Fig. 1. Earth’s rotation measured using entangled photons.

(A) A rotatable 715-m² Sagnac fiber interferometer is built in a laboratory located in Vienna, Austria. (B) Simplified schematic of the experimental setup. Orthogonally polarized photon pairs are converted to path-entangled N00N states in the Sagnac interferometer via a half-wave plate (HWP) followed by a polarizing beam splitter (PBS). The frame angle Θ is defined as the angle between Earth’s angular velocity vector ${\overrightarrow{\mathbf{\Omega }}}_E$ and the fiber loop area vector $\overrightarrow{\mathbf{A}}$ . The signal is extracted by observing the phase shift of quantum interference fringes induced by Earth’s rotation, using a set of quarter-wave plates (QWP) and a HWP, in combination with single-photon coincidence counting (&). (C) An optical switch (OS) is used to toggle Earth’s rotation signal on and off independent of the frame angle Θ. This is achieved by controlling the propagation direction (clockwise or counterclockwise) of photons in one half of the fiber spool.