  The polarization of light conveys crucial information about the spatial
ordering and optical properties of a specimen. However, precise polarization
measurement in challenging conditions, including constrained spaces, low light
levels, and high-speed scenarios, remains a severe challenge. Addressing this
problem, we introduce a real-time polarization measurement method accurate down
to a single-photon level that provides complete information about the
polarization state. Free of moving components, the polarization sensor utilizes
a few-mode fiber followed by a fiber array and a detector array. The
calibration of the sensor relies on a neural network yielding unprecedented
accuracy across all polarization states, including partially polarized light.
We validate the approach by visualizing the polarization structure of a
biological specimen. Our method offers an efficient and reliable solution for
real-time polarization sensing and microendoscopy under low-light conditions.
