Can Extending the Delay in a Quantum Eraser Experiment Alter Past Events?

[insightforge]

Now take a modification of the experiment where many photons are sent to the interferometer and hit the detector so that a pattern could emerge (and those are the idlers?).

While the signal(?) photons are still traveling for such a time that the last idler has hit the detector before the first choice is made on a signal photon.

Then what might you see on the detector if measured prematurely?

Given the point above, I am still confused about why you can only interpret the results at the detector as a Gaussian pattern? With a sufficient delay you would be able to generate a set of points at the detector that should either fit an interference pattern or not fit. I see a lot of comments about the necessity of coincidence counting with the detector results. If coincidence counting is the game breaker about FTL communication couldn’t you overcome the issue by setting up a standard method for how you interact with the signal photons?

For example, let’s assume you are not constrained by the length of the delay that you can add to the system (you use a Bose Einstein condensate to slow down the signal photon significantly or bounce it of an object far away in space). Given your large time margin in your ability to decide whether or not to measure the spin on the signal photons you now have significant time to add some complexity to how you send and interpret the message. Setup the system so that you do everything (measure the signal photons & analyze the idle photon detector patterns) in discrete time intervals. This interval would most likely be the amount of time required to collect enough data at the detector to determine, with a high probability, if the patter was either an interference pattern or not (assuming that you also knew whether or not you were measuring the signal photon for the same interval). Now, with these discrete timeframes of measurement at the detector, you would also use the same discrete time intervals to either measure the spin on the signal photons or not measure the spin. Based on this system, it seems that you could take the data that you collect at the detector over these discrete intervals and apply two coincidence set, one in which you assume there was a measurement of the signal photon for the discrete period and one where you assume there was no measurements. If in fact there was no measurement (ie you did not decide to measure the signal photon in the future during the same discrete interval) then the assumed coincidence count for the measurement model (ie you only have the detector data, but you apply a coincidence count as if you had the signal data with a measurement of the spin taking place) for the same discrete interval should produce an interference pattern at the detector. Finding an interference pattern under the assumed coincidence count for the discrete interval assuming there was a measurement would not fit the data for a series of photons with a collapsed sping passing through a perpendicular double set of polarized filter in front of the double slit. There for it seems that you would know that you did not measure the signal photon for that discrete interval in the future. Given your ability to discern you interaction with the signal photon over discrete intervals in the future could you not then setup a binary, morse code communication system to pass information back to the past?

Disclaimer – I have no formal education in quantum physics, and this particular experiment has always interested me. I am sure there is some specific reason why the proposed method does not work for FTL communication. I was thinking maybe it has something to do with a requirement of knowing if the signal photon does or does not pass through the measurement filter. Another way to create the same proposed system would be to measure the number of photons hitting the detector for the discrete intervals. If there was a measurement of the signal photons there should be half as many photons making it through for the same period. It seems to work that way too. Please let me know why I am wrong as I am sure that what I am suggesting is a bunch of junk.

 

[msumm]

insightforge: from what others are saying, it sounds like you will not see an interference pattern when you look at the cases in which you didn't determine the which-path info. you only see an interference pattern when you look at a subset of those cases -- in terms of the diagram mentioned earlier in this thread, you only see the interference pattern when you look at the cases when a photon was registered at detector D1 (or only D2). someone could correct me on this, I'm no expert, I'm just reading the thread because i had similar questions in the past.

in that diagram you could think of 4 different cases:
1.a.) idler photon detected at D3 and therefore we know the signal photon passed through "slit a"
1.b.) idler photon detected at D4 and therefore we know the signal photon passed through "slit b"
2.a.) idler photon detected at D1 and therefore we do NOT know which-way info
2.b.) idler photon detected at D2 and therefore we do NOT know which-way info

I think what they are saying is that if you look at cases 2.a and 2.b together, the signal photons do NOT show an interference pattern, instead that actually show the same pattern as is seen in 1.a or 1.b. If you look at the pattern produced in cases 2.a only, then you do indeed see an interference pattern, but you cannot know that you are in case 2.a until after you detect the idler at D1 (and then it's too late to change the measurement, ...). similarly for 2.b, and evidentially the interference pattern for 2.b is slightly different so that when you add it to the interference pattern from the 2.a cases you get the same pattern seen in the 1.a./1.b. cases.

