The Quantum Eraser

Richard Feynmann noted more than once that complementarity is the central mystery that lies at the heart of quantum theory. Complementarity rules the world of the very small… the quantum world, and surmises that particles and waves are indistinguishable from one other. That they are one and the same. That it is nonsensical to think of something, or even try to visualize that something as an individual “particle” or a “wave.” That the particle/wave/whatever-you-want-to-call-it is in this sort of superposition, where it is neither particle nor wave. It is only the act of trying to measure what it is that disengages the cloaking device and the particle or wave nature is revealed. Look for a particle, and you’ll find a particle. Look for a wave instead, and instead you’ll find a wave.

Complementarity arises from the limits placed on measuring things in the quantum world with classical measuring devices. It turns out that when you try to measure things that are really really really small, some issues come up… some fundamental issues.  For instance, you can’t really know exactly where a sub-atomic particle is located in space. You can only know where it is within a certain probability, and this probability is distributed through space in the form of a wave. Understanding uncertainty in measurement is key to avoiding the disbelief that hits you when thinking about complementarity.

This article is a continuation of the one linked above. I shall pick up where I left off, in that everyone agrees that measurement on the quantum scale presents some big problems. However, not everyone agrees what these problems mean. Some, such as Albert Einstein, say that just because something cannot be measured doesn’t mean it’s not there. Others, including most mainstream physicists, say the opposite — that if something cannot be measured, it for all practical purposes is not there. We shall continue on our journey by using modern technology to peer into the murky world of complementarity. But first, a quick review.

The Double Slit Experiment — Where It All Began

Firstly, there are purists out there that will disagree with my approach to explaining these concepts. I must plead with you that it is not my goal to submit this article to The Scientific Journal for review. My goal is simply to rip away the complexities that naturally follow this advanced topic, and present it in an easy-to-understand format that anyone can enjoy and learn from. But by all means feel free to expand on anything in the comments!

Complementarity was developed to help understand the results of laboratory experiments. Today, the idea of complementarity resides at the heart of what is known as the Copenhagen interpretation of quantum mechanics. There are other interpretations out there, but the Copenhagen model is the most widely accepted.


The laboratory experiments I speak of revolve around the double slit experiment, which can differentiate between a particle and a wave. Imagine you’re at a gun range and you put up a large target. In between you and the target is erected a large steel wall with two narrow slits…maybe six inches wide and two feet apart. You fire a few hundred rounds with your machine gun, and then observe the pattern on the target. You will find an obvious pattern – two narrow lines where the bullets went through the slits.

Now let us take our large steel wall with the two slits and stick it in a lake, so that the slits are just above the surface. Behind the wall, we’ll place some type of detector that can detect waves. We toss a large rock into the lake and watch the resulting wave emanate from the point of impact and strike the wall. On the other side of the wall, two other waves appear from the slits. The slits will each act like a wave source. The waves from each source will interfere with each other and produce a distinct pattern on our detector wall. It’s known as an interference pattern, and consists of several lines of different intensities.


Now, you should see where we’re going with this. If we have an unknown substance, and we want to know if it is made of particles or waves, we can perform this experiment. Light, for instance, will produce an interference pattern. And that makes perfect sense – it’s an electromagnetic wave. One would think that sub-atomic particles would produce a pattern like our machine gun bullets did – two distinct lines. It turns out that this is not the case. They will produce an interference pattern as well. And that most certainly does not make sense.


But physicists are clever, and decided to try firing one particle at a time at the double slit. About one particle per hour. But it yields the same result — an interference pattern! The particle is acting like a wave, as if it went through both slits at the same time! That’s impossible! We must take a closer look. We will observe the single particle to see which slit it goes through. Turns out that when you do this, you will get the double line pattern like you expected. If we look at it, we will see a particle. If we don’t look at it, we will see a wave. And thus was born the concept of complementarity.

The idea that “observation determines reality” gets into a philosophical quagmire that I’m not touching with a 39 foot HF antenna. But we can probe deeper into this mystery with an experiment. What if we could observe the particle/wave/whatever AFTER it goes through the slit and BEFORE it hits the detector wall? This is precisely what the quantum eraser experiment does.

The Quantum Eraser

Like several concepts in quantum theory, originally thought experiments were developed to explore an idea or approach, but technology has advanced to the point where we can actually carry out some of them. The quantum eraser experiment is one such experiment, and was carried out at the University of Maryland in 1999.

