After reviewing the more than half dozen questions on the DCQE here, this doesn't seem to have been asked or answered directly - if I've missed this piece in one of the other questions or answers please point me in the right direction and we can close this.

Using the detector notation from the DCQE Wikipedia article for brevity:

Let $D_0$ = idler photon collector plate

Let $D_1$ = no-path information detector 1

Let $D_2$ = no-path information detector 2

Let $D_1$ and $D_2$ be time-like separated from $D_0$ such that $D_0$ will register photon arrival hits some finite, measurable time prior to $D_1$ or $D_2$.

For the purposes of this question, we can ignore the cases where which-path info is collected. Frankly, we can ignore the "choice" to insert a beam splitter or not entirely. For the purposes of this question, a half-silvered mirror will remain in place (no which-path information is available).

After many runs, when I go back to correlate the arrival times of clicks at $D_0$ with clicks at $D_1$ and $D_2$, I note that 2 interference patterns are recoverable, with the location of $D_1$ idler entangled photons aligning with troughs of $D_2$, just as expected.

Since I now know the locations of the respective peaks and troughs on $D_0$ correlated with $D_1$ and $D_2$ detection, I wipe my detection plates clean and start again. I note that my first "click" at $D_0$ is in a location that corresponds to a peak in the $D_1$ fringes.

My question is simple:

Based on this knowledge, is the signal photon more likely to be detected at $D_1$? (my $D_1$ detector hasn't clicked yet due to the time-like separation).

It seems the answer must be yes.

If it IS yes, then why is there any discussion at all about backwards causation? If I can note the location of photon recording on $D_0$ and predict a higher or lower probability of recording an entangled sister photon on $D_1$ or $D_2$, then that's the ballgame. Where's the spookiness that has this experiment come up again and again?

If the answer is no in this setup, I struggle to see why, and so any clarification would be greatly appreciated.

  • 4
    $\begingroup$ A downvote with zero comments and zero answers doesn't help this community. It's a straightforward question. $\endgroup$
    – JPattarini
    Jul 7, 2017 at 14:22
  • $\begingroup$ I think you mean to say that $D_1$ and $D_2$ are time-like separated from $D_0$, so that we can unambiguously say that the detection at $D_0$ happens before the others. $\endgroup$
    – user5174
    Jul 9, 2017 at 1:46
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    $\begingroup$ I'm not sure why, but there's a pattern with the DCQE of people taking the notation in Wikipedia and assuming that it's completely universal and will be understood instantly without the need for a diagram or even a reference for where the notation comes from, but in case it wasn't obvious: the notation is not universal, you always need to specify any external sources, and for anything this complicated you do need a diagram just to set a clear common ground. $\endgroup$ Jul 10, 2017 at 18:10
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    $\begingroup$ note to the downvoters that do not comment: Your downvoted questions will be upvoted $\endgroup$
    – lurscher
    Jul 10, 2017 at 18:48
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    $\begingroup$ @lurscher A blanket upvote policy seems to me to be an enormously counter-productive strategy, particularly on questions with flaws as obvious as this one's. There's plenty of good reasons why downvoting is anonymous, and a blanket upvote policy just helps promote bad content on the site. $\endgroup$ Jul 10, 2017 at 20:11

5 Answers 5


The short answer is that you're right. The delayed choice experiment doesn't require backwards-in-time shenanigans, and all the pop-science articles implying this to be the case are basically garbage.

For example, here is the example delayed choice eraser circuit from my quantum circuit simulator Quirk:

delayed choice circuit

The green boxes are state displays that show the probability of each possible measurement result, optionally conditioned on some other qubits' possible measurement results.

The top wire is the "choice" qubit. The second wire is the "which slit" qubit. The rest of the wires are the "where did it hit on the screen" qureg. The first two operations setup some entanglement between the which-slit qubit and the screen qureg.

The four displays on the right show that, if you group the screen measurements by the choice qubit and the which-slit qubit, then within the groupings you will see an interference pattern if and only if the choice qubit is ON.

