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i am a year 12 student and as i was reading a book written by Brain Clegg, i hit a roadblock. It would be great if somone could help me out with this.

when we conduct young's double slit experiment with a single photon , it is seen that we get an interference pattern due to the probability of the photon passing through either of the slits. Now, if we were to place a detector before the slits, we would know for sure through which slit the photon would pass. In the experiment does the results still show an interference pattern even though we know the location of the photon or does it disappear? And is the interference pattern still produced without a detector?

Thank you

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  • $\begingroup$ More on single photon in YDSE. $\endgroup$ – Qmechanic Oct 25 '15 at 12:19
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    $\begingroup$ "The photon" does not pass trough either slit. The photon is simply the outcome of a measurement on a quantum field, i.e. it and it doesn't even exist as a physical object until after you perform the measurement. After you have performed the measurement, the photon is simply a bunch of numbers (time of arrival, detector location and polarization), so it doesn't exist as a physical object any more than the center of mass coordinates of a classical object exists as "an object", either. The conceptual problem with the double slit is a direct result of the meaning of the word "photon". $\endgroup$ – CuriousOne Oct 25 '15 at 14:55
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A common misunderstanding is that the wavefunction describes a particle (e.g., a photon or an electron), and that interference is something that happens to a particle.

Neither is true. In fact, the square of wavefunction at a point describes probability of detection of the particle at that point. Interference is evidence that the "probability of detection" for all points follows a wave equation. That implies that interference can only be detected by performing a large number of identical detections, and not by performing a single detection.

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There is no way to measure the photon without disturbing it. So the version of the double-slit experiment that you describe above has to be done with an electron, which you can measure in flight. It's typically called the "which way" experiment.

In your description above, the interference pattern disappears. What you are describing is a variant of the quantum eraser experiment. I have not been pleased with the descriptions and explanations of the experiment that I have found, but I'm told it makes sense if you look at the math.

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