Patent ID: 8514478

Claim:
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; first and second dichroic mirrors disposed along said incident direction and oriented to enable said pump photon to either pass there-through or else perpendicularly reflect to parallel corresponding first and second photon-reflection directions, each being substantially perpendicular to said incident direction; a reflector disposed beyond said dichroic mirrors along said incident direction and opposite said pulse laser; a down-conversion crystal disposed between said dichroic mirrors along said incident direction, said crystal being non-critically phase-matched with down-conversion probability from said pump photon to en entangled pair of signal and idler photons controlled by temperature of said crystal and alternate probability of pump detection along said first photon-reflection direction for pump detection, said signal and idler photons in corresponding entangled states with both said photons and each having half of said pump angular frequency, said entangled states being orthogonal to each other; and a beam-splitter disposed along said second reflection direction to probabilistically reflect one of said pair that passes through said crystal and alternatively transmits said one along said second photon-reflection direction for scatter detection.