Questions on a revised Afshar Experiment

[StevieTNZ]

In regards to the experiment mentioned in this paper - http://arxiv.org/abs/1001.4785

(1) I understand that the particle goes along both paths to cause interference when the two paths cross each other... at that stage we don't have definite which-path information. But after the crossing, each path leads to the appropriate detector. Only one detector will click. Can we be sure that if detector 1 clicks, the particle really followed the path indicated in the diagram? Is it possible that sometimes detector 2 will click in this experiment (and that if detector 2 does click, the particle followed the opposite path to if detector 1 clicked)?

(2) Would it be correct to say that there is no violation of the complementary principle, because interference was observed at a different time to when we were certain we knew which path the particle followed? I'd would assume that interference is no longer there after the intersection of paths is finished. The particle should continue towards a detector and not stay in the intersection area.

In a way, we know by what detector clicks we can tell which path the particle followed. But we can only be definitely sure once we know what detector has clicked. Otherwise we can only say 50% of the time detector 1 will click, and 50% of the time detector 2 will click. I can't say that is which-path information in the sense that it is definite.

Should there be interference in the first place, if in fact we are in principle able to tell which path the particle went along (despite this measurement occurring after interference is seen)? Being able to distinguish the paths, in principle, should not allow interference to occur at all, right?

 

[Bill_K]

It depends on what you use to observe the interference fringes. If you interfere enough to see the fringes, then some particles will appear at Detector 2. If you don't, then they'll all go to Detector 1.

 

[eaglelake]

In regards to the experiment mentioned in this paper - http://arxiv.org/abs/1001.4785

(1) I understand that the particle goes along both paths to cause interference when the two paths cross each other... at that stage we don't have definite which-path information. But after the crossing, each path leads to the appropriate detector. Only one detector will click. Can we be sure that if detector 1 clicks, the particle really followed the path indicated in the diagram? Is it possible that sometimes detector 2 will click in this experiment (and that if detector 2 does click, the particle followed the opposite path to if detector 1 clicked)?

(2) Would it be correct to say that there is no violation of the complementary principle, because interference was observed at a different time to when we were certain we knew which path the particle followed? I'd would assume that interference is no longer there after the intersection of paths is finished. The particle should continue towards a detector and not stay in the intersection area.

In a way, we know by what detector clicks we can tell which path the particle followed. But we can only be definitely sure once we know what detector has clicked. Otherwise we can only say 50% of the time detector 1 will click, and 50% of the time detector 2 will click. I can't say that is which-path information in the sense that it is definite.

Should there be interference in the first place, if in fact we are in principle able to tell which path the particle went along (despite this measurement occurring after interference is seen)? Being able to distinguish the paths, in principle, should not allow interference to occur at all, right?

As shown in Fig. 1, there is no interference exhibited in this experiment. When detector 1 clicks we know the particle was reflected from mirror 1. This happens 50% of the time. Likewise, 50% of the time a particle from mirror 2 triggers detector 2. We have which-way information and there is no interference

There are no interference fringes ever seen at the crossover point where the beams overlap. Even with the thin wires placed at the crossover point, we still get the same result: 50% of the time detector 1 is triggered and 50% of the time detector 2 is triggered. There is no evidence of any interference effects.

Afshar apparently refuses to accept a) that the results of quantum experiments are determined by the entire apparatus and b) that quantum experiments require a measurement result. These are fundamental characteristics of a quantum experiment. If, instead the apparatus is modified to exhibit interference, then we might get detector 1 triggered 100% of the time and detector 2 never triggered. But we must not speculate about this modified experiment while actually doing the non-interference experiment shown. If we want to show interference, then we must do the actual interference experiment. That is not done here! No particles are ever detected at the crossover point and no fringes are seen, in spite of what is shown in the figure. What might have been, if we had done something differently, does not tell us anything about what the particle was doing before it was detected in the experiment actually performed.

Answers to your specific questions:
1) In this experiment both detectors have the same count rates. There is no interference. When detector 1 clicks, we know the particle took route 1. When detector 2 clicks, we know the particle took route 2.
2) Interference was never observed. There is no violation of complimentary. You are correct, and it has been well established that which way information is incompatible with interference. When Afshar shows us pictures of the interference fringes while simultaneously triggering the detectors equally, then he will have violated complimentarity. For the apparatus shown, we violate complimentarity when we see detector 2 triggered 50% of the time (no interference) and also see detector 2 never being triggered (destructive interference). This is, of course, impossible in the same experiment.
Best wishes