Device that detects if someone is looking at the device?

[Syamsu]

TL;DR Summary

Based on the understanding of variations of the double split experiment, wouldn't it be possible to manipulate the observation causing the wave collapse, to create a device which detects if it is observed?

How this would supposedly work is the non-interactive detector at one slit, and then this detector sends out a detection signal to the environment.

The pattern coming from the slits is measured, and supposedly when someone is observing the detection signal, then the interference pattern would disappear.

To work in a fuzzy way, with large numbers of electrons going through the slits.

 

[Vanadium 50]

to create a device which detects if it is observed?

No.

And since you can't. the question how it would work is moot.

 

[Syamsu]

No.

And since you can't. the question how it would work is moot.

Which means you assert that if you send the detection signal out to the environment, without someone directly looking at it, that none of the which way information would get lost, remain undecided.

 

[Baluncore]

The pattern coming from the slits is measured, and supposedly when someone is observing the detection signal, then the interference pattern would disappear.

To reinforce, and especially to cancel, the streams of information must be identical. If you transmit a noise signal by one route, and that same noise signal modulated with data via another route, then when recombined you can extract your data stream from the two noisy signals.

When someone "looks" at something, the energy is lost to their sensor, rather than to the alternative environment. Such monitoring cannot be detected, since the energy has been lost either way.

Glass fibres are very slightly non-linear. If you shine a light across the fibre, you can measure the strength of the signal travelling along the fibre. It is unlikely you would detect that form of monitoring, because it does not involve interception or disturbance of the information flow.

If you communicate by transmitting entangled photons, interception eavesdropping can be detected because the removal of a photon from the optical link cannot be reliably corrected by inserting a like photon having the same spin.
https://en.wikipedia.org/wiki/Quantum_entanglement

Monitoring the traffic through the non-linearity of the fibre cannot tell which spin of photon was passing without collapsing the photon.

 

[Syamsu]

When someone "looks" at something, the energy is lost to their sensor, rather than to the alternative environment. Such monitoring cannot be detected, since the energy has been lost either way.

But if the detector signal is observed all of the time, then only half of the time any energy is sent from the detector to the observer, because the detector is just at 1 slit, and half of the time the photon goes through the other slit.

 

[Vanadium 50]

Detectors don't know if someone is looking at them. If you are instead asking about some more complex situation, e.g. configuration changes, entanglement, etc. you should ask that question.

 

[PeterDonis]

if the detector signal is observed all of the time

There is no such thing as "observed all the time". "Observed" means that a detection occurred. It does not mean "watching to see if a detection occurs". There is no way to always "watch" a quantum system without detection events occurring.

 

[Syamsu]

Detectors don't know if someone is looking at them. If you are instead asking about some more complex situation, e.g. configuration changes, entanglement, etc. you should ask that question.

It's the standard double slit experiment, with a non interactive detector at one of the slits. And a screen which shows the pattern of photons going through the slits.

So then if you sent out the signal from the detector to the environment, without anybody looking at it, would you then get an interference pattern on the screen, or not?

I then speculate, that there would be an interference pattern, because of nobody looking at it. Because it was demonstrated in a lab, that the which way information from a detector can be erased, causing an interference pattern to appear. So I speculate that the which way information would also be erased naturally, at least in part, if it was sent out to the environment.

 

[PeterDonis]

a non interactive detector

What does this even mean? Any "detection" is an interaction.

if you sent out the signal from the detector to the environment

What does this even mean?

I then speculate

You should not speculate. Personal speculation is off limits here. Questions about what happens when experiments like the one you describe are done can and should be answered using the math of QM, not speculation.

I suggest that you reformulate your question using the math of QM. Using vague ordinary language is only going to mislead you (as indeed it appears to already have done).

 

[Syamsu]

Basically the detector lights up, anytime it detects a photon passing through the slit. And this light is then sent out to the environment.