 

[YLW]

Hi, I'm new and I've just been reading about the DCQE. I have very little physics knowledge so please bear with me if I ask stupid questions. It was explained many posts (and quite some time) ago in this thread by JesseM that the experiment does not show that the 'choice' made later in time does influences the earlier positioning of the signal photons.

But even if we have to look at the subsets of correlations between D1/D2 and related signal photons to see interference, doesn't that suggest that there is some kind of influence made on the earlier detection of the signal photons by a later "choice"? I mean, how would the signal photons which hit D0 arrange themselves in a way that would show interference when taggedwith their twin photons which hit D1 when at the time the signal photons hit D0, we do not even know that their twin photons would hit D1?

Does that suggest that the signal photon that hits D0 already knows that its twin will hit D1 and if it does, wouldn't there at least be a suggestion that there is some backwards-in-time information transfer? Does a violation of causality mean that one would need to tell from looking at signal photons where entangled twin is going to land or is it enough for there to be a suggestion that an event in the present is influenced by an event in the future?

Thanks so much. So sorry if I wasted everybody's time.

 

[hankaaron]

When a particle is vibrating, it is more wave than particle.

However, when the particle back coils from the energy of a

photon, or in the case of a photon, it loses energy after

interaction with an electron, it temporarily ceases to vibrate

and instead becomes a point particle- that is, it is more

particle than wave.

So in the double slit experiment, the undisturbed particle is

mostly a vibrating wave, and hence interference is seen. If the

particle is observered (i.e., measured) then it ceases vibration

and becomes a point particle.


In the Quantum Erasure Experiment, if one of the entangled

particle is tagged, then so is the other. This is because there

is no complimentary value to tagging. If one of the entangled

pair is tagged, then so is the other. So when the first

entangled particle of the pair is measured, the other one

becomes a point particle at the same time.

But in the Delayed Choice Quantum Erasure experiment, why does

the signal detection sensorshow interference? It is because of

discontinues. That is, the particle begans to vibrate again

when some other observable is unknown. It stops vibrating when

a complimentatry observable is known. So in other words, the

photon stops vibrating when tagged.

But what happens when the idler photon is further away? The

signal detector should have already been either a interference

pattern or non interference pattern long before the idler photon

completes its path (into a detector with path info or one

without path info).

To understand what is happening, it is necessary to continue to

view the two split photons as only still one. In one reality,

the photon went straight to the signal detector screen. In the

other reality, it went back to the idler detectors.

Moreso, the idler photon went to another splitter, and that

photon went to both an idler detector that discerns path, and it

went to an idler detector without path info. This is why the

signal detector can accurately reflect both interference and non

interference results. The idler detectors branch off into

different realities (or universes), but the signal detector is

"connected" to all three.

For L&R path, the photon vibrates. For Left or for Right path,

it stops vibrating. This isconsistent with the prevailing

theory behind Quantum Computing.

Signal detector (present, simutaneous with idler dectors)
/ | \
Left L&R Right (present, simutaneous with signal detector.

Causality is still observed).

The which-path information is irrelevant since all paths are

exercised.

But what stops the photon from vibrating when the path is known?
That is the one question I can not find an answer for. It seems

that as soon as we understand one aspect of QM, other questions

arise!

 

[JesseM]

Hi, I'm new and I've just been reading about the DCQE. I have very little physics knowledge so please bear with me if I ask stupid questions. It was explained many posts (and quite some time) ago in this thread by JesseM that the experiment does not show that the 'choice' made later in time does influences the earlier positioning of the signal photons.

But even if we have to look at the subsets of correlations between D1/D2 and related signal photons to see interference, doesn't that suggest that there is some kind of influence made on the earlier detection of the signal photons by a later "choice"? I mean, how would the signal photons which hit D0 arrange themselves in a way that would show interference when taggedwith their twin photons which hit D1 when at the time the signal photons hit D0, we do not even know that their twin photons would hit D1?