The experiment starts with visible light photons traveling through a double slit. The exiting light immediately hits a prism which splits a single photon into an entangled pair.  A lens then directs one of the photons to detector D0. The other photon goes to another prism. What happens next depends on which slit the original photon came through. If it came from the top slit (path pictured in red), it will go to a half-silvered mirror BSb. If it came from the bottom slit (path pictured in blue), the prism will direct it to half-silvered mirror BSa. Note that “BS” stands for “Beam Splitter”: a half-silvered mirror will allow 50% of the photons to pass, and will reflect the other 50%.


The BSb mirror will send 50% of the photons from the top slit to detector D4 and the other 50% to the mirror Mb. The photons from Mb head to another half-silvered mirror BSc. This mirror will send 50% of the photons to detectors D1 and Drespectively.

A similar action occurs with photons coming through the bottom slit. They will hit BSa, which sends photons to detector D3 and mirror Ma. From Ma, they will go to mirror BSc, which takes half of the photons to D1 and the other half to D2.

In the end, photons from the top slit will go to detectors’ D1, D2 and D4. Note that no photons from the top slit can reach detector D3. Photons from the bottom slit will go to detectors’ D1, D2 and D3. No photons from the bottom slit can reach detector D4. Note that it is not possible to determine which slit the photons that hit D1 and D2 originated from. So this is what we have:

  • Top slit = D4
  • Bottom slit = D3
  • Unknowable = D1 and D2

er_06Detector D0 lies on the shortest path, so a photon will strike it approximately 8 nanoseconds before its entangled partner reaches another detector. The Coincidence Counter allows us to assign a photon that strikes D0 to its entangled partner, which strikes D1 – D4.

So we put 12v on the Arduino Uno and let the photons loose. This is what we find — D3 and D4 (labeled “R0n” in the Wiki images) show a particle pattern. D1 and D2 show an interference pattern. And this makes sense. We cannot know which slit the photons detected at D1 and D2 came through. So they act as a wave. And we know which slit the photons detected at D3 and D4 came through, so they act like particles. But this is not the point of the experiment.

Note how the bands are opposite of each other in R01 and R02. This corresponds to the R01 and R02 waveforms in the top image. See the Wiki for more information on why this occurs.

The neat stuff is going on at detector D0. For every photon that hits D1 – D4, it has an entangled partner that hits D0 8ns earlier. Like the other detectors, D0 can resolve a particle or wave pattern. This is doing exactly what we wanted to do — we’re looking at the particle AFTER it goes through the slit (via D0), but BEFORE it hits the detector wall, which in this case is made of detectors’ D1 – D4.

What they found was that the photons that hit D0 always — as in 100% of them — correlated to their partner photons. And this, my fellow hackers, should be impossible. Why? Because:

  • The photons hit D0 8 nanoseconds before D1 – D4.
  • The photons have a 50/50 chance of hitting D1/D2 or D3/D4.

How then can the photon that hits D0 know if its entangled partner went to D1/D2 or D3/D4? We are forced to consider an impossible scenario:

  • The photons that end up at D1 and D2 must be sending information 8ns into the past to tell its entangled partner at D0 to become a wave.
  • The photons that end up at D3 and D4 must be sending information 8ns into the past to tell its entangled partner at D0 to become a particle.

This, my friends, is a simplified explanation of what the quantum eraser is all about. It “erases” the past, preventing us from ever knowing which slit the photon came through. Bohr was right: complementarity is real, impossible as it might seem. However, our problem lies not with what appears to be undeniable time travel. Our problem is that how we view the natural world is not compatible in the quantum realm. To ask if it is a wave or particle is nonsense. To ask if it’s even there is nonsense. There is no such thing as “there” in the quantum realm. Concepts like time and space, cause and effect …have different meanings there… meanings that we’re still not sure of to this very day.

Encourage your sons, daughters, nieces and nephews to take the helm and study quantum theory. Stir their curiosity… there are stories yet to be told, and discoveries that remain to be made. Many of which are surely greater than the greatest fiction, but whose fantastic implications are rooted in a very real reality — the next frontier of modern science.

64 thoughts on “The Quantum Eraser

  1. Great post! Very clear explanation of something that is beyond reason. If our pitiful brains could ever really understand this we will know the secrets of the universe (42 maybe?)

