Quantum Teleportation

interference of 2 photons
at a beamsplitter

If one overlaps two particles at a beamsplitter, interference effects determine the probabilities to find the two particles incident one each from a and b either both in one of the two outputs c and d or to find one in each output.

Only if two photons are in the state

they will leave the beamsplitter into different output arms. If one puts detectors there, a click in each of them, i.e. a coincidence, means the projection of the two photons onto the state PSI-

For the other three Bell states both photons will exit together through one of the two output arms. Here polarisation analysis can separate the state PSI+

from the states PHI+-

. (This feature of interferometric Bell-state analysis was used in the experiment demonstrating Quantum Dense Coding.)

The identification of all 4 Bell-states will be possible if one couples the two particles by some interaction. This should be soon possible for various systems (photons, atoms or trapped ions) which will also form the first 2-qubit quantum logic gates.