The red and blue laser beams (on top and bottom) represent two possible paths.

Photons start off in the |1,0⟩ state. The lower beam splitter applies an operation on the photon state and the phase shifters apply additional operations. The top beam splitter applies the same operation as the first beam splitter. All the operations are unitary, meaning they are reversible. In the absence of phase shifts, 100% of the laser beam intensity goes to the top detector.

According to classical optics, the beam should always be split in half at the end, but quantum mechanics violates the classical picture.

This experiment shows the inherent probabilistic nature of quantum effects. In principle, the experiment should work with a single photon.