interaction of the iSWAP gate in a passive manner, by exploiting the engineered coupler level structure, and demonstrate a two-qubit interaction fidelity of 99.87 Æ 0.23% with a 30-ns gate duration
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
