Quantum-limited determination of refractive index difference by means of entanglement

TL;DR

This paper demonstrates a quantum optical method using entangled photons to measure the refractive index difference in fibers with unprecedented precision, surpassing classical techniques and enabling better fiber design.

Contribution

It introduces a quantum Hong-Ou-Mandel interferometry approach for measuring refractive index differences with higher accuracy than classical methods.

Findings

Achieved a fourfold accuracy improvement in quantum regime

Demonstrated a precision of 6×10^{-7} in refractive index difference

Benchmarking shows an order of magnitude better precision than classical methods

Abstract

Shaping single-mode operation in high-power fibres requires a precise knowledge of the gain-medium optical properties. This requires accurate measurements of the refractive index differences ($\Delta$n) 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 accuracy enhancement, exhibiting a state-of-the-art $\Delta$n 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.