Because our and our ancestors' experiences have been largely formed by the logic of classical physics for millions of years, we find quantum mechanics counter-intuitive.Update:Years after this blog post was written, I learned about a totally sensible paper by Bram Gaasbeek which explains why the experiment contains no paradoxes, no retrocausality, and no new physics. Quantum mechanics with the normal Born's rule etc. is enough. Correlation doesn't imply causation. The moment of the co-collapse may be different for different observers but at the end, they will agree on facts they should agree upon.

Update 2: See also a 2016 blog post focusing on (non-existent) retrocausality in this experiment.

However, for everyone who is interested in physics, there should eventually come a moment in which he realizes that quantum mechanics works.

He (or she) should learn how it works, he should appreciate that the predictions are uniformly confirmed by observations, he should see that the quantum predictions approximately reduce to the classical ones whenever they should, and that there is nothing paradoxical or retrocausal about quantum mechanics.

Feynman has said that all the surprising wisdom of quantum mechanics is hiding in the double slit experiment. If you think about it carefully enough, you will ultimately figure out all the important and amazing new features of the world that quantum mechanics uncovers.

However, many people disagree and they tend to expect that every time they add a new prism or a new laser or a beam splitter to an experiment, the situation becomes more confusing than ever before and there is a new "hope" that a disagreement with quantum mechanics will be found.

I disagree with that. More complex experiments only make the situation more contrived - but the basic scheme of how quantum mechanics works is unchanged. Also, all the errors that lead most people to believe that there is something paradoxical about quantum mechanics keep on repeating themselves.

**The list of experiments**

The four basic experiments that I want to mention are

- The ordinary double-slit experiment
- Wheeler's delayed choice experiment
- An EPR experiment with a single pair of particles
- Delayed choice quantum eraser

But before we begin to analyze the particular experiment, let us say a few general words about quantum mechanics and its aspects that are widely misunderstood and that are relevant for the four experiments above - and many related experiments.

**Key principles of quantum mechanics**

People make several basic mistakes before they incorrectly determine that there is something paradoxical about quantum mechanics. Most of the mistakes arise because people fail to appreciate some (or all) of the following four fundamental and universal points about quantum mechanics:

**Quantum mechanics only predicts the final results of experiments:**it is not possible to say what the "real properties of the system" were prior to the measurement; and it's unsurprising that everything that occurs before a measurement may influence its outcome**All the predictions of quantum mechanics for the outcomes of the measurements are probabilistic and we can't influence the random generator:**there can't be any hidden variables that decide about the exact fate of individual particles, not even in principle, and whenever some phenomena are random in the quantum sense, the data is genuinely random and produced by Nature at the given moment; we should never think that it was "us" who made the decision**Nature never forgets about any correlations:**if the equations of quantum mechanics predict two objects "A, B" or their properties to be correlated - i.e. the probabilities of combinations of outcomes "P(A1, B3)" can't be universally written as "P(A1) P(B3)", then Nature never forgets about this correlation, not even after a long time (or at a very different place); so Nature's random generator that decides about the outcome of the measurement of "B" uses the conditional probabilities in which all quantities that have already been measured - e.g. "A1" - are already assumed; after "A" is measured to be "A1", the relevant probabilities for outcomes in "B" are the conditional probabilities assuming "A1"; this prescription may be visualized as a "collapse of the wave function" but this collapse is not a real physical process in any sense, it is just a rule for us to know which amplitudes are relevant**Correlation is not causation:**the fact that two spatially or temporarily separated measurements are correlated doesn't mean that one of them has physically influenced the other; instead, in all EPR-like experiments above, the correlation between the two measurements appears because both measurements share a common past - or a common "cause", if you wish; this comment is the typical and nearly universal reason why there's never any propagation of "faster than light" signals in any similar experiments as many people incorrectly say

**The double-slit experiment**

This experiment is too easy and I kind of need to assume that the reader is familiar with this basic stuff. (He or she may need a basic lecture on entanglement, too.) There are two slits in a plate. The individual particles coming from the source and going through the plate are predicted by quantum mechanics - and observed - to create an interference pattern on the photographic plate. It follows that we can't say that the particle went only through slit "1" or only through slit "2".

