MFW I realized most quantum experiments in introductory books are just lies to children, they're thought experiments at best. The literal setups as described either never work in practice, or don't really prove their results due to loopholes.

An interferometer with a single photon is the worst offender. Ordinary light sources like lamps or lasers can never work as single-photon sources. Even if you add a huge attenuator and make the light intensity lower than the energy of a single photon, there's still a non-negligible probability that it will emit multiple photons. In fact all experiments before the 1980s had this loophole. Creating true "anti-bunching" single-photon sources requires esoteric techniques. Still, this probability can only approach but not reach 0% in practical devices.

For the same reason, the BB84 quantum key distribution protocol often don't work in practice in its original form. Source or detector imperfections make the original protocol often totally broken. physics.unm.edu/Courses/Deutsc