Why assume that the signal photons are influenced by the later event of the idler being received at D1 or D2, as opposed to the other way around? Can't you equally well imagine that depending on which D0 position a given signal photon is detected, that influences the probability that the idler will later end up at D1 or D2? If the signal photon is detected at a position that is closer to a peak of what will become the D0/D1 interference pattern, and closer to a valley of what will become the D0/D2 interference pattern, then that might influence the idler to make it more probable it will end up at D1 and less probable it will end up at D2. I think this is exactly what you'd find in the Copenhagen interpretation where the detection of the signal photon "collapses" the wavefunction for the signal/idler pair, altering the probabilities that the idler will later be detected at different detectors. Analyzing what's going on in the Bohmian interpretation or the many-worlds interpretation would probably be a bit more complicated (see this paper for a Bohmian analysis), but there shouldn't be a need for any backwards causation at any rate.

 

[hankaaron]

The key point that I am trying to make is that you must not think of the two entangled particles and wholly separate enties. They are, in essence, one particle. For each photon split through the crystal, try to think of the split particles as one. Then apply MWI of QM.

 

[DrChinese]

The key point that I am trying to make is that you must not think of the two entangled particles and wholly separate enties. They are, in essence, one particle. For each photon split through the crystal, try to think of the split particles as one. Then apply MWI of QM.

Welcome to PhysicsForums, hankaaron!

I think you make a great point. Entangled particles, as well as unobserved "split" probability waves: these do not take on a classical form and do not follow classical rules. Often, when people get confused with things like DCQE or similar experiments, the issue is they are applying a classical view. Entangled photons are not really separate. And photons going through a double slit (if which path not known) do not really go through a single slit.

 

[JesseM]

Welcome to PhysicsForums, hankaaron!

I think you make a great point. Entangled particles, as well as unobserved "split" probability waves: these do not take on a classical form and do not follow classical rules. Often, when people get confused with things like DCQE or similar experiments, the issue is they are applying a classical view. Entangled photons are not really separate. And photons going through a double slit (if which path not known) do not really go through a single slit.

But it depends on the interpretation, no? In the Bohmian interpretion, the particles are discrete separate entities but they are both steered by a "pilot wave" which in some sense allows things that happen to one particle to influence the other particle FTL. And what about an advocate of the MWI who is also committed to the idea that the MWI should be understood in a way that preserves locality? Here it seems like it'd be more complicated to say to what extent the detection of the two particles should be viewed as causally "separate" if the detection-events have a spacelike separation...

 

[YLW]

Thanks JesseM but isn't the choice of whether to go to D1 or D2 made randomly by a beam splitter?

So sorry if I misunderstood anything...

 

[JesseM]

Thanks JesseM but isn't the choice of whether to go to D1 or D2 made randomly by a beam splitter?

It's random in the sense that the experimenter can't control it, but it's not random in the sense that the probability a given idler goes to D1 or D2 must always be 50/50, even if you already know where the corresponding signal photon was detected.

 

[YLW]

Thanks once again JesseM, I really have little or no physics understanding so in your view it is less likely that there is any backward causality or information traveling backwards in this case?

 

[JesseM]

Thanks once again JesseM, I really have little or no physics understanding so in your view it is less likely that there is any backward causality or information traveling backwards in this case?

I don't think there's any real need to suppose such a thing, but it really depends on what interpretation of QM you prefer, the transactional interpretation does feature some form of backward causality while the others do not.

 

[YLW]

Sorry, one more question. So what do the majority of accepted theories advocate? And what's your view?

 

[JesseM]

Sorry, one more question. So what do the majority of accepted theories advocate? And what's your view?

The problem is that all of the different "interpretations" of quantum mechanics are identical in terms of their experimental predictions, so they can't be treated as distinct scientific theories that we can test to see which is right...which one you prefer depends more on philosophical preferences than anything else. Personally I think the many-worlds interpretation is actually the simplest conceptually and so I would favor it based on [PLAIN , but that's a philosophical argument rather than a scientific one.

