Shaping single-mode operation in high-power fibers requires a precise knowledge of the gain-medium optical properties. This requires precise measurements of the refractive index differences (Δn) between the core and the cladding of the fiber. We exploit a quantum optical method based on low-coherence Hong-Ou-Mandel interferometry to perform practical measurements of the refractive index difference using broadband energy-time entangled photons. The precision enhancement reached with this method is benchmarked with a classical method based on single photon interferometry. We show in classical regime an improvement by an order of magnitude of the precision compared to already reported classical methods. Strikingly, in the quantum regime, we demonstrate an extra factor of 4 on the precision enhancement, exhibiting a state-of-the-art Δn precision of 6 × 10⁻⁷. This work sets the quantum photonics metrology as a powerful characterization tool that should enable a faster and reliable design of materials dedicated to light amplification.

在高功率光纤中塑造单模操作需要对增益介质光学特性有精确的了解。这需要精确测量折射率差异（Δn) 在光纤的纤芯和包层之间。我们利用基于低相干 Hong-Ou-Mandel 干涉法的量子光学方法，使用宽带能量-时间纠缠光子对折射率差异进行实际测量。用这种方法达到的精度提高是以基于单光子干涉测量的经典方法为基准的。与已经报道的经典方法相比，我们在经典方案中展示了一个数量级的精度改进。引人注目的是，在量子方案中，我们证明了精度增强的额外因子 4，展示了6 × 10 ^-7的最先进的Δn精度. 这项工作将量子光子学计量设置为一种强大的表征工具，应该能够更快、更可靠地设计专门用于光放大的材料。