But correlation goes both ways. Instead of thinking about how the choice qubit and which-slit qubits predict the screen measurement, we can think about how the screen measurement predicts the which-slit qubit's state.

It would be tedious to set up $2^7$ bloch sphere displays, each conditioned on a different screen measurement. Instead, let's use a single condition but cycle through offsets to the screen measurement. This circuit makes it very clear that the landing position on the screen is correlated with different states of the which-slit qubit:

screen landing predicts state of which-slit qubit

This diagram also answers your main question:

[given a landing position] is the signal photon more likely to be detected at D1 [than D2]?

Yes. The value analogous to the D1-vs-D2-given-position likelihood is shown in that green box in the top right of the diagram. The chance is changing as we focus on different positions.

Notice that the qubit is spinning like crazy regardless of whether we applied the $X^{1/2}$ rotation that will be controlled by the choice qubit. All the $X^{1/2}$ rotation does is switch which axis the qubit is spinning around as I vary the screen measurement position being focused on. If there is no $X^{1/2}$ rotation, the spinning goes around the measurement axis, and so doesn't affect the probability of measuring ON-vs-OFF (i.e. we've picked a rather poor measurement axis). But if the $X^{1/2}$ is applied, then the spinning is around the Y axis instead of the Z axis, and so does translate into changes in computational-basis measurement probability.

So here is a forward-in-time collapse interpretation of the delayed choice experiment:

  1. We create a situation where the which-slit qubit is entangled with a screen-landing position.
  2. We measure the screen-landing position. This collapses the system into a state where the which-slit qubit is pure, but the specific direction it points is determined by the landing-position measurement.
  3. We choose to measure along an axis that picks up these variations-from-landing-measurement, or to measure along a totally uninformative axis.
  4. We act surprised that our choice determined whether we see correlations between the which-slit qubit measurement and the screen landing-position measurements.
  5. We switch to thinking about the correlations in the opposite direction and start talking about time travel, because that's what brings in the sweet sweet clickbait ad revenue.
  • $\begingroup$ For posterity, I agree retrocausal inferences are promoted especially on youtube as clickbait in order to increase clicks either for popularity or ad revenue. But it may also be due to the ignorance of the true nature of the experiment and entanglement at large. Most people take DCQE as not what it is as per QM but what they erroneously think it is. (Tail wagging the dog). Youtube is not a good place for reliable science. Most DCQE-related Youtube videos instead of solving the puzzle try to aggravate the mysteriousness of the puzzle for the sake of delivering sheer thrill to the viewer. $\endgroup$ Jun 18, 2023 at 5:08

With the caveat that I don't really understand the views I'm about to describe, I'm going to try and explain where talk of retrocausality comes from.

Historically, there had been a lot of resistance to the picture of reality painted by quantum mechanics, and people were looking for less well-defined but more palatable ways to explain experimental behavior. (with, of course, the hopes of eventually discovering a well-defined theory)

The more famous experiments are about killing these various alternative ideas. For example, the various Bell tests that killed local hidden variable theories.

The main idea at play here, I think, originates from the basic double slit experiment. You normally see an interference pattern, but if you add something to the experiment to discern which-way information, the interference pattern goes away.

The potential idea that spawned is that the photon is normally in some "I will interfere" state, but the presence of which-way apparatus tells the photon to switch to an "I won't interfere" state.

This may sound squirrelly, but remember that this isn't in the "I have a concrete idea that makes precise, testable predictions!" phase of science, it is in the early stages of the "I am trying to figure out a way to make sense of the data!" phase of science.

The quantum eraser experiments were a big step towards killing these ideas; despite the eraser afterwards, the which way apparatus is still there to spoil the interference. But these ideas can still be rescued in various ways; e.g. by positing that it still takes some time for the which-way apparatus to spoil the interference, or the presence of the eraser tells the photon not to switch to the "I won't interfere" state.