    1. Indeed — I’m always extremely fascinated by the parts of quantum mechanics that sound like nature aggressively stating “these are my secrets, you don’t get to know them, fuck off”. It’s something completely foreign to our previous understanding of the world and to so many of our concepts.

    1. The “particle/wave” duality stuff? Ignore that. They’re not either.

      They’re quantized excitations of the underlying field. So to create the photon, you set up an oscillation in the field, and it propagates forward. If nothing actually *interacts* with it, then it propagates through the double-slit, and it interferes with itself: so then later, past the double-slit, if you try to interact with it *again*, only places where it hasn’t cancelled itself out still “look like” that quantized oscillation. So those are the only places you can interact with it, and you get an interference pattern.

      If something *does* interact with it as it passes through the slit, then that means the original excitation is *destroyed*, and a new excitation propagates forward, one which doesn’t interfere with anything (since it’s past the slit) and you don’t see an interaction pattern at all.

        1. No, the ‘field’ here is the electromagnetic field (or, if you want to get silly, the portion of the vector B field that’s perpendicular to the polarized vacuum’s direction). In quantum field theory, all particles are underlying excitations of a field.

      1. So when you put a detector right before one of the slits, it creates a new excitation, just, well, going thru that slit you detected it from? They usually just use 1 detector per slit, right? No need for two.

        So if there’s only 1 detector at 1 slit, why when it’s going thru slit 2 without a detector, does it not interfere with itself to create a wave on the sheet past the slits? Wouldn’t you see a solid “bar” of hits lined up with slit 1, and then a skewed wave pattern of sorts along with it, since only a portion were destroyed then re-created?

        OR is it that ANY detector lined up alongside a slit is going to destroy the the excitation wave covering the whole area anyway?

    2. I ‘get’ it in the sense that the explanation was reasonable and the conclusion was clear. The gist is simply that based off of the experiment, the results detected by the four sensors followed suit with the results of observation/non-observation effect like the double-slit experiment. But even *before* these four detectors had photons hit them and make the expected patterns, the trajectory of an exact photon copy was recorder 8 nanoseconds BEFORE they could even be detected, and the results are still the same. It is basically as if the photons that hit the four detectors went back in time, and corrected the trajectory of the first detector’s photon so that the results lined up. Every single time.

      It warps your concept of things happening in a procedural way through time, as they clearly do NOT at the quantum level. Some things in the future affect some things in the past.

  2. I can’t recall every reading or hearing Feynman say “complimentarity” in this context. Plus it is a weak and vague concept that no one uses today. I think it was one of Bohr’s ideas. People will say uncertainty, or Heisenberg uncertainty, but I think you will only find “complimentary” in papers on the history of QM or biographies.

  3. What would happen if you replace the 8ns delay line with a very long delay line (several seconds at least, say it’s bouncing back and forth between parallel mirrors for a while) and replace the half-split mirrors with mirrors that can be controlled to hit either detector (say a small electrostatic mirror cell like from a DLP, or just a fast servo controlled mirror) could it be possible to send information back into the past? And perhaps chain several machines together (all started sequentially) in order to send it back further? Just thinking out loud, would it work?

    1. I just barely understand this, but I assume there’s some quantum ridiculousness that makes this impossible (extremely unlikely?). And since we both thought of it independently, I assume (hope) that someone who knows what they’re doing has already tried.

    2. You’re not sending information anywhere! This part is totally wrong in the article. There’s no information being sent. It’s just a correlation (that was created by the Glan-Thompson prism) that gets observed later. Altering the polarization angle of the detectors destroys the correlation.

    3. No, because there is no actual information transmitted, just “coincidence”. If you look at each of the set of detector results alone, nothing special happens. It’s when you look at the combinations of detector results that you see the magic.

      A very simplified way of looking at it is to think the basic entangled photon experiment, but instead of seeing results spread out across two different places, you are seeing it as spread out across two different points in time. And just with entangled photons where you don’t get instantaneous communication across space, you don’t get communication across time.

      1. And I’ll just add this tidbit, it’s the Copenhagen interpretation that requires such mental gymnastics when dealing with entangled photons. The Copenhagen interpretation is still the most commonly taught interpretation as it’s the easiest to understand and has been the most popular among physicists, but it’s popularity has been waning. Recent surveys show De Broglie–Bohm as the most popular.

