Encoding quantum information in quantum states with disjoint wave-function support and noise-insensitive energies is the key behind the idea of qubit protection. While fully protected qubits are expected to offer exponential protection against both energy relaxation and pure dephasing, simpler circuits may grant partial protection with currently achievable parameters. Here, we study a fluxonium circuit in which the wave functions are engineered to minimize their overlap while benefiting from a first-order-insensitive flux sweet spot. Taking advantage of a large superinductance (𝐿∼1  μ⁢H), our circuit incorporates a resonant tunneling mechanism at zero external flux that couples states with the same fluxon parity, thus enabling bifluxon tunneling. The states |0⟩ and |1⟩ are encoded in wave functions with parities 0 and 1, respectively, ensuring a minimal form of protection against relaxation. Two-tone spectroscopy reveals the energy-level structure of the circuit and the presence of 4⁢𝜋 quantum-phase slips between different potential wells corresponding to 𝑚=±1 fluxons, which can be precisely described by a simple fluxonium Hamiltonian or by an effective bifluxon Hamiltonian. Despite suboptimal fabrication, the measured relaxation (𝑇1=177±3  μ⁢s) and dephasing (𝑇E2=75±5  μ⁢s) times not only demonstrate the relevance of our approach but also open an alternative direction toward quantum computing using partially protected fluxonium qubits.

中文翻译：

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使用 Bifluxon 隧道保护 Fluxonium Qubit


使用不相交的波函数支持和对噪声不敏感的能量在量子态中编码量子信息是量子比特保护理念背后的关键。虽然完全受保护的量子比特有望提供针对能量弛豫和纯分相的指数保护，但更简单的电路可能会使用当前可实现的参数提供部分保护。在这里，我们研究了一个磁通电路，其中的波函数被设计为最小化它们的重叠，同时受益于一阶不敏感的磁通量最佳点。利用大超电感 （L∼1 μH），我们的电路在零外磁通量时集成了谐振隧穿机制，该机制以相同的磁通量奇偶性耦合状态，从而实现双磁通隧穿。状态 |0⟩ 和 |1⟩ 分别用奇偶校验 0 和 1 的波函数编码，确保一种最小形式的防止松弛。双音光谱揭示了电路的能级结构以及对应于 m=±1 磁通量的不同电位阱之间存在 4π 量子相位滑移，这可以用简单的磁通量哈密顿量或有效的双磁通量哈密顿量来精确描述。尽管制造方式不理想，但测得的弛豫 （T1=177±3 μs） 和去相 （TE2=75±5 μs） 时间不仅证明了我们方法的相关性，而且为使用部分保护的磁通量子比特的量子计算开辟了另一种方向。