Two-photon absorption switch having which-path exclusion and monolithic mach-zehnder interferometer

A two-photon absorption (TPA) switch for minimizing which-path information in quantum optic interference, comprising:

• a pulse laser that emits a pump photon in a state having pump angular frequency, said pump photon traveling along a photon-incident direction;
• a complimentary pair of right-isosceles triangular prisms 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;
• a plurality of trombone reflector units, each reflector unit forming a right-isosceles mirror that rigidly faces a corresponding surface of said four outer side surfaces of said rectangular prism;
• and
• a down-conversion crystal disposed between a pair of said plurality of trombone reflector units along said incident direction, said crystal being non-critically phase-matched with down-conversion from said pump photon to signal and idler photons controlled by temperature of said crystal, said signal and idler photons in corresponding entangled states with both photons and having half of said pump angular frequency, said entangled states being orthogonal to each other.