Parametric iSWAP Gate at Zero Flux Boosts Qubits

Researchers at RIKEN and Toshiba Corporation have demonstrated a high-fidelity iSWAP gate operating at zero flux bias between superconducting qubits. The circuit uses a capacitively shunted double-transmon coupler (CSDTC) which eliminates the need for static flux biasing, a common source of error in these systems, while maintaining high performance. Achieving an average gate fidelity of 99. 92(2)% in just 112 nanoseconds, the work addresses challenges posed by large flux pulses prone to distortion and decoherence, and “boost[s] further progress in the research of superconducting quantum computers.”

Capacitively Shunted DTC Enables Zero-Flux iSWAP Gates

Unlike conventional designs requiring precise, constant magnetic fields to maintain qubit performance, this new approach operates effectively around zero flux, simplifying system calibration and potentially reducing error rates. This is achieved through the CSDTC’s ability to realize a minimal residual ZZ interaction across a broad range of flux values, a feature that allows for high-fidelity gate operation without the need for static biasing. Traditional CZ gates employing DTCs and CSDTCs rely on baseband flux pulses with substantial amplitudes, creating vulnerabilities to pulse distortion and decoherence caused by strong qubit-coupler hybridization.

The demonstrated iSWAP gate circumvents this issue by employing a simple flux-drive waveform, avoiding the need for complex pulse shaping or predistortion techniques. This simplification is critical for scaling up quantum systems, as it reduces the demands on control electronics and minimizes potential sources of error.

The design’s effectiveness is further supported by numerical simulations that accurately reproduce both the experimentally observed iSWAP interaction rate and the effective ZZ interaction. This agreement between theoretical modeling and experimental results confirms the applicability of the underlying physics and validates the CSDTC’s performance characteristics.

“The trade-off can be overcome by mediating interactions through a dedicated coupler applicable to highly detuned qubits,” the authors write, highlighting the importance of coupler design in achieving robust quantum operations. The ability to accurately model time-domain dynamics, such as gate operations, alongside spectral information, provides a powerful tool for optimizing future qubit architectures and control sequences.

Fixed-Frequency Transmons Mitigate Flux Noise in Entangling Gates

A capacitively shunted double-transmon coupler (CSDTC) design allows for high-fidelity quantum gate operations without relying on precise, static magnetic fields, a common challenge in superconducting qubit systems. The ability to operate effectively away from static flux biasing is particularly significant because it reduces susceptibility to low-frequency flux noise, a major source of decoherence in these delicate systems. The resulting iSWAP gate achieved 99. The design’s success hinges on a second-harmonic response of the coupler, generating the necessary exchange interaction while maintaining operation at the zero-flux sweet spot.

This specific operational parameter is key, as it ensures the qubits remain insensitive to 1/f flux noise, a pervasive issue in superconducting circuits. The researchers used a CSDTC device previously developed in their lab, consisting of two fixed-frequency transmon qubits coupled via two coupler transmons.

Detailed parameters of the device, including qubit frequencies and coupling strengths, were remeasured to ensure accurate calibration and performance. “By activating the interaction without introducing a static offset from the zero-flux sweet spot, this approach is compatible with tunable-coupler architectures,” the researchers explain, emphasizing how the design works with other systems. The demonstrated iSWAP gate addresses a critical bottleneck in scaling quantum systems: errors in two-qubit gates.

Fixed-frequency transmons, favored for their robustness against low-frequency flux noise and extended coherence times, often face a trade-off between gate efficiency and residual ZZ interaction. However, the CSDTC design, combined with the parametrically driven iSWAP gate, mitigates this trade-off, offering a pathway toward more scalable and reliable quantum processors. The team’s work builds on previous characterization of the CSDTC device, providing a comprehensive understanding of its properties and performance.

Parametric Drive Avoids Baseband Pulses & Decoherence

This is achieved by realizing a small residual ZZ interaction over a wide range of flux values around zero flux, a departure from conventional approaches that typically demand precise, static magnetic field settings. 92(2)%. The team’s success hinges on a specific modulation technique.

By applying a sinusoidal flux drive at frequency ${\omega }_d$, the energy level of the coupler is modulated at twice that frequency, resonantly driving the exchange interaction between the qubits when $2{\omega }_d$ is approximately equal to the energy difference between the qubit states. The observed iSWAP interaction rate and effective ZZ interaction align with numerical simulations, validating the theoretical model’s applicability to both spectral information and time-domain dynamics, including gate operations.

