Abstract
Entangling gates with error rates reaching the threshold for quantum error correction have been reported for CZ gates using adiabatic longitudinal control based on the interaction between the |11〉 and |20〉 states. Here, we design and implement nonadiabatic CZ gates, which outperform adiabatic gates in terms of speed and fidelity, with gate times reaching 1.25/(2sqrt 2 g_{01,10}), and fidelities reaching 99.54 ± 0.08%. Nonadiabatic gates are found to have proportionally less incoherent error than adiabatic gates thanks to their fast gate times, which leave more room for further improvements in the design of the control pules to eliminate coherent errors. We also show that state leakage can be reduced to below 0.2% with optimisation. Furthermore, the gate optimisation process is highly feasible: experimental optimisation can be expected to take less than four hours. Finally, the gate design process can be extended to CCZ gates, and our preliminary results suggest that this process would be feasible as well, if we can measure the CCZ fidelity separate from the initialisation and readout errors in experimental optimisation.
Highlights
Our result demonstrates that the difficulties of nonadiabatic gates can be overcome to the point that gate fidelities exceed those of adiabatic gates reported in the literature, which themselves have already passed the threshold for quantum error correction using the surface code
We find that state leakage is suppressed if the Z-pulse distortion calibration is performed well
Our experiment has proved that finding high-fidelity pulses is realistic and viable
Summary
Motivated by the quest for quantum error correction and expanding the set of realisable circuits,[1,2] there has been a great effort to improve the design of entangling gates,[1,2,3,4,5,6,7,8,9,10,11] and there is a rich array of design choices in a variety of quantum computing modalities, including superconducting quantum circuits,[12] trapped ions,[13] quantum dots[14] and NV diamonds.[15,16]. The best results for entangling-gate fidelity using longitudinal control is 99.44%,3 and using transverse control is 99.1%.20 The former result reaches the surface code threshold error rate.[3,23]. Incoherent errors, on the other hand, can not normally be eliminated within the domain of designing and calibrating pulses, and the best, most direct way to mitigate this source of error is to make the gate as fast as possible. Normalised by the coupling strength Ts = 1/(2g11,20), our gates are significantly faster than their adiabatic counterparts, reaching 1.25Ts (40 ns), compared to adiabatic gate times, which range from 1.66 to 1.87Ts.[3] The faster gate times mean the upper bound set by decoherence during the operation is higher, which leaves more room for improvement of overall gate fidelity by focusing on coherent errors. Our simulations find that the fastest possible gate is 1.06Ts, but in practice, technical limitations will cause the gate time to increase
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
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 callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