If someone is looking at the screen, and sees an interference pattern emerge, then they can know, nobody was looking at the slit. Alternatively, if no interference pattern shows up, they know that someone must have been looking at the slit.

 

[DrChinese]

It's the standard double slit experiment, with a non interactive detector at one of the slits. And a screen which shows the pattern of photons going through the slits.

So then if you sent out the signal from the detector to the environment, without anybody looking at it, would you then get an interference pattern on the screen, or not?

Maybe the below version of the double slit will help you...

It is possible to obtain which path information in a double slit setup in which the photons are NOT absorbed or otherwise directly detected (as per usual).

The technique is as follows: Place a polarizer over each of the 2 slits (2 polarizers total) that can be oriented either parallel or perpendicular relative to each other. When the polarizers are parallel, there WILL be a traditional interference pattern. When the polarizers are perpendicular (orthogonal), there will NOT be a traditional interference pattern. And if you vary the polarizer settings between 0 and 90 degrees, you get a mixture of the two patterns. The intensity of the pattern is not affected by the polarizer settings, only the pattern formed. So the "detector" in this case is the relative polarizer settings, which serve to "mark" the which slit information onto the photons which emerge. That marking itself is not looked at by anyone, and in fact no effort at all is made to embed it somewhere or otherwise read it.

Young’s double-slit experiment with single photons and quantum eraser

In this configuration: you don't actually learn the which path information, it is enough that you COULD learn that information for the patterns to change. So to compare to your example: you should be able to see that the parallel is a "detector with no one looking at it". If there were such a thing, the pattern would change regardless of whether anyone looks at it.

 

[PeterDonis]

the detector lights up, anytime it detects a photon passing through the slit.

How would such a detector work?

If someone is looking at the screen, and sees an interference pattern emerge, then they can know, nobody was looking at the slit. Alternatively, if no interference pattern shows up, they know that someone must have been looking at the slit.

What are you basing this on? Your description of the detector, even assuming an actual detector that works as you described can be built, said nothing about anyone looking at the slit.

 

[Syamsu]

How would such a detector work?

It would work in a non-interactive way, so it detects the photon without interacting with it. Because the interaction would collapse the wave, and then you would never get an interference pattern.

 

[PeterDonis]

It would work in a non-interactive way, so it detects the photon without interacting with it.

How do you know such a thing is even possible?

the interaction would collapse the wave, and then you would never get an interference pattern.

The detection you describe, with a light lighting up when a photon passes through, would destroy the interference pattern.

Again, you are misleading yourself by using vague ordinary language instead of the actual math of QM.

 

[Syamsu]

How do you know such a thing is even possible?

There is a wellknown experiment by Kwiat from 2006 in which they used non interactive measurement of a photon, in order to search a database without running the search algorithm.

It has been demonstrated that which way information can be erased in a lab. So then mabye at least some of the which way information would be erased naturally, when it is sent out in the environment.

 

[Vanadium 50]

It would work in a non-interactive way

A detection is a kind of interaction. You can have it either way, but not both.

 

[PeterDonis]

There is a wellknown experiment by Kwiat from 2006 in which they used non interactive measurement of a photon, in order to search a database without running the search algorithm.

Can you give a link to the paper? I am unable to find one online.

Generally speaking, the "non-interactive measurements" (or "interaction-free measurements") that you speak of don't work quite the way you appear to think they work. In particular, they are not magical devices that let you send out a signal every time a quantum system passes by, without interacting with that system. There is no such thing. Again, you are misleading yourself by using vague ordinary language (although in this case, to be fair, the vague and misleading ordinary language description is a very common one) instead of looking at the actual math.

It has been demonstrated that which way information can be erased in a lab.

Sort of. You will find a number of threads here on PF discussing the various subtleties involved with "quantum eraser" experiments.

So then mabye at least some of the which way information would be erased naturally, when it is sent out in the environment.

