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arXiv:2603.11692

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

Silu Zhao, Li Li, Weiping Yuan, Xinhui Ruan, Jinzhe Wang, Bingjie Chen, Yunhao Shi, Guihan Liang, Shi Xiao, Jiacheng Song, Jinming Guo, Xiaohui Song, Kai Xu, Heng Fan, Zhongcheng Xiang, Dongning Zheng

quant-phSubmitted Mar 12, 2026• Announced Mar 13, 2026• 0 citations
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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.

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We demonstrate high-fidelity single-qubit gates on a C-shunt flux qubit that simultaneously combines a large anharmonicity (A/2π=848 MHz\mathcal{A}/2π=848~\mathrm{MHz}A/2π=848 MHz) with long relaxation time (T1=23 μsT_1 = 23~μ\text{s}T1​=23 μs). 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.
We demonstrate high-fidelity single-qubit gates on a C-shunt flux qubit that simultaneously combines a large anharmonicity (A/2π=848 MHz) with long relaxation time (T1=23 μs). The large anharmonicity significantly suppresses leakage to higher energy levels, enabling fast and precise microwave control. Using derivative removal by adiabatic gate 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.
Superconducting devices as quantum bits (qubits) are one of the most promising candidates for implementing a practical quantum computer. Over the last decade, quantum coherence times of superconducting qubits have been continuously improved. Operating qubits in noise insensitive points, eliminating the participation of the lossy materials, and better isolation from environment increased the decoherence times from nanoseconds to tens of microseconds. One recent approach on the qubit design is to shunt the Josephson junctions with large capacitors to reduce the sensitivity to noise and to redistribute the stored electromagnetic energy in a well-engineered area. This approach was first applied on charge qubits and later adopted to other types of superconducting qubits. \n \nIn this thesis, we present the design and the characterization of a three-Josephson junction superconducting circuit with three large shunt capacitors. We present experiments on decoherence in the circuit used as a qubit. The qubit relaxation time T₁ is reaching as high as 47 μs and the spin-echo dephasing time T₂ = 9.4 μs. In addition, we present spectroscopy experiments and decoherence characterization in the qutrit subspace formed by the lowest three energy levels. The spectroscopy data is in excellent agreement with the complete circuit model based on the system capacitance matrix. At the flux symmetry point, the circuit has a large anharmonicity, defined as the difference between the 1-2 and 0-1 transition frequencies, approaching 2π × 3.7 GHz, which is enabling fast single qubit operations. We performed randomized benchmarking with the qubit gates with a duration of 1.62 ns and an average gate fidelity of 99.9%. \n \nMoreover, we present experiments with multi-level control in qutrit subspace, formed by the lowest three energy levels. We demonstrate a single step experimental implementation of a generalized Walsh-Hadamard gate. We use a decomposition of the quantum gate into two unitary operations, one implemented by an off-diagonal Hamiltonian and the other implemented by a diagonal Hamiltonian. The off-diagonal Hamiltonian is obtained by the simultaneous driving of the transitions between levels 0-1, 1-2, and 0-2, with the latter being a two-photon process. The diagonal Hamiltonian is effectively implemented by appropriately shifting the phases of adjacent pulses. The gate is characterized using tomography process and the average fidelity exceeds 90%, in good agreement with numerical simulations that take into account the multi-level structure of the system.
We present the design and characterization of a three-Josephson-junction superconducting loop circuit with three large shunt capacitors. Used as a qubit, the circuit shows long energy-relaxation times, of the order of $40\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{s}$ and a spin-echo dephasing time of $9.4\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{s}$. The circuit has high anharmonicity, of $2\ensuremath{\pi}\ifmmode\times\else\texttimes\fi{}3.69\phantom{\rule{0.2em}{0ex}}\mathrm{GHz}$. We extract the multilevel relaxation and dephasing rates of the circuit used as a qutrit and discuss the possible sources for the decoherence. The high anharmonicity allows for fast qubit control with nanosecond-range gate durations and a measured average gate fidelity of 99.92%, characterized by randomized benchmarking. These results demonstrate interesting potential use for fast nanosecond-time-scale two-qubit gates and multilevel quantum logic.
We report a demonstration of a self-shunted superconducting flux qubit (SSFQ) as an ultracompact, strongly anharmonic, and long-lifetime qubit by using the full epitaxially grown NbN/AlN/NbN Josephson junctions. The NbN-junction-based SSFQ achieved a high anharmonicity of 797 MHz as well as drastically reduced footprint of the qubit compared to capacitively shunted (C-shunt) flux qubits with a large shunt capacitor. The lifetime of the SSFQ reached 13 - 25 μμμs, which is comparable to or relatively higher than that of NbN-based C-shunt flux qubits and Al-based flux qubits with more than 10 times smaller junctions. This indicates that the large epitaxial NbN-based junction can be adopted for SSFQ as well as merged-element transmon with the relatively large junctions. The results presented in this work will offer a new choice of the scalable qubits as well as the material platform toward the realization of large-scale superconducting quantum computers.
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We proposed and theoretically investigated the self-shunted superconducting flux qubits (SSFQs) with relatively large Josephson junctions with the diameter over1  μm\text{1}\; \mu \text{m}1μm. Because of the large self-capacitance of several 10 fF in the junctions, the large shunt capacitor with dimensions of several 100$ \mu \text{m}oftheconventionalcapacitivelyshuntedfluxquantumbits(qubits)canbeeliminated,resultinginsignificantreductionofthefootprint.WesystematicallycalculatedtheSSFQperformances,suchastheanharmonicity,andinvestigatedtheirdependencesonthecircuitparameters.Asaresult,itwasshownthattheSSFQshavethelargeanharmonicityof400–2000MHzforthereasonableparameters,suchasthelargejunctionareaofaround1of the conventional capacitively shunted flux quantum bits (qubits) can be eliminated, resulting in significant reduction of the footprint. We systematically calculated the SSFQ performances, such as the anharmonicity, and investigated their dependences on the circuit parameters. As a result, it was shown that the SSFQs have the large anharmonicity of 400–2000 MHz for the reasonable parameters, such as the large junction area of around 1oftheconventionalcapacitivelyshuntedfluxquantumbits(qubits)canbeeliminated,resultinginsignificantreductionofthefootprint.WesystematicallycalculatedtheSSFQperformances,suchastheanharmonicity,andinvestigatedtheirdependencesonthecircuitparameters.Asaresult,itwasshownthattheSSFQshavethelargeanharmonicityof400–2000MHzforthereasonableparameters,suchasthelargejunctionareaofaround1 \mu \text{m}^{2}andthesmallcriticalcurrentdensityofaround10and the small critical current density of around 10andthesmallcriticalcurrentdensityofaround10\text{A/cm}^{2}$. Furthermore, we derived and estimated the relative relaxation time of the SSFQs due to the charge and flux noises. As a result, we found that the high anharmonicity over 800 MHz could be achieved without degrading the relaxation time from that of the high-coherence flux qubits. The SSFQs offer the new choice of the highly scalable qubits, which achieve both the small footprint and high anharmonicity.