发表机构
Quantum Advanced Research Center, CSIC; Instituto de Ciencia de Materiales de Madrid, CSIC; Laboratoire Collisions Agrégats Réactivité, UMR 5589, FERMI, Université de Toulouse, CNRS（西班牙科学研究中心量子先进研究中心; 西班牙科学研究中心马德里材料科学研究所; 图卢兹大学FERMI碰撞聚合反应实验室（UMR 5589，CNRS））
机构由 AI 辅助整理，请以论文原文为准。AI 中文总结
该研究针对强驱动两能级系统的Floquet准能简并，解析推导并数值验证了其携带的ℤ₂拓扑荷，提出可分离几何相位并观测该荷的实验方案。
AI 中文摘要
强驱动两能级系统的准能谱随驱动参数变化呈现锥形交叉，该交叉由隐藏的非时间局域对称性导致。本文证明，每个此类交叉携带一个量子化拓扑荷：沿环绕锥形的绝热回路获得的Floquet-Berry相位等于一个ℤ₂取值的荷。我们进一步识别出第二类简并，其发生在驱动振幅为零时，此时能级分裂匹配场的m个能量量子。沿斯塔克移动的共振线，最小准能隙以|A|^m形式打开，且仅当m为奇数时荷是非平凡的。我们通过对处于有效二维磁场中的自旋-1/2粒子进行微扰约化，解析推导了这两个结果，并通过Bargmann不变量数值验证。此外，我们提出一种基于手性的方案，该方案可抵消动力学相位以分离几何相位，以及一种基于辅助比特的Ramsey读出方案，可使拓扑荷直接可观测。
英文摘要
The quasienergy spectrum of a strongly driven two-level system as a function of the driving parameters exhibits conical intersections, which are enabled by hidden time-nonlocal symmetries. We show that each such crossing carries a quantized topological charge: the Floquet--Berry phase acquired along an adiabatic loop around a cone is equal to a $\mathbb{Z}_2$-valued charge. We further identify a second family of degeneracies that occurs at vanishing driving amplitude, when the level splitting matches $m$ energy quanta of the field. Along the Stark-shifted resonance line, the minimum quasienergy gap opens as $|A|^m$, and the charge is nontrivial only for odd $m$. We analytically derive both results from a perturbative reduction to a spin-$1/2$ in an effective two-dimensional magnetic field and confirm them numerically through the Bargmann invariant. Moreover, we propose a chirality-based protocol that cancels the dynamical phase to isolate the geometric one, and an ancilla-based Ramsey readout that renders the topological charge directly observable.
Comments10 pages