Each particle had to behave as a "wave" before it was detected on the photographic plate. The wave interfered with itself. The wave was interpreted as the probabilistic wave function for the particle. The squared absolute value of this amplitude determined the probability density that an individual particle appears at any particular place of the photographic plate.

There's only one particle - no room for correlations - so we apparently only test the rules 1,2. The particle is always allowed to do many things before it's seen. All the histories interfere. And at the end, the particle is observed at some point of the photographic plate. Quantum mechanics does predict the probability that it will be one place or another. Many particles with the same initial treatment eventually draw an arbitrarily sharp interference pattern on the photographic plate.

It works for photons - because many photons may be interpreted as a classical electromagnetic wave that interferes because of the good old classical reasons - but it also works if you send photons one by one or if you send any other elementary particles. The wave-particle duality is universally valid for all particle species.

While I have said that there was just one particle and no room to test the correlations, it's just not quite true. There are also many atoms in the photographic plates etc. And their state - the question whether the pixels on the photographic plate change their color etc. - is ultimately correlated with the particle that may have been detected at a given place.

Also, Nature and quantum mechanics guarantee that the particle is never detected at two places of the photographic plate simultaneously. For electrons, a reason is that the charge is conserved and the electron can't be doubled. This observation actually verifies the rule 3 as well: Nature never forgets any correlation.

If we only send one electron, there is a correlation between the number of electrons that appear at two places of the photographic plate: the laws prohibit the electron(s) from appearing on both points at the same time. This is a test of the claim that Nature never forgets about any correlations.

In some sense, the double slit experiment is testing the rule 4 as well. For example, our actions that may depend on seeing the electron at a given place of the photographic plate can't be interpreted so that we "forced" the electron to appear on the given place. The causation really goes in the opposite direction here. However, we only perform one measurement with the electron, so we can't derive any contradiction with causality.

**Wheeler's delayed choice experiment**

Obviously, if one decides to observe each photon's path - whether it took one slit or the other - it follows that the photon couldn't have gone through the other slit. Because it only went through one slit, whose identity we know, it couldn't have interfered. And indeed, quantum mechanics predicts that the interference pattern disappears whenever we detect which path was taken. Experiments confirm this prediction - like all other predictions of quantum mechanics.

John Wheeler designed a thought experiment - that has already been realized in practice - in which we may decide whether we want to "look" at the photon after it goes through one of the slits (or "both"). Needless to say, the photons for which we know which slit they took can't contribute to the interference pattern while those whose "which slit" information wasn't measured do interfere and contribute to the interference pattern.

Now, the reason why this looks counterintuitive to many people is the very same mistake that has already been discussed in the context of the ordinary double slit experiment. They simply violate rule 1 - that only the results of experiments should be predicted. No "objective properties of the system" exist prior to the measurement.

In the case of the ordinary double-slit experiment, it would be a mistake to think that the photon had to take one particular slit, or the other particular slit, because such an assumption would prevent the particle from contributing to the interference pattern. Instead, the photon is allowed to try all histories at once - before it's measured.

However, in this Wheeler's delayed choice experiment case, the mistake of the people who are surprised is pretty much analogous. Much like they thought that the particle had to objectively possess the property "took slit 1" or "took slit 2" before anything was measured about this particle, in this Wheeler's case, the same people make a similar wrong assumption that the particle is either "behaving as a particle" or "behaving as a wave" at the very moment when it is going through the slit(s).

But it's a wrong assumption - and qualitatively speaking, the mistake is completely isomorphic to the mistake in the ordinary double slit experiment. Wheeler's setup may be a bit more complex but it doesn't really raise any physically new issues. It just uses two other intermediate types of histories - "particle behaving as a particle", "particle behaving as a wave". There's no objective answer which behavior the particle chooses when it's going through the slits, much like there's no objective answer to the question whether it's going through one slit or the other.

**EPR experiments**

In the Einstein-Podolsky-Rosen experiments, a particle is split into two particles that are consequently entangled. For example, if a spin-zero particle (e.g. a positronium in a spin-zero state) decays into two photons flying into opposite directions, spin conservation implies that only "RR" and "LL" final states can appear. Here, "R" and "L" stand for "right-handed" and "left-handed", respectively.

