Curator's Take
This research demonstrates a significant advance in superconducting qubit gate operations by achieving a 99.92% fidelity iSWAP gate using a novel capacitively shunted double-transmon coupler that operates at zero magnetic flux bias. The breakthrough addresses a key challenge in quantum computing hardware: previous high-fidelity gates required large flux pulses that were prone to distortion and decoherence, but this new approach uses simple parametric driving to achieve exceptional performance with minimal qubit-coupler hybridization. Operating at zero flux is particularly valuable because it eliminates the need for precise static flux biasing, which has been a source of noise and complexity in superconducting quantum processors. This work represents important progress toward more robust and scalable quantum gate operations, offering a cleaner path to building larger, more stable superconducting quantum computers.
— Mark Eatherly
Summary
A double-transmon coupler (DTC) enables a fast, high-fidelity CZ gate between two highly detuned, fixed-frequency transmon qubits. Moreover, a recently proposed capacitively shunted DTC (CSDTC) realizes a small residual ZZ interaction over a wide flux-bias range around zero flux, eliminating the necessity of static flux biasing while maintaining high CZ-gate fidelity. However, CZ gates with the DTC and CSDTC require baseband flux pulses with large amplitudes, which are vulnerable to pulse distortion and decoherence due to large qubit-coupler hybridization. To address these issues, we experimentally demonstrate a parametrically driven iSWAP gate operated at zero flux bias between highly detuned, fixed-frequency transmon qubits coupled through a CSDTC. Using a simple flux-drive waveform without predistortion, we realize an average gate fidelity of 99.92(2)% at a total gate time of 112 ns. The observed high-fidelity performance is consistent with small qubit-coupler hybridization and small effective ZZ interaction during the gate. Our numerical simulations reproduce the experimentally observed iSWAP interaction rate and effective ZZ interaction, demonstrating the applicability of the theoretical model not only to spectral information but also to time-domain dynamics such as gate operations. These results boost further progress in the research of superconducting quantum computers.