From Quantum mysteries
In fact, the team has carried out several tests of the stranger predictions of quantum theory, but the most dramatic is what they call the "quantum eraser". In this variation on the Young's slit theme, the experiment is first set up in the usual way, and run to produce interference. Quantum theory says that the reason why interference can occur, even if light is a stream of photons, is that there is no way to find out, even in principle, which photon went through which slit. The "indeterminacy" allows fringes to appear. But then Chiao and his colleagues ran the same experiment with polarising filters in front of each of the two slits. Any photon going one way would become "labelled" with left-handed circular polarization, while any photon going through the other slit is labelled with right-handed circular polarization. In this version of the experiment, it is possible in principle to tell which slit any particular photon arriving at the second screen went through. Sure enough, the interference pattern vanishes -- even though nobody ever actually looks to see which photon went through which slit. Now comes the new trick -- the eraser. A third polarising filter is placed between the two slits and the second screen, to scramble up (or erase) the information about which photon went through which hole. Now, once again, it is impossible to tell which path any particular photon arriving at the second screen took through the experiment. And, sure enough, the interference pattern reappears! The strange thing is that interference depends on "single photons" going through both slits "at once", but undetected. So how does a single photon arriving at the first screen know how it ought to behave in order to match the presence or absence of the erasing filter on the other side of the slits? All of these experiments were carried out using beams of individual photons, and there is no way in which the results can be explained by using classical physics. They lay bare the mysteriousness of quantum mechanics in all its glory, and in particular demonstrate its "non local" nature -- the way in which a photon starting out on its journey behaves in a different way for each experimental setup, as if it knew in advance what kind of experiment it was about to go through. |
My point is that more "things" do not necessarily connote more "information". In the QM experiment the evidence of information is a direct pattern.