Remote fiber sensing by quantum spectroscopy using frequency entangled photons via a real-world fiber network

Quantum optics has attracted significant interest owing to its potential for novel applications in fields such as computing, communication, and sensing. With advancements in single photon generation and detection in the telecom band, quantum remote applications utilizing the nonlocality of entangled photon pairs are becoming increasingly viable. In this study, we demonstrate quantum remote spectroscopy by transmitting frequency entangled photons through a real-world fiber network. In the experiment, entangled photon pairs in the telecom band (photon 1 and photon 2) were generated via spontaneous parametric down-conversion in a type-II PPMgSLT crystal using a mode-locked Ti:Sapphire laser (76MHz repetition rate, 792nm center wavelength). The photon pairs were separated with a fiber polarization beam splitter. Photon 1 was transmitted through a 2.6-km real-world fiber network and programmable spectral filter, serving as the sample under investigation. Both photons were detected using superconducting single photon detectors (SSPDs). Quantum frequency correlation, termed joint spectral intensity (JSI), was obtained as a two-dimensional histogram utilizing coincidence counting and time-stretch methods. The spectrum of photon 2, which did not transmit fiber network and spectral filter, was obtained by projecting the JSI. Although photon 1 interact with the fiber network and programmable filter, the spectrum of photon 2 exhibited strong agreement with the reference spectrum defined by programmable filter. Finally, one photon of the pair was transmitted over a 2.6km real-world fiber network with a fiber sensor. Using quantum frequency correlation, the spectral information of the fiber sensor was deduced remotely from the spectrum of the counter part of the photon, which did not directly interact with the sensor. This demonstrated the feasibility of remote spectroscopic sensing using frequency entanglement via a real-world fiber network. The findings pave the way for practical quantum remote spectroscopy, obviating the need for sophisticated spectroscopic equipment at the remote site.