Question

I am a beginner in this quantum-mechanics stuff. I understand the quantum eraser only from an experimental view. So I didn't understand the formalism that describes the quantum eraser. But what does the experiment tells us? Does the photon know that there is somebody watching it? And this is why it behaves in another way? Does the photon also see the future?

Answer 1

No, the photon doesn't see anyone watching it. And the photon doesn't see its future, either. In fact, the photon doesn't exist in any classical sense prior to its observation.

All of its properties - e.g. which slits it could be taking; whether it behaves more as a particle or a wave etc. - are encoded in the wave function until the very moment of the measurement which is why they may always be "changed back" to the previous answers. For example, in quantum eraser, the photon is ordered to behave as a wave again, even though a premature argument could lead a sloppy person to think that the photon has already decided to behave as a particle forever.

When you measure the photon, it is finally possible to think of its properties classically and the wave function allows one to calculate all probabilities that the outcome will be something or something else. In the case of the quantum eraser, we restore the interference pattern. But any attempt to "imagine" that the photon has obtained a classical property at any moment before it was measured would lead to wrong predictions.

It is always essential to appreciate that the photon always behaves according to the laws of quantum mechanics and we're never allowed to approximate it by any classical intuition because the classical intuition fails. This strict requirement that classical mechanics is wrong may only be partly circumvented after the photon is actually detected (because then it interacts with a classical object that quickly decoheres) - but not earlier than that. In other words, quantum mechanics always holds: that's the main lesson of this experiment (and many others).

Sb1 says that it was remarkable that the experiment behaved as Scully and Druhl predicted. I disagree with this wording. The prediction could have been made by any father of quantum mechanics - no new physics was used whatsoever and they could predict the behavior of any setup of this kind. It could have been remarkable in the 1920s but after the 1920s, all such experiments were mundane physics.