We present the detection of the highly forbidden $2{{}^{3}S}_1\to 3{{}^{3}S}_1$ atomic transition in helium, the weakest transition observed in any neutral atom. Our measurements of the transition frequency, upper state lifetime, and transition strength agree well with published theoretical values and can lead to tests of both QED contributions and different QED frameworks. To measure such a weak transition, we develop two methods using ultracold metastable ($2{{}^{3}S}_1$) helium atoms: low background direct detection of excited then decayed atoms for sensitive measurement of the transition frequency and lifetime, and a pulsed atom laser heating measurement for determining the transition strength. These methods could possibly be applied to other atoms, providing new tools in the search for ultraweak transitions and precision metrology.

中文翻译：

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直接测量氦中2 ^ {3} S_ {1}→3 ^ {3} S_ {1}的原子跃迁。

我们提出了高度禁止的检测 $2{{}^3\mathrm{小号}}_{\mathrm{1个}}\to 3{{}^3\mathrm{小号}}_{\mathrm{1个}}$氦中的原子跃迁，是任何中性原子中观察到的最弱的跃迁。我们对跃迁频率，上态寿命和跃迁强度的测量与已发表的理论值非常吻合，并且可以导致对QED贡献和不同QED框架的测试。为了衡量这种微弱的过渡，我们开发了两种使用超冷亚稳（$2{{}^3\mathrm{小号}}_{\mathrm{1个}}$）氦原子：低本底直接检测激发的然后衰变的原子，以便敏感地测量跃迁频率和寿命，以及脉冲原子激光加热测量来确定跃迁强度。这些方法可能会应用于其他原子，从而为寻找超弱跃迁和精确计量提供了新的工具。