  4. The explanation as to why this likely occurs is trivial, but the circular-referenced dogma of modern physics in general makes curricular changes nearly impossible.

    It seems the heliocentric theories will persist for another generation, as people will argue over which fantasy is more appealing.

      1. He’s likely a crank but I can’t help but feel we need something new and special to unpick physics at a modern level.

        Maybe that’s string theory? The whole M-theory thing is pretty effing mind blowing.

        Those 5 theories you’ve been working on? Yeah, same theory. Also this completely unrelated one? Same thing. And also there is one more. When a handful of related but different threads of research and a completely unrelated one suddenly merge you know good things should be a foot (in the next few decades!)

        1. Several generations of physics grad students have already been wasted on string theory. It doesn’t predict anything new. I cringe when I see new students being sucked into that dead end.

          1. I think physics can be more about the pursuit than the thing itself. (Contrasted with engineering). So I reckon many of those who went into string theory didn’t feel themselves wasted.

            Maybe they do? I’m in no position to know.

            As an aside certainly string theory is at least productive from a mathematical point of view.

      2. “Ah thank you”

        Was “Crank” meant as pejorative slang, or asserting an informal fallacy?
        Note, my false dichotomy also alludes to the initial observation about modern academic focalism.

  5. This part’s wrong.

    “And this, my fellow hackers, should be impossible. Why? Because:

    The photons hit D0 8 nanoseconds before D1 – D4.
    The photons have a 50/50 chance of hitting D1/D2 or D3/D4.”

    The problem is that you’re pretending the photons that go through D1/D2 and D3/D4 are the same thing as free photons. They’re not. When they were created (along with the photon that hit D0), they weren’t created with enough phase space to populate two free photons.

    This is what leads you to the seemingly crazy conclusion of “the photons that end up at D1 and D2 must be sending information 8ns into the past” (or the opposite). This conclusion is insane, obviously. The information *came from when the photons passed through the beam splitter in the first place*. The correlation you’re seeing is what allows you to conclude that the beam must have had a phase relationship in the past.

    The time delay doesn’t matter. It could be a billion years, so long as the photons haven’t had their phase screwed with since then, because you’re still measuring the same bits of information that correspond to that first creation moment.

    The amazing part isn’t “information traveling into the future or past,” because there is no information travelling into the future or past. The amazing part is that you’re able to encode information into 2 photons in such a way that you can *only* recover it by measuring the same two photons.

      1. No, it’s just the photons in the experiment. I hate the phrase “entanglement” because it implies there’s a physical connection between the particles. There’s not. It should be called “quantum encoding,” because that’s really what’s going on. You’re encoding information in the photons that you can recover later but a) only once, and b) only if you do exactly the right thing to both photons.

        (This wording also makes it obvious why it’s useful for quantum cryptography works.)

          1. No, this is completely separate from that (In my mind ER=EPR is the stupidest conjecture in modern physics, and note that it’s “ER,” not EP – Podolsky was the one who didn’t write the paper on wormholes).

            The idea that you need to have some way of connected entangled particles makes absolutely no sense. You don’t need any connection between them. When the two photons get created, their polarizations are linked, because you created 2 of them from something with a single polarization.

            The 2 of them, together, don’t have the same information content as 2 free photons, because they weren’t created that way. So when you consider the two of them, together, through any experiment, you can’t expect them to *act* like 2 separate free photons, so why are you describing them as 2 separate free photons at all?

            Mathematically, you probably could describe entangled particles as being connected by wormholes in a quantum gravity description. But this would just be math, not anything useful, and it would just serve to confuse people as thinking you could send information along it (you couldn’t – it’d be a nontraversable wormhole, which would be a way for the math to represent the connection closing as soon as one side’s state is altered, and it wouldn’t be a wormhole in the underlying spacetime, it’d be a wormhole in the field space). It’d just be a way of describing two coupled photons in terms of free photons and a field connection, but that’s really just a decomposition. Doesn’t really change anything. It’d be like saying “I can describe you as ‘me minus the difference between me and you'”.

    1. I see what you’re saying Pat. I did not touch on entanglement because I wanted the reader to understand the basics of the erasure experiment and what it was trying to do. Thanks for all of you comments, however. Keep them coming. You know your stuff!

      1. The problem is that all of the weirdness in the “eraser” experiment comes entirely from the idea that you’re trying to nail down the behavior of that initial photon through the slit.

