A monolithic Mach-Zehnder Interferometer (MZI) is provided for photon beam-splitting and beam-combining with accurate super-positioning of the outgoing beams, which creates the interference. The MZI includes a complimentary pair of right-isosceles triangular prisms, and several reflector units. The triangular prisms are configurable to physically join together along associated hypotenuse surfaces that form a beam-splitter interface, thereby producing a rectangular prism having a square cross-section with four outer side surfaces. Each reflector unit forms a right-isosceles mirror that rigidly faces a corresponding surface of the four outer side surfaces of the rectangular prism. The MZI further includes a spacer disposed between the corresponding surface and the each reflector unit.