Please see attached image below.

enter image description here The DEVICE is beaming two streams of entangled particle pairs (B1 and B2) in opposite directions (e.g.: Particle no. 1 (p1) goes left, Particle no. 2 (p2) goes right, where p1 and p2 are making up an entangled particle pair). Let us follow the left particle beam (B1) for now. When B1 encounters the Double Slit Apparatus (A1) in its way, an interference pattern will form on the Detector Screen (S1) behind. There is a switchable Which-way Detector (D1) that can monitor Double Slit Apparatus (A1). If D1 is switched off, the interference pattern prevails; however, if D1 is active, the wave function collapses and draws a ballistic pattern over time.

My questions are:

  1. what pattern is detected by S2 if D1 is active?
  2. and what pattern emerges on S2 when it is turned off?

Thank you!

  • $\begingroup$ entangled on which state ? $\endgroup$ – user46925 Jan 10 '16 at 22:29
  • $\begingroup$ @igael How does the state relate to the patterns detected on the screens? $\endgroup$ – Balázs Jan 10 '16 at 23:05
  • $\begingroup$ The double slits won't care about the entanglement. You could correlate single photon detections on the screens between A and B... but this is just another quantum Rube Goldberg machine that teaches nothing about physics. $\endgroup$ – CuriousOne Jan 10 '16 at 23:10
  • $\begingroup$ @Balázs : the following question is 'what is the relation between this entanglement and the 2 slits exp. ?' $\endgroup$ – user46925 Jan 11 '16 at 4:21

Most importantly, the pattern detected at S2 will not depend on the detector D1. Otherwise, this could be used for faster-than-light signalling, which is impossible within quantum mechanics.

Beyond that, what you will see at S2 will depend on which degree of freedom has been entangled. Generally, one would expect to see an interference pattern, but if you manage to entangle a degree of freedom which relates to the slit taken by the photon, there will be no interference pattern.

  • $\begingroup$ What is the reason why S2 observations will not "depend" on D1 collapsing p1's superposition when determining which way it went? Why would its entangled pair p2 not show correlation with p1 when measured at S2? $\endgroup$ – Balázs Jan 11 '16 at 0:33
  • $\begingroup$ The observation at site 2 will possibly depend on the outcome of the which-way measurement (if it is done), but not on the fact whether it is done. So there is correlation of results, but no causal dependence between actions. $\endgroup$ – Norbert Schuch Jan 11 '16 at 0:49
  • $\begingroup$ Could you elaborate please how entanglement on different degrees of freedom in a pair of particles may produce different results on S2? $\endgroup$ – Balázs Jan 11 '16 at 0:55
  • $\begingroup$ @Balázs If the degree of freedom which is entangled determines the slit taken, it will destroy the interference pattern, otherwise it won't. $\endgroup$ – Norbert Schuch Jan 11 '16 at 10:17

When entangled photons are created in Bell-type experiments, it is their polarizations which are entangled. It is not clear to me that their positions or momenta are entangled in the same way. But anyway, engtanglement does not violate "no action at a distance" and in particular there is no way to see at location S1 what measurement is being made at location S2. Therefore it is clear that an interference pattern observed at location S2 cannot change according to how the photons are measured at location S1.

  • $\begingroup$ If there is no way to see at location S2 what measurement is being made at location S1, how can measurement correlations between the entangled particle pairs be established? In other words: how would you confirm that particles of the entangled pairs were in fact entangled? $\endgroup$ – Balázs Jan 10 '16 at 23:13
  • $\begingroup$ In your experiment, you are assuming entanglement. With that understood, I am not aware that you can check for entanglement once you started the experiment. $\endgroup$ – Ed Yablecki Jan 10 '16 at 23:17
  • $\begingroup$ @Balázs Correlation is not equal to causation. $\endgroup$ – Norbert Schuch Jan 10 '16 at 23:33
  • $\begingroup$ @EdYablecki when one half of an entangled particle pair decoheres to a certain state, the other half is always found having (anti)correlated states. I won't be able to name the specific experiment that confirmed this, but this is kind of a fact (if I understand it right), which means the scientists had to have a way to measure both ends. $\endgroup$ – Balázs Jan 11 '16 at 0:12
  • $\begingroup$ @Balázs, you may be mixing up entanglement and the observer effect. This is what the experiment is testing if photons that are entangled are also entangled for the observer effect. The answer is no, observer effect is not an entangle state, polarization is. $\endgroup$ – Ed Yablecki Jan 11 '16 at 1:03

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