Note that the angular momentum of a right-handed photon flying in the opposite direction is the same as the angular momentum of a left-handed photon flying in the original direction - which is why the two circular polarizations are equal rather than opposite. In "RR" and "LL" and similar expressions, the first letter always corresponds to the photon that flies to the left, and the second letter refers to the photon flying to the right.

Now, the final state of the two photons is something like "RR-LL" divided by square root of two. It's not shocking that both "RR" and "LL" are equally represented (it follows from parity, the left-right symmetry, of QED); the phase between the "RR" and "LL" terms depend on conventions and axes.

It's not too shocking that if one photon is left-handed, the other has to be left-handed as well. And if it is right-handed, the other one is right-handed as well. Nature won't forget about correlations - rule 3 - and we will see the same handedness of both photons. Rule 4 assures us that there's no action at a distance. After all, even in clasical physics, the angular momentum conservation law would imply a similar correlation. The two photons just emerged from the same "parent particle" so it's not shocking that their properties are correlated.

However, what's funny in quantum mechanics is that it predicts correlations of other properties as well. The state "RR-LL" over "sqrt(2)" may also be rewritten in terms of linear polarizations and we get something like "xy + yx" over "sqrt(2)". I am not sure about the relative sign and I am not sure whether it should be "xy + yx" or "xx + yy". Sorry. I will continue to assume that the state is "xy + yx" over "sqrt(2)".

But whatever the exactly right state is, it is clear that the final state of the two photons actually also predicts a 100% (anti-)correlation between the linear polarizations. If one experimenter detects "x", the other one will detect "y", and vice versa. Again, rule 4 emphasizes that correlation isn't causation.

But it's true that this "multiple correlations" wouldn't be possible in classical physics. Einstein believed that once the photons leave the positronium, they must already have some particular state. And each of them has to have a separate identity.

So Einstein believed that as soon as the photons left the positronium, they had to be either in the state "RR" or "LL" - and this single classical bit of information already had to be in the air. In his opinion, it was seemingly needed to explain the correlation in the measurements of the circular polarizations (angular momentum), without any superluminal action at a distance that would contradict relativity.

So without a loss of generality, Einstein thought that he could assume that the photons left the positronium as "RR". A right-handed photon has 50%:50% odds to be measured as x-polarized or y-polarized if we decide to measure the linear polarizations. But the two photons should be independent - because they've been independent since the decay of the positronium, he thought.

So Einstein thought that the probabilites of the outcomes "xx", "xy", "yx", "yy" had to be 25% each. For two right-handed photons, each of the four combinations of the two linear polarizations was equally likely.

However, quantum mechanics vigorously disagrees - and experiments agree with quantum mechanics. A simple exercise in linear algebra (a tensor product of two simple basis conversions) shows that there are only "xy" and "yx" states represented in the final state - so there's no way how the two photons could have the same linear polarizations: "xx" and "yy" are not allowed.

In my framework, you can see what is Einstein's mistake: it's nothing else than the complete denial of rule 3: correlations are never forgotten by Nature. The final state can be shown - by a simple linear algebra - to predict a 100% anti-correlation between the linear polarizations of the photons. So this correlation can't be forgotten, not even after years that the photons may spend before they're detected by two detectors that measure the linear polarizations.

There is no propagation of signals that are faster than light, as rule 4 guarantees. The circular polarizations of the two photons are totally correlated; the linear polarizations of the two photons are totally anti-correlated as well. All these correlations and anti-correlations arise because of the common origin of the two photons.

Quantum mechanics carefully remembers the amplitudes for all combinations of the possibilities for both photons (and all particles in the world). In most cases, the probabilities can't be just factorized - into the product we use for independent probabilities. But the correlation is never a result of a superluminal influence; it's always a manifestation of the common past.

Because quantum mechanics allows us to "rotate" states in the Hilbert space and measure many kinds of properties and quantities that correspond to non-commuting operators, the same quantum states may be seen to produce "many new kinds of correlations" - including correlations that violate (exceed) Bell's bounds. But that doesn't imply that superluminal signals are needed.

**Delayed choice quantum eraser**

Here is the delayed choice quantum eraser, a more convoluted experiment that combines the previous three experiments in a bigger setup. Some people find it more confusing but all the insights we must be careful about have actually been discussed in the three experiments above.

