Double slit experiment -- Question about the influence of our measurement equipment

  • #1
Ebi Rogha
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TL;DR Summary
In double-slit experiment, it is said when we turn on the sensor/detector/instrument to measure/detect the behaviour of the particle, it shows particle behaviour (otherwise, it shows wave behaviour).

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I would like to know, how can we be sure this is not due to the influence/impact/interference of our measurement, not necessarily the intrinsic nature of photons?

In most reference books, it seems it is a given and it is not discussed
 
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  • #2
hutchphd
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The point is that it is intrinsic to any measurement you can devise. The use of photons iss presented as the most obvious example.
 
  • #3
Ebi Rogha
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The point is that it is intrinsic to any measurement you can devise. The use of photons iss presented as the most obvious example.
Any measurement has an impact (influence) on the measured quantity. Am I right?
For example, any measurement on an extremely unstable system will make the system collapse.
 
  • #4
hutchphd
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The "system" doesn't collapse.
But the measurement process necessarily introduces disturbance sufficient to negate any attempt to beat the Heisenbeg relations in a meaningful way. This is true for every system.
In my mind the "collapse" of the wavefunction is simply one way to formulate the measurement vis a vis Quantum Mechanics.
 
  • #5
DrChinese
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Summary:: In double-slit experiment, it is said when we turn on the sensor/detector/instrument to measure/detect the behaviour of the particle, it shows particle behaviour (otherwise, it shows wave behaviour).

.

I would like to know, how can we be sure this is not due to the influence/impact/interference of our measurement, not necessarily the intrinsic nature of photons?

In most reference books, it seems it is a given and it is not discussed
It is NOT due to the "influence/impact" of the measurement itself. You may or may not be aware of double slit experiments where a polarizer is placed at each of the 2 slits. When the polarizers are aligned parallel, there IS interference. When the polarizers are aligned perpendicular (orthogonal), there is NO interference. When aligned at other angles relative to each other, there is SOME interference and this can be adjusted as desired.

In all cases, the detected photons pass through a polarizer, so there is no more or less "influence/impact" being performed; and the overall intensity of the pattern does not change (50% of the light always gets through). The variable result here is due to whether or not you could have learned which-slit information IF you later looked for polarization information from each photon striking the detection screen. It doesn't matter whether you actually obtained such information.

https://sciencedemonstrations.fas.h...-demonstrations/files/single_photon_paper.pdf
 
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  • #6
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It is NOT due to the "influence/impact" of the measurement itself. You may or may not be aware of double slit experiments where a polarizer is placed at each of the 2 slits. When the polarizers are aligned parallel, there IS interference. When the polarizers are aligned perpendicular (orthogonal), there is NO interference. When aligned at other angles relative to each other, there is SOME interference and this can be adjusted as desired.

In all cases, the detected photons pass through a polarizer, so there is no more or less "influence/impact" being performed; and the overall intensity of the pattern does not change (50% of the light always gets through). The variable result here is due to whether or not you could have learned which-slit information IF you later looked for polarization information from each photon striking the detection screen. It doesn't matter whether you actually obtained such information.

https://sciencedemonstrations.fas.h...-demonstrations/files/single_photon_paper.pdf


The system that is being measured in a sense appears to 'know' more about the particular setup and the world than the experimenter. Or anyone else. It's impossible to outsmart it, isn't it? At least no one has been successful so far.

This aspect is particularly puzzling and deserves much more attention in terms of the nature of the system being measured.
 
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  • #7
hutchphd
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Can you spell this out a little bit more for a particular system? Specific particle specific measurement so we don't get lost in generalities.
 
  • #8
Cthugha
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No one mentioned the quantum eraser experiment. It is an enhanced version of the double slit experiment that explores your question about whether the measurement act itself changes the outcome. The quantum eraser can irretrievably erase which-slit information gathered via measurement. If the gathered information is erased and rendered inaccessble before it has been observed, the double slit interference results are the same as if the measurement had not been done at all. The conclusion is the act of measuring does not influence the result, but rather access to (observability of) the which-way information does.
This is pretty flawed and misleading.

