[论文解读] Coherent oscillations between classically separable quantum states of a superconducting loop

Vladimir Manucharyan, Jens Koch+3 more|ArXiv.org|Oct 16, 2009

Quantum Information and Cryptography参考文献 29被引用 5

一句话总结

该论文在超导通量onium量子比特中实现了经典可分离通量态之间的相干宏观量子振荡，通过高电感回路和一种新颖的非共振读出方案避免了Purcell效应，实现了580的相干性品质因子。该系统表现出亚GHz量级的振荡且退相干极低，验证了在一种稳健、可重置的量子比特平台中存在宏观量子相干性。

ABSTRACT

Ten years ago, coherent oscillations between two quantum states of a superconducting circuit differing by the presence or absence of a single Cooper pair on a metallic island were observed for the first time. This result immediately stimulated the development of several other types of superconducting quantum circuits behaving as artificial atoms, thus bridging mesoscopic and atomic physics. Interestingly, none of these circuits fully implements the now almost 30 year old proposal of A. J. Leggett to observe coherent oscillations between two states differing by the presence or absence of a single fluxon trapped in the superconducting loop interrupted by a Josephson tunnel junction. This phenomenon of reversible quantum tunneling between two classically separable states, known as Macroscopic Quantum Coherence (MQC), is regarded crucial for precision tests of whether macroscopic systems such as circuits fully obey quantum mechanics. In this article, we report the observation of such oscillations with sub-GHz frequency and quality factor larger than 500. We achieved this result with two innovations. First, our ring has an inductance four orders of magnitude larger than that considered by Leggett, combined with a junction in the charging regime, a parameter choice never addressed in previous experiments. Second, readout is performed with a novel dispersive scheme which eliminates the electromagnetic relaxation process induced by the measurement circuit (Purcell effect). Moreover, the reset of the system to its ground state is naturally built into this scheme, working even if the transition energy is smaller than that of temperature fluctuations. As we argue, the MQC transition could therefore be, contrary to expectations, the basis of a superconducting qubit of improved coherence and readout fidelity.

研究动机与目标

• 在超导环中实现并观测A.J. Leggett所提出的两个经典可分离量子态之间的宏观量子相干性（MQC）。
• 克服MQC系统中两大退相干挑战：(i) 测量电路引起的阻尼（Purcell效应）；(ii) 环路中的通量噪声。
• 开发一种超导量子比特架构，实现空间分离的通量态之间长寿命、相干的振荡，且波函数重叠极小。
• 展示一种读出与重置机制，能够保持相干性，并实现高保真度初始化与测量。

提出的方法

• 设计一种电感比以往RF-SQUID实验高出四个数量级的通量onium量子比特，以降低对外部通量噪声的敏感性。
• 在小约瑟夫森结中工作于充电 regime，以最小化隧穿频率并增强态的可分性。
• 采用高品质因子（Q ≈ 400）的传输线谐振器实现非共振读出方案，避免电磁弛豫和Purcell阻尼。
• 使用叉指电容将量子比特耦合至谐振器，实现强且可调的耦合，耦合强度g ≈ 2π × 135 MHz。
• 施加重置脉冲将量子比特初始化至基态，且该非共振读出机制本身具备自然重置能力。
• 通过Ramsey干涉测量MQC相干性，使用两个相隔自由演化时间的π/2脉冲，观测|L⟩与|R⟩态之间的振荡。

实验结果

研究问题

• RQ1 能否在超导环中观测到两个经典可分离通量态之间的相干振荡，且退相干极低？
• RQ2 能否在保持高保真度读出与重置的同时，避免超导量子比特中的Purcell效应？
• RQ3 高电感、低隧穿频率的通量onium架构是否能抑制由通量噪声和环境耦合引起的退相干？
• RQ4 此类系统中MQC振荡的实际相干时间是多少？与理论噪声限制预测相比如何？
• RQ5 该系统能否作为测试宏观系统中量子力学极限的稳健平台？

主要发现

• 在时间域中观测到左、右通量态|L⟩与|R⟩之间的相干振荡，Ramsey条纹衰减时间T_Ramsey = 250 ns。
• 测得的相干性品质因子为Q_MQC = T_Ramsey × ω_01 = 580，显著高于典型超导量子比特。
• 测得的相干时间（250 ns）远短于弛豫时间T_1 > 5 μs，表明存在除能量弛豫外的主导退相干通道。
• 尽管存在典型通量噪声水平（1 Hz时为10⁻⁶ Φ₀/Hz¹/²）和临界电流噪声（1 Hz时为10⁻⁶ I₀/Hz¹/²），理论预测的相干时间仍超过10 ms，表明存在未被解释的退相干机制。
• 系统相干性在远离半通量量子点时增强，表明在Φ_ext = Φ₀/2处，电感的导数dω₀₁/dL最小。
• 该量子比特的低跃迁频率（亚GHz）和强健的重置能力，使其在与纳米机械系统耦合及未来拓扑量子比特架构中具有广阔前景。