We know that in the double slit experiment, the reading out of the 'which route' information will destroy the interference pattern.

But the reading out of the 'which route' information can be achieved by an interaction free measurement based strategy, which means that the location of the particle can be detected in an interaction-free style. I mean here that the 'which route' information is regarded as the quality of the bomb in the quantum bomb tester so that its location can be read out by the interaction free mechanism.

I am wondering, in such a setup, will the reading out of the 'which route' information also destroy the interference pattern?

A quantum computation circuit based analysis shows that the interference pattern is destroyed. But what's interesting is that even the 'interaction free measurement' has not been carried out(no photon is sent in the bomb tester to read out the information), the interference pattern is also destroyed. So it seems that even an intention to read out the information(by just setting up a bomb tester behind a slit) will destroy the interference pattern.

As shown in the following picture, the particle of the double slit experiment (an electron with blue circle) passes one of the slits and the interaction free bomb tester tests the existence of the electron behind each slit using photons(red dots). This is somehow similar with the delayed choice experiment.

Interaction free measurement based readingout

  • $\begingroup$ So you mean that the 'which route' concept is a wrong concept? OK, we can modify the term 'which route' to 'a measurement of the state of the particle'. Then a measurement on the particle state will destroy the interference pattern. Then the interaction-free measurement can measure the state. I believe that no matter how you measure the state of the particle, the reading out of the information will destroy the interference pattern. But it seems even a setting up of a bomb tester (without shooting a photon to carryout the measurement) will destroy the interference pattern. @John Forkosh $\endgroup$ – XXDD May 30 '17 at 5:04
  • $\begingroup$ I agree that setting a Vaidman bomb tester behind the slits and reading out the location information of the particles in the double slit experiment WILL destroy the interference pattern. My question is : If we just set up the configuration (a double slit experiment with one or two bomb tester behind one or both of the slits), but during the double slit experiment we do not shoot photons in the bomb tester to read the particle location information. Will this also destroy the interference pattern? My analysis shows the answer is YES. This sounds strange.@John Forkosh $\endgroup$ – XXDD May 30 '17 at 5:10

If "which route" information is collected, positively or negatively, the interference pattern will vanish. Thus, if you have a live bomb, you will only see a single band of light (and then the bomb will eventually explode when you get unlucky and the photon takes the bombed path). If you have a dud, you will see the interference pattern.

Of course, this is only astonishing if you choose not to think in QM terms when looking at the result of a decidedly QM experiment. In QM terms, the live bomb itself can be thought of as being in a superpositon of states as well. The photon is put in a superposition of taking both slits, which then interacts with the live bomb, putting it in a superposition of states such that whether it explodes or not is directly correlated to which path the photon takes. And, of course, the photon does not get the opportunity to interact with itself to create interference patterns, because there is only one path which actually reaches the screen (the live bomb intercepts the other path)

For a real mind bender, take a look at the delayed choice quantum eraser. Same game, but vastly more brutal. Like this one, it has the same nature: the experiment seems impossibly paradoxical until you actually look at it from a QM perspective and then its result is natural. If anything, all these do is show that there are some circumstances that, if you don't think in QM terms, you get the wrong answer.


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