Again, you should not be waving your hands and speculating. The "quantum eraser" experiments can only "erase" information that has not been sent out in the environment.

 

[Syamsu]

Can you give a link to the paper? I am unable to find one online.

https://pubmed.ncbi.nlm.nih.gov/16495993/

Generally speaking, the "non-interactive measurements" (or "interaction-free measurements") that you speak of don't work quite the way you appear to think they work. In particular, they are not magical devices that let you send out a signal every time a quantum system passes by, without interacting with that system. There is no such thing. Again, you are misleading yourself by using vague ordinary language (although in this case, to be fair, the vague and misleading ordinary language description is a very common one) instead of looking at the actual math.Sort of. You will find a number of threads here on PF discussing the various subtleties involved with "quantum eraser" experiments.Again, you should not be waving your hands and speculating. The "quantum eraser" experiments can only "erase" information that has not been sent out in the environment.

It is shown that which way information from a detector can be erased. How do you know that the environment would preserve which way information 100 percent of the time?

 

[PeterDonis]

It is shown that which way information from a detector can be erased if it has not yet been sent out in the environment.

See the bolded qualifier above. It's crucial.

How do you know that the environment would preserve which way information 100 percent of the time?

Because "preserving which way information"--i.e., making it irreversible--is sending it out to the environment. They're the same thing. (Look up "decoherence" for much more literature on this.)

 

[Syamsu]

See the bolded qualifier above. It's crucial.Because "preserving which way information"--i.e., making it irreversible--is sending it out to the environment. They're the same thing. (Look up "decoherence" for much more literature on this.)

Maybe sending is not the right word, the information is channelled to the environment. Maybe information is not the right word, because at this point it hasn't been decided yet which slit it went through.

I still don't see how you can know that nothing in the environment, is like what they used in the lab to erase the which way information. I guess in the lab, with the delayed quantum eraser experiment, they channel the information to some kind of black box. I don't see why things in the environment would not function as some kind of black box just the same.

 

[PeterDonis]

Maybe sending is not the right word, the information is channelled to the environment. Maybe information is not the right word, because at this point it hasn't been decided yet which slit it went through.

This isn't a matter of choice of words. The physics is that what you are proposing is impossible. You cannot both send which way information to the environment and have an interference pattern.

I still don't see how you can know that nothing in the environment, is like what they used in the lab to erase the which way information. I guess in the lab, with the delayed quantum eraser experiment, they channel the information to some kind of black box. I don't see why things in the environment would not function as some kind of black box just the same.

Yes, I get that you don't see these things. That is because you are trying to understand these experiments based on vague ordinary language and "guessing" instead of reading and understanding the actual experimental papers and doing the math.

 

[DaveC426913]

Has anyone mentioned The Observer Effect?
https://en.wikipedia.org/wiki/Observer_effect_(physics)
Any kind of detection of a quantum phenomenon will affect the outcome of the experiment.

So it's explicitly stated that you can't observe a phenomenon without altering it (i.e. passively).

 

[Syamsu]

Putting the data of the pattern in a black box.
And putting the data of the which way information in another black box.

Then if the data of the which way information is observed, and then the pattern data is observed, it shows no interference pattern.

If the which way data is erased from the black box, and then the pattern data is observed, then an interference pattern is shown.

So then, if at least part of the environment erases the which way data, then you can tell the difference whether someone is looking at at the which way data or not.

 

[PeterDonis]

Putting the data of the pattern in a black box.
And putting the data of the which way information in another black box.

Then if the data of the which way information is observed, and then the pattern data is observed, it shows no interference pattern.

If the which way data is erased from the black box, and then the pattern data is observed, then an interference pattern is shown.

So then, if at least part of the environment erases the which way data, then you can tell the difference whether someone is looking at at the which way data or not.

At this point you are just waving your hands and speculating. Personal speculation is off limits here. Your original question has been answered.

Thread closed.