Absorption measurement is an exceptionally versatile tool for many applications in science and engineering. For absorption measurements using laser beams of light, the sensitivity is theoretically limited by the shot noise due to the fundamental Poisson distribution of the photon number in laser radiation. In practice, the shot-noise limit can only be achieved when all other sources of noise are eliminated. Here, we use seeded squeezed light to demonstrate that direct-absorption measurements can be performed with a sensitivity beyond the shot-noise limit. We present a practically realizable scheme, where intensity-squeezed beams are generated by a seeded four-wave mixing process in an atomic rubidium-vapor cell. More than 1.2 dB quantum advantage for the measurement sensitivity is obtained at faint absorption levels ($\le 10\mathrm{\%}$). We also present a detailed theoretical analysis to show that the observed quantum advantage when corrected for optical loss would be equivalent to 3 dB. Our experiment demonstrates a direct sub-shot-noise measurement of absorption that requires neither homodyne or lock-in nor logic coincidence-detection schemes. It is therefore very applicable in many circumstances where sub-shot-noise-level absorption measurements are highly desirable.

中文翻译：

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籽晶压缩光的量子优势，用于吸收率测量

吸收测量是科学和工程学中许多应用程序的一种非常通用的工具。对于使用激光束的吸收测量，由于激光辐射中光子数的基本泊松分布，理论上，灵敏度受到散粒噪声的限制。实际上，仅当消除所有其他噪声源时才能达到散粒噪声限制。在这里，我们使用种子压缩光来证明可以以超出散粒噪声极限的灵敏度执行直接吸收测量。我们提出了一种切实可行的方案，其中在原子rub蒸气池中通过种子四波混合过程产生了强度压缩的光束。在微弱的吸收水平下，获得了超过1.2 dB的测量灵敏度量子优势（$\le 10\mathrm{％}$）。我们还提出了详细的理论分析，以表明在校正光损耗时观察到的量子优势将相当于3 dB。我们的实验表明，不需要吸收零差或锁定，也不需要逻辑符合检测方案即可直接测量吸收的子散粒噪声。因此，它非常适用于非常需要亚散粒噪声级吸收测量的许多情况。