Single-layer transition metal dichalcogenides provide a promising material system to explore the electron’s valley degree of freedom as a quantum information carrier. The valley degree of freedom can be directly accessed by means of optical excitation. However, rapid valley relaxation of optically excited electron-hole pairs (excitons) through the exchange interaction has been a major roadblock. Theoretically such valley relaxation is suppressed in dark excitons, suggesting a potential route for long valley lifetimes. Here we develop a waveguide-based method to detect time-resolved and energy-resolved dark exciton emission in single-layer WSe₂, which involves spin-forbidden optical transitions with an out-of-plane dipole moment. The valley degree of freedom of dark excitons is accessed through the valley-dependent Zeeman effect under an out-of-plane magnetic field. We find a short valley lifetime for the dark neutral exciton, likely due to the short-range electron-hole exchange, but long valley lifetimes exceeding several nanoseconds for the dark charged excitons.

单层过渡金属二卤化物提供了一种有前途的材料系统，可探索电子的谷自由度作为量子信息载体。谷自由度可以通过光激发直接访问。然而，通过交换相互作用的光激发电子-空穴对（激子）的快速谷松弛已经成为主要障碍。从理论上讲，这种暗谷弛豫在暗激子中被抑制，这暗示了长谷寿命的潜在途径。在这里，我们开发了一种基于波导的方法来检测单层WSe _2中时间分辨和能量分辨的暗激子发射，其中包含自旋禁止的光跃迁以及平面外偶极矩。在平面外磁场下，通过依赖于谷的塞曼效应来获得暗激子的谷自由度。我们发现暗中性激子的谷寿命很短，这可能是由于短距离电子-空穴交换引起的，而暗电荷激子的谷寿命却超过了几纳秒。