        And you start saying “aha! when my D0 photon strikes, if it’s in ‘particle mode’ (e.g. in an interference null) it needs to force the other photon to have gone through D3/D4!” – that’s where the misunderstanding comes from.

        In fact, it’s important to realize *none of the detectors show interference patterns on their own!* None of them! They all look like single diffraction patterns. It’s only in looking at *correlations* between them – where did it hit D0 versus on D1, etc.

        And see that ‘180-degree phase shift’ between D1/D2? In other words, if you add the two, they add up… to a single diffraction pattern. So in fact, the “D3/D4” versus “D1/D2” means *nothing*.

        It might make things clearer if I explain the experiment a different way:

        We shoot one photon through a 2-slit grating. It interferes with itself, and then encounters a Glan-Thompson prism which generates 2 entangled photons. There is *some information encoded in the entire spatial extent of the beam exiting that prism* (N.B.: this is not a ‘hidden variable theory’- the information now gets encoded into the *pair*, not one).

        Collecting the beam from one entangled partner gives you some information, but since it’s not *all* of the information of the original photon, you can’t get that information back. Then you take the other beam, and split it, and rejoin one leg, but not the other.

        This means that *one* of those legs (D1/D2), plus D0, contains all the information from the original photon. The other doesn’t. So you can recover it (the interference pattern) from one set, but not the other.

    2. Many thanks for your far better explanation.

      I dislike description that first focus on the wrong conclusion and then later try to correct the issue. I know it’s mainly how quantum physics has been discovered historically, but this is stuff for high end guy in there field of study. For peoples not studying quantum physics, it’s a far better way to simply describes the underlying reality like you do: the conclusion of the experiment are then very simple to understand.

      Unfortunately most articles about quantum physic are there only to satisfy the author ego by hiding the real facts to there readers, pretty much like magician do on a show.

  6. Quantum entanglement is the subject of much subjugated and misinformed concepts. One particle can exist in an infinite number of model visualizations in an infinite number of places at one time while being nowhere at all.

    The cosmic joke of quantum mechanics is that as one looks more closely at the particles in question, sheogorath himself makes the particle smile, stick out there tongue and give the said scientist, or inquisitor the raspberry.

    If one examines particles one will find themselves under the microscope. Ultimately, the greater power of the individuals belief system will show the individual the true nature of omnipotence. An indestructible, volatile, existing everywhere at once molecule that has no name but can exist in future and past tense.

    If one were so inclined, they’d have better luck predicting a weather pattern than figuring this one out. This is where we break out the ouji board, pendulum, and try the DMT. Maybe the mechanical elves can answer this equation where Einstein couldn’t? Hell man, at this quantum level, if you don’t have a bowl lit, some mushrooms, some LSD, and munchies, you just won’t get it man. Maybe peyote, ahyaausca, and some concentrated DMT?

  7. There is a Human “Anthropocentric” obsession with the concept of “Time” – this is holding Us back today. It’s like an eradicable disease as long as Humans exist in organic form. Once Humans are replaced by self-replicating machines; this barrier will fall – and the horizons of discovery will (finally) be limitless. The result will be “Beings” that fully understand “Being” [except for the “Retro-Beings” – who insist on being Arduino Uno Compatible”].

  8. You really need to explain the object labeled BBO, after the double slit but before the prism. That object is fundamental to what happens. It is a crystal or other medium that produces two entangled photons when it is struck by a single photon. That property is what allows you to fire one photons, and then measure two in different places.

    As for why you can’t delay a photon until next week and encode the winning lottery numbers with them, PBS SpaceTime explains it well. If you just look at D0, you will see an even distribution. It is only when you look at the recordings made at D0 and correlate them with the data from D1-D4 that you get two opposite interference patterns and two single photon patterns. You can’t know which group a photon belongs to, even though it’s hit D0, until the measurement at D1-D4 is made.

  9. “What they found was that the photons that hit D0 always — as in 100% of them — correlated to their partner photons. And this, my fellow hackers, should be impossible.”

    It’s only impossible because we’re so used to thinking in Newtonian physics.
    One thing you have to remember about photons is that they exist AT the speed of light, so the human concept of “time” doesn’t really apply. Relativity tells us that for traveler light speed, time is frozen — so a photon doesn’t care “when” you measured it — it’s only whether you measured it or not.