The quantum eraser cannot erase information gathered via measurement. A measurement is an irreversible interaction. In the quantum eraser, one creates a REVERSIBLE change of the state (relative phase shifts and corresponding polarization shifts are most common), which would result in which-way information if a measurement was performed. One can only reverse these reversible changes and the corresponding information about the path (which one would get if one performed a measurement) prior to this measurement. Once a measurement has been done, this is not possible. The information can also not be considered "gathered" before the measurement has been performed. It would be available if one had already performed a measurement, but that is a very different thing.
 
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  • #10
Cthugha
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In the quantum eraser experiment, the eraser is in the apparatus for a reason -- to erase the information derived from a measurement. After all, if no measurement had been made, there would be nothing for the eraser to erase.

The quantum eraser underscores the importance of observation in QM and, in this case, the critical distinction between measurement and observation.

Every interaction (measurement) between two objects, A and B, results in a change of state for the involved objects. If third party C cannot observe the resulting measurement information, from C's perspective, it is as if that measurement, which did indeed occur, had not taken place. In such case, C remains in superposition relative to the change of state of A and B.

In the quantum eraser experiment, which-way measurement occurs within the instrumentation, but that which-way information is subsequently erased such that it is not accessible to the experimenter. Consequently, from that experimenter's point of view, things remain as if the measurement had not occurred.
To get back to this point: It is not that easy. There are two basic "kinds" of quantum eraser experiments.

1) The typical version attributed to Walborn puts wave plates at the double slit positions. These serve as which way markers. For example, they shift horizontal polarization to right-circular polarization, vertical polarization to left-circular polarization,right-circular polarization to vertical polarization and left-circular polarization to horizontal polarization. This is a reversible interaction and it is essentially loss-free. All of the light passes the waveplates. This approach also works for delayed choice experiments.
One could also perform an irreversible interaction by putting polarizers at the slit positions. These are filters that just let a part of the light field pass. This obviously constitutes a measurement for the photons that do not pass the filter. However, these do not form an interference pattern anyway. The photons that pass the filter will have a polarization that matches the filter orientation. However, there is no way to know whether a photon is indeed present unless we actually detect it. Accordingly, if we place perpendicular polarizers at the two slits, the photons that make ith through will not show an interference pattern if detected directly. One can now insert another filter oriented at 45 degrees to the two initial filters. Again, some photons will not make it through the filter. Those that make it through will show a polarization aligned with the filter and will show interference when detected by a detector placed behind the double slit and all of the filters. For the photons that actually make it to this detector, the absorption at the detector is the one measurement (strong measurement, to be more precise) we have here. Beforehand, information about the path would have been available to us if we had performed a measurement. However, we did not. Thus, it would be more accurate to say that we erased the possibility to gain which-way information instead of erasing which-way information. In order to get real which-way information along the way, we would need some kind of signal or trace that actually tells us that a photon has just passed a certain flter. This is, however, not how they work. We can just take note of the photons that get absorbed at these filters. This whole kind of setup does not work well in delayed choice scenarios, which is why they are not used in sophisticated scenarios.


2) The Scully version instead uses a very clever design that relies on nonlocal filtering. However, as you mentioned only the quantum eraser and not the delayed choice version, discussing this might be out of the scope of the discussion.

Also, even more importantly it is not true that "Every interaction (measurement) between two objects, A and B, results in a change of state for the involved objects". However, it is a very important and fundamental point to understand why that is the case which really helps in understanding quantum mechanics. Simply consider a photon bouncing off a mirror. If this "interaction" changed the state of both the photon and the mirror, this would already constitute which-way information. We could just check the state of the mirror and it would tell us which way the photon went. Mach-Zehnder interferometers would not work if that was indeed happening. What indeed happens is that the mirror is in some initial state that is characterized by some position and momentum and some uncertainty of both (at the very least due to Heisenberg uncertainty). This is the state of the mirror. When reflecting off the mirror, the photon will transfer momentum to the mirror, but it will be so little momentum that the amount is small compared to the initial momentum uncertainty of the mirror. The initial and the final state of the mirror have a large overlap - usually much more than 99.99% - so that one cannot say whether an interaction happened judging from a single photon reflection process. Taken the other way around, this also defines what a "good" detector or measurement process is. If the initial and the final state of your "detector" before and after the interaction are orthogonal (have 0 overlap), then one consider the action of this detector as a measurement in the most common sense in QM. For example, if you make the mirror much much lighter and mount it on a spring, the momentum transferred by a single photon might be sufficient to put the mirror into a different state. Some precision measurements work this way.

