Quantum-enhanced phase sensitivity in an all-fiber Mach-Zehnder interferometer

Abstract

Recent advances in quantum photonics have enabled increasingly robust protocols in optical phase estimation, achieving precisions beyond the standard quantum limit and approaching the Heisenberg limit. While intrinsic losses hinder the realization of unconditional super-sensitivity, reaching quantum advantage, defined as sensitivity surpassing that of any classical counterpart with identical resources, remains achievable. Here we experimentally demonstrate such an advantage using a fully fibered Mach-Zehnder-type interferometer operating at telecom wavelengths, free of post-selection. The scheme relies on the conversion of polarization-entangled photon pairs, a degree of freedom commonly favored for experimental convenience, into energy-time entanglement, which is particularly well suited for scalable fiber-based sensors. All system imperfections, including asymmetric losses and detector inefficiencies, are accounted for in the Fisher information analysis, yielding a measured quantum advantage of 10%. This result highlights the practicality of compact, alignment-free quantum interferometers for real-world sensing applications.

Figures (4)

• Figure 1: a) Experimental setup for quantum advantage measurement. It features three parts i) the creation part to transform the polarization entangled state to an ET state ii) the manipulation part consisting of a folded Franson interferometer and iii) the detection part made of filters and detectors. PC : polarization controller. PBS : polarizing beam splitter. BS : beam splitter. WDMs : wavelength-division multiplexers. SNSPD : superconducting nanowire single-photon detectors. TDC : time-to-digital converter. $D_1$ to $D_4$ are the four detectors used for the measurement. The numbers indicated on the WDMs refer to standard telecom wavelength ITU channels.
• Figure 2: Experimental coincidences measured at the output of the interferometer. The error bars represent the standard Poissonian distribution. The solid lines represent a fit from Eq. \ref{['eq: fit vis']}. The difference in amplitude of each of the curves is induced by the asymmetric losses of the system.
• Figure 3: a) FI per photon as a function of the phase extracted from the measured coincidence peaks. The orange curve is the experimental data while the shaded area is the $3\sigma$ (99.7%) confidence region derived from the fit uncertainty. The blue and grey dashed lines are the theoretical expected $\mathcal{F}_1$ and $\mathcal{F}_2$, respectively. The red line corresponds to the standard quantum limit. b) Zoom in the region of interest. The orange and blue horizontal dotted lines are the maximum of the functions.
• Figure 4: Folded Franson interferometer in a Mach-Zehnder configuration. BS: Beam Splitter; PNRD: Photon Number Resolving Detector.