Linear optics comprises one of the best examples for demonstrating quantum physics. It works at room temperatures, and can be observed with relatively simple devices. Linear optics involves physical processes that conserve the total number of photons. In the ideal case, if there are 100 photons at the beginning, no matter how complicated the physical process is, there will be exactly 100 photons left in the end.
Source:
https://phys.org/news/2019-06-physical-limit-quantum-optics-mystery.html