92% iSWAP Fidelity Achieved in 112ns with CSDTC

A parametrically driven iSWAP gate achieved 99. The CSDTC architecture realizes a minimal residual ZZ interaction over a broad range around zero flux, eliminating static flux biasing requirements while maintaining high gate fidelity. The team’s approach uses the second-harmonic response of the coupler to generate the exchange interaction, essential for the iSWAP gate operation. This method allows for operation at the “zero-flux sweet spot,” a critical parameter for suppressing coherent ZZ errors during the gate.

Characterization of the gate revealed a ZZ interaction of -35. 3 kHz at zero flux bias, remaining relatively small across a wide flux-bias range, reaching a minimum absolute value of -13. 8 kHz at a specific flux excursion. This broad range is a key advantage of the CSDTC, easing constraints on precise flux control.

To quantify gate error, the researchers employed randomized benchmarking, fitting sequence fidelity data to a model that accounts for leakage. The researchers report that the experimental results align with the model, strengthening confidence in the design and control parameters used. The team’s work demonstrates a pathway towards more robust and efficient quantum gates, essential for building larger and more complex quantum processors.

Small Hybridization & ZZ Interaction Enhance Gate Performance

This architecture’s advantage extends to a newly demonstrated parametrically driven iSWAP gate, operated at zero flux bias between fixed-frequency transmon qubits, circumventing the large baseband flux pulses traditionally required for CZ gates. The CSDTC’s wide-range small static ZZ interaction, combined with cancellation by the dynamically induced ZZ interaction from the flux drive, contributes to this suppression. The observed performance is dominated by incoherent errors, with both leakage and coherent ZZ errors strongly suppressed, indicating a robust gate operation.

“Notably, this performance was obtained using a simple analytically defined waveform without predistortion,” the researchers state, highlighting the simplicity of the control scheme. The iSWAP gate’s activation at the zero-flux sweet spot further reduces sensitivity to low-frequency flux noise, enabling high fidelity despite a relatively long gate duration.

The team attributes the success to the CSDTC’s small static ZZ interaction, further minimized by the dynamical ZZ interaction induced by the flux drive, and the use of a small flux-pulse amplitude that avoids strong qubit-coupler hybridization. “We attribute the high-fidelity performance of the iSWAP gate to two key features of the proposed scheme,” the researchers explain, emphasizing the synergistic effect of these design choices.

The simulations, employing a Cooper-pair-number basis in a multi-transmon system, demonstrated significant agreement with experimental data, extending beyond energy spectrum and ZZ interaction to encompass iSWAP interaction strength, resonant frequency and dynamical ZZ interaction.

Second-Harmonic Activation at Zero-Flux Sweet Spot Demonstrated

Baseband flux-controlled gates traditionally rely on net-zero flux pulses to minimize sensitivity to flux noise, but these require substantial flux excursions that introduce incoherent errors during operation. The need for time-consuming calibration of flux-line distortions, a process that becomes increasingly difficult as system size grows, further complicates their implementation. An alternative approach, flux-driven parametric gates activated by first-order modulation, avoids calibration issues but often necessitates a flux bias away from the zero-flux sweet spot, reintroducing the trade-off between gate speed and dephasing.

This technique allows for the use of an alternating current flux-drive pulse while maintaining a flux-insensitive idle point, a critical advantage for tunable-coupler architectures like the capacitively shunted double-transmon coupler (CSDTC). The CSDTC is designed for zero-flux-bias idling, offering first-order insensitivity to 1/f flux noise, making second-harmonic activation particularly appealing.

Researchers experimentally demonstrated a parametrically driven iSWAP gate at the zero-flux sweet spot, coupling highly detuned, fixed-frequency transmons through a CSDTC. The device used in the experiment is the same as that detailed in a previous publication, and analysis of the dynamical ZZ interaction, detailed in an appendix, further supports the observed high fidelity.

This result demonstrates the viability of second-harmonic activation as a method for creating high-performance quantum gates without the drawbacks of traditional flux-based approaches. The ability to operate at the zero-flux sweet spot, combined with the CSDTC architecture, offers a promising pathway toward building more stable and scalable superconducting quantum computers.

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