 

[Frame Dragger]

Sorry, one more question. So what do the majority of accepted theories advocate? And what's your view?

For better or worse, SQM formalism dominates, with The Copenhagen Interpretation as the backdrop. I'm not advocating that, but it's certainly the most popular.

I'd say the most popular non-standard Interpretation is The de Broglie-Bohm Pilot Wave theory, which is the only commonly accepted Hidden Variable theory to survive non-locality.

 

[JesseM]

For better or worse, SQM formalism dominates, with The Copenhagen Interpretation as the backdrop. I'm not advocating that, but it's certainly the most popular.

I'd say the most popular non-standard Interpretation is The de Broglie-Bohm Pilot Wave theory, which is the only commonly accepted Hidden Variable theory to survive non-locality.

From what I've gathered the MWI is more popular among physicists than the Bohm interpretation, perhaps because of its bare-bones quality of just taking the equations for wavefunction evolution between measurements in the Copenhagen interpretation, and saying those equations prevail at all times, with nothing special happening during measurements (I think some physicists who advocate the MWI as a way of thinking about QM don't actually care about whether the other branches of the universal wavefunction are 'real' in an ontological sense, though...look at Stephen Hawking's first comment here for example). Look at the poll mentioned here for example, or at Steven Weinberg's first comment in this exchange, where he follows the suggestion of a Bohm advocate to poll his colleagues on their opinions of the Bohm interpretation:

I have carried out the experiment you requested. At the regular weekly luncheon meeting today of our Theory Group, I asked my colleagues what they think of Bohm's version of quantum mechanics. The answers were pretty uniform, and much what I would have said myself.

First, as we understand it, Bohm's quantum mechanics uses the same formalism as ordinary quantum mechanics, including a wave function that satisfies the Schrodinger equation, but adds an extra element, the particle trajectory. The predictions of the theory are the same as for ordinary quantum mechanics, so there seems little point in the extra complication, except to satisfy some a priori ideas about what a physical theory should be like.

There is also the point that it does not seem possible to extend Bohm's version of quantum mechanics to theories in which particles can be created and destroyed, which includes all known relativistic quantum theories.

It is not true that the only alternative to Bohm's version of quantum mechanics is the Copenhagen interpretation, for which I share your reservations. These days most physicists who think about it at all understand quantum mechanics in terms of the Everett many-histories approach. In any case, the basic reason for not paying attention to the Bohm approach is not some sort of ideological rigidity, but much simpler --- it is just that we are all too busy with our own work to spend time on something that doesn't seem likely to help us make progress with our real problems.

 

[Frame Dragger]

From what I've gathered the MWI is more popular among physicists than the Bohm interpretation, perhaps because of its bare-bones quality of just taking the equations for wavefunction evolution between measurements in the Copenhagen interpretation, and saying those equations prevail at all times, with nothing special happening during measurements (I think some physicists who advocate the MWI as a way of thinking about QM don't actually care about whether the other branches of the universal wavefunction are 'real' in an ontological sense, though...look at Stephen Hawking's first comment here for example). Look at the poll mentioned here for example, or at Steven Weinberg's first comment in this exchange, where he follows the suggestion of a Bohm advocate to poll his colleagues on their opinions of the Bohm interpretation:

From what I've seen dBB proponants tend to see MWI as complementary to dBB. MWI adherents who are NOT dBB adherents definitely are the majority. Frankly I didn't realize that dBB was so widely discussed these days until I came to these PFs! In that sense, MWI is hugely popular as an alternative to TCI, and more so becuase it gets all of those Bohmians as well! Of course Zenith or Demystifier would know more about the dBB than most on the forums.

 

[hitthelimit]

To Spacezilla

"So what happens if we delay it long enough to change whether we'll detect or erase the information about the second photon's after observing the results?"

Nothing. Their is no way to say what type of event you see on the screen – this particular spot may be either of interferences or of corpuscular pattern.