    1. The corollary to this is that you can’t *alter* a photon (since that would give it a concept of time). If you interact with it, the original is gone. It’s a new photon now. Some parts of the original photon can be ‘copied’ onto the new photon, but if you try to alter the portion that’s entangled, the entanglement is destroyed. (Which is why I call it ‘quantum encoding’: obviously if you try to encode *new* information in that part of the state, the old information is destroyed.)

      That’s essentially what the beam splitters are doing – they’re creating a new photon that’s still got a polarization relationship to the D0 photon, but that only works because they’re polarization-blind.

      So then you see the idea of “what if you *force* the D1-D4 photons to hit a certain detector to send information” is obviously nonsense – if you alter the polarization of the D1-D4 photons, you lose the link to D0’s polarization.

  10. This is the same aspect that Einstein called “spooky action at a distance.” It is based upon the assumption that there is a hidden state that is “communicated” faster than light….or backward in time. Not really. Can’t modulate information over that seeming channel. The following reference is a great explanation of this:
    1. Salart D, Baas A, Branciard C, Gisin N, Zbinden H. Testing spooky action at a distance. 08083316. August 2008. doi:doi:10.1038/nature07121.

  11. “Impossible” you keep using that word. I do not think it means what you think it means.

    Seriously, the word is used several times in the article. *Clearly* the observations you are describing are possible.

    We do a huge disservice to our “sons, daughters, nieces and nephews” by describing quantum physics as “impossible”, “weird”, “strange”. You’re describing reality; there’s nothing weird or impossible about it. Only our understanding is flawed. (As a side note (which this margin is too small to contain), multiverse theories quite nicely account for everything in this article with no time-travel, acausality, ftl signaling, non-linearity etc. required). If we want the young ones to figure this out, we should teach it to them without the mental block that comes with telling them it’s an “impossible” problem.

  12. one assumes the arrow of time but that only happens with ergodic changes.

    Imagine some kind of game “God Mode” if you could rewind time to before the original slit as long as it didn’t hit D3 or D4, but when you hit play it might go through the other slit. When the dual entangled photons emerge, rewinding would combine them (one should see if you could take entangled but not determined pairs of photons to recombine them in the same kind of crystal that split them)

    The interference patters or lack thereof are in SPACE, not time. Until it hits a detector – including D1 and D2, you can rewind and going forward can produce a different result and you could do this an unlimited number of times in your meta-time.

    1. To clarify, I’m saying time doesn’t have an arrow most of the time. Just like the speed of light is constant, but that means length, mass, and time get stretched or shrunk, I’m saying that time itself can reverse for a bit then return to the normal flow until an ergodic event happens to it which freezes its state.

      Another thing which might prove useful is to do a variant to allow instantaneous communications by sending entangled photons to Mars and on either end have a telegraph key that either leaves them entangled (interference pattern) or forces a state (particle pattern). Apparently it has been done and allows FTL – instantaneous – communication. Maybe SETI should look for entangled (or not) particles and photons

      1. All other reality-bending aside, the entangled photons still have to travel to Mars and back, which is not instantaneous. Therefore the latency of the information transfer is still limited by the speed of light.

      2. “Another thing which might prove useful is to do a variant to allow instantaneous communications by sending entangled photons to Mars and on either end have a telegraph key that either leaves them entangled (interference pattern) or forces a state (particle pattern). Apparently it has been done and allows FTL – instantaneous – communication. ”

        Nope. Not in the slightest. There’s no information being communicated. You’re just encoding information, which you recover at the end. There’s no way you can actually affect the partner photons by doing anything to the first one. You try, and all that happens is you break the entanglement.

        See why ‘quantum encoding’ is better terminology? There’s nothing connecting the two except that they have the same piece of information – the original polarization of the photon – encoded as the combination of their polarizations, and *any* messing with them destroys that. That’s why D3/D4 don’t show an interference pattern (their polarization state was destroyed) and D1/D2 do (theirs was preserved).

        “Maybe SETI should look for entangled (or not) particles and photons”

        You can only maintain the encoding if you can avoid any interactions that would alter the encoded information. That’s very hard to do in lab conditions over big distances. Sending them across space would be impossible.

        1. Your explanation makes sense, but it still leaves me wondering where the initial ‘spooky interpretation’ came from? Insufficient analysis by the experimenters? Did they really not think of this and resorted to magical thinking?

