Now, the same photon is moving through two different parts of the device. The photon that passed through the mirror is now on the "lower path". It may or may not encounter a bomb, which is designed to explode if it encounters a single photon. The photon that was reflected off the mirror is now on the "upper path". Both photons next encounter a normal mirror. The "lower-path" photon is reflected ninety-degrees upward (if it did not detect a bomb). The "upper-path" photon is reflected back ninety degrees so that it is returned to its original trajectory.