These attempts at salvage suffer a mortal blow from the delayed choice quantum eraser experiments; the photon is forced to "choose" before the choice to erase has been made, so any attempts to stick to these sorts of alternative explanations are forced to resort to retrocausality.

  • $\begingroup$ See my comments to Emilio's answer below, they apply equally to your answer $\endgroup$
    – JPattarini
    Jul 12, 2017 at 19:05
  • $\begingroup$ Note that the idler photons still "choose" before the choice to erase has been made. Half of the photons that arrive at D1/D2 don't interfere because they "travel over a single path". They have "chosen" a single path but take "the other path" that does not lead to D3/D4. $\endgroup$
    – fishinear
    Jun 22, 2023 at 12:45

According to your description I agree with you that a prediction, that idler photon is more likely detected at D1 or D2, based on knowledge where signal photon hit D0, is possible.
In your simplified experiment a 50% beam splitter (BSc) has remained. I think that reason why discussion about retrocausality has started, is the following:

  • A 50% beam splitter is a device which reflects 50% of photons, the other 50% can pass through. So in case of a laser beam which consists of a huge amount of photons, it is predictable that 50% of the photons will be reflected.
  • BUT it is impossible to predict for an individual photon, whether it will be reflected or not. This is pure objective coincidence.

Now, according to your description it is possible to do a prediction for a single photon (the idler photon) which is arriving at BSc (some nanoseconds after entangled signal photon hit D0), whether it will be reflected at the splitter or not.
This would be an astonishing result as such a prediction should not be possible at all.
Retrocausality comes into picture if such a prediction of an individual event is unacceptable and hence still considered as impossible. In such case, idler photon would unpredictably hit at D1 or D2.
As a logical consequence, result at D0 would have to be changed retrocausal, so that proper interference pattern would appear at the end.

  • $\begingroup$ "BUT it is impossible to predict for an individual photon, whether it will be reflected or not. This is pure objective coincidence." - That's not impossible at all. If it is a polarising beam splitter, for example, then sending a polarised photon will enable you to predict which way it will go. $\endgroup$
    – fishinear
    May 28, 2023 at 17:02

Old question, and the physical part of it has already been explained sufficiently by the other answers. I'd like to answer your question:

If it IS yes, then why is there any discussion at all about backwards causation?

The main reason is because people do not understand that the pattern at D0 does not actually change. People who have heard of the experiment, but have not actually looked at the details of it, think that which D1-D4 detector the photon lands determines where its twin lands (has landed) at D0. The D1/2 photons show an interference pattern at D0, while the D3/D4 photons show a bulge pattern at D0, so where the D1-D4 photon lands must influence where its twin lands at D0, right? I think it is an easy misinterpretation to make by a layman, and it even happens to respected presenters like PBS Digital Studios: https://www.youtube.com/watch?v=8ORLN_KwAgs&t=157s.

They don't understand that each signal photon always has an interference probability pattern at D0, and nothing changes in that based on what happens at the other side. It is just that if you take the interference patterns of the subset of signal photons whose partner lands at D3 or D4, then they happen to add up to form a bulge pattern. And if you add up the interference patterns of the different subset of photons whose partner lands at D1 or D2, then they happen to form an interference pattern.

  • $\begingroup$ Do you mean whatever we do to idler photon doesn't affect signal photon whatsoever? Does that mean where signal photon lands on x axis of the plot is decided only by path of the signal photon? $\endgroup$ May 28, 2023 at 9:38
  • $\begingroup$ @DukeWilliam Correct. $\endgroup$
    – fishinear
    May 28, 2023 at 16:57
  • $\begingroup$ Thanks, but if we change the idle photon path setup will it affects later photons that emerge from the BBO crystal? $\endgroup$ May 28, 2023 at 17:13
  • 1
    $\begingroup$ @DukeWilliam A change in the idler photon path setup, will of course affect the idler photon. But if you don't change anything in the signal photon's path, then it will not affect the signal photon. $\endgroup$
    – fishinear
    May 28, 2023 at 17:18

This answer makes the assumption (unwarranted as of v5 of the question), that you're using the notation from the Wikipedia article on the topic.