      3. “sending entangled photons to Mars and on either end have a telegraph key that either leaves them entangled (interference pattern) or forces a state (particle pattern)”

        That is an excellent example of why this experiment doesn’t work.
        The detector D0 in this experiment could be on Mars. The telegraph key could be placed at the position of the mirrors BSa, BSb. Then you could send data to Mars as you suggest, faster than light. Which is known to be impossible.
        If you set that up, the recipients would see only a smooth curve (see the experimental results on this page). They could not see an interference pattern unless you were to send them the readouts of the detectors D1, D2, Then they could sort their photons into 2 groups and see the interference.
        But, since you have to send them that information after you press the “telegraph key”, you may as well send your message directly!

  13. Can someone please elaboratly explain how a single photon through a double slit experiment produce an interference pattern??
    The wavefuction associateed with a single photon is guassian ,which implies it’s nature should be practicle like..?

  14. There is a mistake in the article. The authors have failed to describe the experiment correctly.

    This article states “D3 and D4… show a particle pattern. D1 and D2 show an interference pattern”

    Now, it you read the original paper, or the summary on Wikipedia, you’ll see that this doesn’t happen.

    The detectors D1 to D4 measure the time of arrival of a photon, but they are not scanned sideways like D0 so they do NOT measure interference patterns or any other patterns.

    This is why the diagram shows an arrow beside D0 but not beside any other detector.

    Fortunately, this mistake doesn’t affect the analysis of the results. I will explain in another comment why they are wrong too.

  15. I’m sorry, everybody, but this experiment does not achieve “delayed choice erasure”. I will explain why not.

    A quick recap: “delayed choice erasure” supposedly erases interference patterns in the past, when you measure something in the present.

    In this experiment, the patterns are seen by a detector called D0, then there’s a brief delay, then a measurement happens at one of the other four detectors.

    Why four ? Logically, they need only three. You should be suspicious.

    They need two detectors to spot photons from the “red” and “blue” routes. A third detector could catch the “unknown” photons. So why are there two detectors doing that job (D1 and D2) ? Bear with me…

    The user “Quin” posted “You really need to explain the object labeled BBO”. Very good, Quin. That’s the key to the trick. BB0 is a crystal, it splits a photon into two photons. The split happens at a random depth inside the crystal, therefore the incoming photon is at a random phase. But the outgoing photons are in phase with each other. Remember that !

    Now, look at the mirror BSc. The “unknown” photons arrive here, one by one. Each photon travels both the red and blue routes (that’s a standard quantum effect), so it’s going to interfere with itself where the routes meet up.

    When the photon hits the mirror, it can go through or it can reflect. And the reflection happens at the top surface. Any physics student will tell you that causes a phase flip in the photons on the OUTSIDE (red line going to D2). It does nothing to photons on the INSIDE (blue line going to D1).

    What does that imply? It means that if you don’t interfere with yourself at D1, you WILL do so at D2. Or the other way around.

    Now, look at D0. The interference pattern appears at D0 if and ONLY if you pick out the photons corresponding to ONE of D1 or D2. And what causes interference? The phase relation of the upper photons’ red and blue versions. But that is the SAME as the phase relation of the lower photons (remember, they are copies!). And, using the mirror, we SORTED those photons by their phase relation.

    I hope you can see it now. This is completely invalid ! The experimenters created the thing they were looking for !
    It’s like saying “Suppose I pick out all the green M&Ms from a bag. They will magically create a green pile”

    This experiment depends on quantum interference (which is weird) and entanglement (even more weird) but it does NOT change the past. It does NOT achieve “delayed choice erasure” as claimed.

    I’ll be glad to discuss this….

  16. Interesting origins of the theory of “diffraction”- starts in the 1600s with Francesco Grimaldi- Grimaldi never mentioned the theory that light particles are reflecting off of the inner surface of the hole. The debate between the theory that light is made of material particles that move mostly through empty space (corpuscular) vs the theory that light is made of material particles that mostly collide with each other conveying a motion (undulatory) was popular through the 1700s. In the early 1800s the undulatory took over and the earth was immersed into that singular universal view. In the 21st century the rise of FitzGerald’s space contraction and dilation took center stage- but Maxwell’s theory about light being a dual transverse wave still holds firmly, at least according to the Encyclopedia Britannica and Wikipedia.

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