This looks very complicated but it's not. I ask you for a little bit of patience:

In the left upper corner, there's a laser. The light instantly gets to a double slit; the two slits are called "a" (red) and "b" (light blue) on the picture. This part of the experiment is trivial. Everything that is convoluted happens after the light goes through the double slit.

The photon takes slit "a" or slit "b" (or "both" in a way that will be discussed). In the picture, it means that the photon is either red or light blue.

You see that the red or light blue photons immediately split into pairs. That's achieved by a "beta barium borate (BBO) crystal" which performs a "spontaneous parametric down conversion". If a photon goes through the BBO crystal, it splits into two entangled photons with 50% of the energy (frequency) each.

So each photon that has made it through the double slit - whether it's red ("a") or light blue ("b") or undetermined - splits into two "lighter", mutually entangled photons. One of them goes to the upper portion of the picture, one of them goes down. The photon going up is called the "signal photon" and it may draw an interference pattern on the photographic plate "D0". Or not.

The photon that went down is called the "idler" because it's interesting to force this photon to wait for a while; don't forget that the idler is entangled with the signal photon. The idler is going to be manipulated by further components of the apparatus that may either preserve of destroy the interference pattern that may be drawn by the signal photon.

There are some grey-green mirrors that only reflect photons and they don't change anything about the information carried by the photons. However, there are also light green beam splitters ("BS") that have a 50% probability to transmit and 50% probability to reflect an incoming photon.

The geometry - including the ordinary geometric prism (incorrectly labeled "Glen-Thomson prism on the picture"; the GT prism is a part of the BBO) - is such that if the photon goes to "BS_b", it had to arrive from the red "a" slit, and when it goes to "BS_a", it had to arrive from the light blue "b" slit. Not sure why they crossed the convention for "a" and "b" for the beam splitters on the picture. ;-)

One half of the idler photons from "BS_a" and "BS_b" are sent to detectors "D3" and "D4", respectively. If you detect an idler in "D3" or "D4", it proves that it had to go through slit "b" or "a", respectively. Because the signal photon is entangled and Nature never forgets any correlations - rule 3 - it follows that the upper, "signal photon" had to go through "b" or "a", too.

Such photons whose path is known couldn't have interfered. And indeed, the signal photons corresponding to idlers seen in "D3" or "D4" are found not to contribute to an interference pattern. If you reconstruct the sub-picture made by the signal photons that were linked to the "D3" and "D4" idlers, you will see no interference pattern at all.

However, one half of the photons going through either "BS_a" or "BS_b" are manipulated in such a way that the "which slit" information will never be detected. We say that this "which slit" information is "erased" and this part of the gadget is the "quantum eraser".

How does it work? You see that the parts of the photons that went through the "BS_a" or "BS_b" splitters but didn't end in "D3" or "D4" continue, through mirrors "M_a" or "M_b", to another beam splitter "BS_c" where the beams from both mirrors "M_a" and "M_b" are re-united into a single beam. Well, more precisely, they're reunified into two beams going to "D1" or "D2" but "D1" or "D2" is not correlated with the "a", "b" slits in any way. So the information about the slit is no longer remembered by the position of the photon.

The beam splitter "BS_c" is designed in such a way that it sends the unified beam either to "D1" or "D2", whether it comes from the mirror "M_a" or "M_b".

Because the "which slit" information was erased by the eraser - by the re-unification of the two beams - the idler photons that ultimately end in "D1" or "D2" were allowed to interfere. Because they are entangled with some signal photons in the upper part of the picture, it follows that the corresponding signal photons could interfere as well.

And indeed, if you reconstruct the positions of the signal photons whose partner idlers were found in "D1" - which could occur much much later - you will find out that the positions of these signal photons did draw a sharp interference pattern. Also, if you reconstruct the positions of the signal photons whose partner idler photons were found in "D2", you also find a sharp interference pattern - but a different one than for "D1".

These two patterns are actually negations of one another - minima for "D1"-partnered signal photons overlap with the maxima for "D2"-partnered signal photons and vice versa. So if you add all the signal photons whose partners appeared either in "D1" or "D2", the interference pattern disappears. Again, recall that the signal photons associated with the idler photons detected in "D3" or "D4" didn't contribute to the interference pattern, either. So if you simply detect all signal photons and make no distinction between them, there will be no interference pattern on the screen.

