Demonstration of High-Fidelity Gates in a Strongly Anharmonic with Long-Coherence C-Shunt Flux Qubit

Abstract

We demonstrate high-fidelity single-qubit gates on a C-shunt flux qubit that simultaneously combines a large anharmonicity ($\mathcal{A}/2π=848~\mathrm{MHz}$) with long relaxation time ($T_1 = 23~μ\text{s}$). The large anharmonicity significantly suppresses leakage to higher energy levels, enabling fast and precise microwave control. Using DRAG pulses and randomized benchmarking, the qubit achieves gate fidelities exceeding 99.9\%, highlighting the capability of C-shunt flux qubits for robust and high-performance quantum operations. These results establish them as a promising platform for scalable quantum information processing.

Figures (3)

• Figure 1: C-shunt flux qubit device. (a) Circuit schematic highlighting the Josephson junction loop (red dashed box). (b) False-color optical micrograph of the device, showing the feedline (yellow) coupled to a readout resonator (pink), the shunt capacitor (blue), airbridges (green) for ground connection, dedicated Z-bias (orange) and XY control (purple) lines for flux and microwave driving, respectively. The inset highlights the junction loop (red). (c) Scanning electron micrographs of the junction area: (i) overview of the junction loop, (ii–iii) enlarged views of junctions with different sizes.
• Figure 2: Performance characteristics of the qubit.(a-b) Qubit transition spectrum of the $\left|g\right\rangle \leftrightarrow |e\rangle$ and $|g\rangle \leftrightarrow |f\rangle$ transitions versus external magnetic flux. (c) Energy relaxation time $T_1$ versus external magnetic flux, showing a maximum around the sweet spot. (d–f) Representative measurements and exponential fits of relaxation and dephasing times at $\Phi_0/2$, yielding $T_1=23~\mu\mathrm{s}$, $T_2^{\mathrm{Ramsey}} = 6.3~\mu\mathrm{s}$, and $T_2^{\mathrm{spinecho}} = 17.4~\mu\mathrm{s}$ at $\Phi_0/2$, which indicate the coherence properties of the device.
• Figure 3: Randomized benchmarking (RB) of single-qubit gates on the C-shunt flux qubit. The plot shows the measured ground-state survival probability $P(|g\rangle)$ as a function of the number of Clifford gates. Data points represent experimental measurements with error bars from statistical uncertainty, while solid lines correspond to exponential fits. The initial survival probability matches the independently calibrated readout fidelity $F_e=0.75$ (see Fig. S6 in supplementary material). The extracted reference Clifford fidelity is $99.68\% \pm 0.02\%$. Single-gate fidelities for the identity gate and the $X_{\pm \pi/2}$ and $Y_{\pm \pi/2}$ rotations are obtained from interleaved randomized benchmarking using Eq. (7). The resulting values are: identity gate ($I$, $99.99\% \pm 0.02\%$), $\pi/2$ and $-\pi/2$ rotations around $X$ ($99.92\% \pm 0.02\%$ and $99.94\% \pm 0.02\%$), and $\pi/2$ and $-\pi/2$ rotations around $Y$ ($99.91\% \pm 0.02\%$ and $99.91\% \pm 0.02\%$).