Given that, in the situation you have described, the answer to your core question,

is the signal photon more likely to be detected at $D_1$?

is yes. If the interference were perfect (i.e. in a noiseless experiment), if you detect a photon in a position where the $D_0\wedge D_2$ coincidence rate has a node, then you can be absolutely certain that the idler will be observed at $D_1$ and not $D_2$. If the experiment is less than perfect, or the detection is slightly away from one of the nodes, then there will be a small probability of detecting the idler at $D_2$ but this will still be smaller than at $D_1$.

Now, as you note, if the $D_1/D_2$ detection is in the causal future of the detection at $D_0$, and in the absence of the possibility of introducing which-way detections, then as far as causality goes the experiment is less challenging, and there are reasonable classical models (which are specific to this experiment, and not extensible to a general theory) that do a good job of describing the dynamics without any weird quantum fuzziness.

However, that's just because you've artificially de-fanged the experiment by removing the core part of the argument ─ the choice in delayed-choice quantum eraser. The standard argument posits that within the data collected at $D_0$ there are already latent interference patterns even though the photons were 'marked' with information that can be used to discern which-path information after the (potentially) interfering photon has already been detected. That seems to bother some people.

In particular, let's take your analysis and run with it: suppose that you've measured the photon at one of the nodes of the $D_0\wedge D_2$ interference pattern, and therefore you're certain that if you measure on the $D_1/D_2$ no-which-way pair then you'll go always see it on $D_1$. Then some enterprising folks might say "aha! now that we know it's interfering, and how, let's not do the $D_1/D_2$ pair, let's do the $D_3/D_4$ which-way pair instead" ─ and, of course, they find out that once they post-select on that the interference patterns disappear.

I agree with you that the delayed-choice quantum eraser is not particularly surprising (or at least, I don't see how it adds anything much beyond the standard quantum-eraser configuration), but you're cutting out key bits of the analysis, and that rarely bodes well.

  • $\begingroup$ To your 2nd to last paragraph: I fail to see how an interference pattern can be said to "disappear" - since a single detection in a node for D1 is also compatible with a single detection location for the general bell-shaped no-interference distribution associated with D3/D4. The whole point is that it's only by correlating either D1 or D2 with their paired D0 hits that interference can be recovered. Coordinate (x,y) on D0 can be compatible with either D1, D3, or D4 (assuming it's at a trough for D2). $\endgroup$
    – JPattarini
    Jul 11, 2017 at 17:53
  • $\begingroup$ My point is in all scenarios there's forward causality regardless of the choice. If a D0(x,y) is recorded, the future options for which detectors can light up for it's entangled sister are reduced to the subset of detectors with which-path info (whose D0 distributions overlap), or the detector without which-path info for which D0(x,y) corresponds to a node. The choice to detect which-way info (or not) further reduces the available subset of detectors in the same manner. $\endgroup$
    – JPattarini
    Jul 12, 2017 at 18:59
  • $\begingroup$ Given this, how retrocausality or similar mysterious ways of talking about this ever got introduced into the way we discuss these experiments is, to me, a mystery (pun intended). $\endgroup$
    – JPattarini
    Jul 12, 2017 at 19:02
  • $\begingroup$ @JamesPattarini I ventured an answer because part of your question was technical and objectively answerable. As for the rest, there's a number of ways to attempt to cope with the weirdness of QM, and retrocausality is one of them. I personally find it so bizarre from the outset that it's not really worth contemplating, but unless and until the measurement problem gets solved, making fun of other people over their ways to cope with QM is not really a polite thing to do, because all interpretations have their issues. $\endgroup$ Jul 12, 2017 at 19:44
  • $\begingroup$ Certainly no poking fun intended. I asked the question because there seemed to be no place for retrocausality, unless my description of events in the post question was wrong. It's hard enough to learn QM without needlessly complicating it, IMO. Appreciate the answer. $\endgroup$
    – JPattarini
    Jul 12, 2017 at 20:02

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