**Why do people find it counter-intuitive?**

And why do they think that the experiment reveals retrocausality (i.e. the ability to influence the past of the Universe)?

Again, it is because these folks violate one of the rules 1,2,3,4 - or several of them. Most typically, they think that we are "retroactively forcing" the signal photons to contribute - or not to contribute - to one of the interference patterns or another by doing something to their partnered (and entangled) idler photons.

However, as the rule 2 emphasizes, it's not "us" who is deciding about the fate of the idler photons. It's the random generator of Nature itself. We have no credentials to influence it. Moreover, the manipulations with the idlers don't influence the predictions for the signal photons at all, as I will explain below where I confirm rule 4.

The logical reasoning above that was needed to determine which signal photons contributed to one of the two mutually negated interference patterns (partners of "D1" or "D2" idlers, respectively) and which signal photons didn't ("D3" and "D4") is respected by the laws of quantum mechanics, especially because of the rule 3 that Nature never forgets any correlations. You shouldn't forget them, either.

Nevertheless, as the rule 4 makes clear, correlation doesn't imply causation. So the later manipulations with the idler photons didn't "force" their partnered signal photons to behave in one way or another. In fact, it is always more sensible to imagine that the causal relationship goes in the other way. But even this is not needed.

Because some signal photons ended e.g. in the near vicinity of the interference minima of one of the interference patterns - which is the case that makes us maximally certain that these photons didn't contribute to the interference patterns - it followed that their idler partners couldn't have been seen in "D1" or "D2", respectively (depending on which pattern's minima we're talking about) which is appropriate for idler partners of non-interfering signal photons or the idler partners of the signal photons that had a maximum at the point.

Typical individual photons can't be sharply said to be "a part of one of the interference patterns" or "not a part of any interference pattern" before their idler twins are detected. As rule 2 tells you, the quantum predictions are probabilistic, and unless the predicted probabilities are 0% or 100%, both answers - Yes, No - may be true.

However, the 0% or 100% probabilities are exactly what we get once we know where the idler photons were detected. Once we know it, it is possible to reconstruct whether the corresponding signal photons did behave as the parts of one of the two interference patterns or parts of the non-interfering boring blob.

**A brief chronological history of a photon pair**

Let us now look at a photon - that becomes a pair of photons - chronologically, to see that no "retrocausality" (modification of our own history) or "faster than light signals" ever appear in such experiments if they are interpreted properly, according to the postulates of quantum mechanics.

A high-energy photon emerges from the laser and goes through the slit. We know that we should associate it with a wave function that has the potential to interfere - unless we're going to deal with the photon in a way that destroys the interference pattern.

The high-energy photon gets through the double slit and gets transformed into a pair of photons. The most general wave function of the high-energy photon is a complex superposition of the states "a" and "b".

The BBO crystal transforms the state "a" into a specific entangled state of two low-energy photons (signal photon and idler) near the "a" slit. In the same way, it transforms "b" into a specific entangled pair of low-energy photons (signal photon and idler) near the "b" slit. One of the two photons in the pair is x-polarized while the other is y-polarized, and Glen-Thomson prism is used to send them in different directions (signal goes up, idler goes down). (The diagram and Wikipedia incorrectly use the term "Glen-Thomson prism" for another prism to be discussed momentarily.)

Linearity of quantum mechanics also tells you what is the state of the two low-energy photons (signal photon and idler) that you get from a general linear superposition of "a" and "b": it's the linear superposition of the corresponding two-photon states, with the same complex coefficients.

Now, we know that the signal photon is going up, attempting to produce something like an interference pattern - or not. We want to measure its exact position on the photographic plate "D0". This signal photon is entangled with the idler that goes down to the prisms, beam-splitters, mirrors, and binary detectors.

Imagine that the signal photon reaches the photographic plate "D0" (well) before the idler gets to the prism. How does the signal photon decide whether it should help to paint an interference pattern or not? It's not yet known whether its partner's "which slit" information will be measured or not, is it? Well, it doesn't matter at all. It's always clear which probabilistic distribution should be used for the first particle. What is the rule?