There are of course other ways to treat the other cases (POVMs and so on), but the example above illustrates the typical strong measurement that people usually have in mind when they talk about measurements in QM.
 
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  • #11
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The quantum eraser cannot erase information gathered via measurement. A measurement is an irreversible interaction.
Exactly. All it shows is in some simple cases of decoherence (which in modern measurement theory is generally thought of as what a measurement is), it is reversible. Because it is reversible, it is not really a measurement, although it can at first 'brush' look like one. Sean Carrol explains it in a slightly amusing way here (he is a many-worlds proponent, which accounts for some of the language he uses):
https://www.preposterousuniverse.com/blog/2019/09/21/the-notorious-delayed-choice-quantum-eraser/

There is also a detailed analysis using the consistent histories approach:
https://quantum.phys.cmu.edu/CQT/chaps/cqt20.pdf

A good way to come to grips with quantum issues is to study an actual interpretation and think it through carefully:
https://quantum.phys.cmu.edu/CHS/histories.html

As far as the original question goes, here is a better explanation than beginning books give on the double-slit:
https://arxiv.org/pdf/quant-ph/0703126.pdf

But exactitude forces me to say the above also has issues - it is just better than what beginning books say.

Thanks
Bill
 
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  • #12
PeroK
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As I understand it, every interaction yields a gain of information for the interacting objects.
This is not the case. A measurement is when a quantum mechanical system interacts (irreversibly) with a macroscopic measuring device.

To take the example of the Stern-Gerlach experiment: the magnetic field does not measure the spin of the electron. Instead, the electron enters the field in a superposition of spin states and exits the field with a different spatial wave-function corresponding to each spin state. No information has been gained at this point and nothing has been measured. It's only when the electron (or, more precisely the silver atom of which it is a part) interacts with a detecting screen that a measurement of its position is made. The nature of electron spin is inferred from these position measurements, rather than measured directly (by the magnet or otherwise).

This is why, if the screen is replaced by another apparatus that combines the two superposed beams, then the result is an electron still in superposition - and not an electron with a definite spin in the direction of the magnetic field.

This is analagous to the double-slit experiment and what happens when you do not gather which-way information. The photon or electron is still in a superposition of spatial wave-functions corresponding to each slit. No measurement has been made by the particle interacting with the double-slit barrier.

This is a vital point.

This is very different from the case where a measurement has been made, but we do not know the result. Owing to decoherence, that situation is represented by classical probabilities; not quantum superposition or probability amplitudes.

Finally, in the quantum eraser you can try to outsmart nature by running your experiment and trying to keep all the idler photons until a definite pattern has been established by the signal photons. E.g. you could try to send all the idler photons across some large distance. This is where quantum uncertainty strikes back and the longer you try to maintain the idler photons, the more uncertainty there is about the information that can be obtained from them. Eventually, there is so much uncertainty that the which-way information is essentially lost.

There are several cases like this in QM where the uncertainty principle asserts itself in different ways in each particular experimental set-up, and when you do the detailed analysis you find you cannot force nature to contradict itself or undo an established measurement.
 
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  • #13
PeroK
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In regards to, "Simply consider a photon bouncing off a mirror. If this 'interaction' changed the state of both the photon and the mirror, this would already constitute which-way information." That's a good example. Under my approach, the bounce did change the state of the mirror and the photon (its direction of travel, for one thing), but that information is merely not observable by the experimenter without his/her further interaction with the photon and/or mirror.
This is simply not what QM says. You cannot say "a photon bounced off a mirror" until you have measured it. The assumption that the event must have happened one way or another is essentially a belief in "hidden variables". That is not QM. In QM the system remains in a superposition of all possibilities until a measurement is made.

And, as others have pointed out, this is the essence of the measurement problem.
 
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  • #14
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Moderator's note: An off topic subthread has been deleted. Please keep discussion in this thread focused on the specific question and scenario introduced by the OP.
 

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