The wave function is defined as a function of all the measurable variables (a maximum set of commuting observables) of both photons. It is

psi(signal photon properties, idler properties)For each combination of signal photon properties - we will care about its position - and the idler's properties - we will care about the qubits that ultimate determine (probabilistically) which detector out of "D1,D2,D3,D4" it can take - there is a complex number.

So what is the probability that the signal photon is detected at a particular spot? Let's imagine that all the variables are discrete. (If they're continuous, and the signal photon location should be continous, the sum should be replaced by an integral and the probabilities are really probability densities.)

The probability that the signal photon chooses location L is given by

Prob(signal photon location is L) =This formula shouldn't be shocking if you have ever calculated any probability in quantum mechanics. The probability of a micro-outcome in quantum mechanics is given by the squared absolute value of the probability amplitude.

= Sum over basis vectors "i" of the idler's Hilbert space of

|psi(signal photon at L, idler has property "i")|^{2}.

But because we are (and Nature is) only interested in a property of the signal photon right now (She has to urgently decide where the signal photon appears), we must sum the probabilities over all independent vectors of the Hilbert space of all other degrees of freedom - in this case, all basis vectors of the idler photon's Hilbert space.

Now, it is important to realize that it doesn't matter which (orthonormal) basis of the idler photon's Hilbert space we choose. The probability of location L of the signal photon described above is completely independent of the choice of the basis for the idler's Hilbert space! That's a simple consequence of the idler's basis' being orthonormal.

The probability that the signal photon's location is equal to L doesn't depend on any idler photon's degrees of freedom. So the formula above implies that it is a kind of "squared length" of a vector in the idler's Hilbert space - a vector that depends on L - and such a "squared length" is independent of the choice of the basis.

Why am I saying that? For a simple reason. James Gallagher who, together with Mephisto, has provoked me to write this blog entry, posted the following comment 20 seconds ago:

[I] look forward to it [this blog entry], but you should say whether an inteference pattern could be retrieved (by a signal/idler match) even if the eraser is removed before the idler photons hit it.He wants me to say "whether an interference pattern could be retrieved even if the eraser is removed before the idler photons hit it".

(I agree orthodox QM predicts the correct result, but I'm puzzled as to why you think removing the eraser before the idlers hit them is not interesting or puzzling to contemplate)

The formula for the "Probability (signal photon location is L)" written above clearly and simply answers all such questions. The answer is, of course, that

the pattern painted by the set of all the signal photons is totally independent of anything we later do with the idlers.This is a simple consequence of the fact that the wave function for both photons has some values before we do - or we don't do - anything with the idlers. And the probabilities for the signal photon are completely independent of our choice of the idler's basis, because of the simple unitarity rule. And we're summing the probability over all possible idler's outcomes.

In practice - and in this experiment, the interference pattern never exists if you just collect all the signal photons - because they're entangled with the idler photons that we "trace over" which destroys the interference. However, in more general experiments, a weak interference pattern could be predicted even with all the photons.

Obviously, whatever we do with the idlers (and the whole "eraser") has absolutely no impact on the picture that is created by all the signal photons. All the signal photons will create a "diluted" interference pattern in general - which can be interpreted as a juxtaposition of an interference pattern and a non-interference pattern. In this particular experiment, there is no interference pattern visible at all if you just collect all the signal photons.

More generally, the relative representation of the interference pattern vs non-interference blob in the mixture is only determined by the properties of the experiment "before the eraser" - e.g. only by the laser, double slit, and BBO - and in the particular example discussed above, the relative weight of the interference and non-interference patterns in the mixture - in the "diluted" interference pattern - is 50%:50%. However, the interfering "D1,D2" signal photons induce opposite interference pictures that cancel if you just add them up - so that the picture drawn by all the signal photons shows no interference at all.

Obviously, without the eraser - or without its measurement of the idlers (whether they are of the "D1/D2" type or "D3/D4" type), we can't exactly say which particular signal photons were contributing to one of the interference pictures. We can only calculate the probability for each signal photon that it was a participant in the "D1" or "D2" interference pattern project. Only for the signal photons that end up "exactly" in the interference minima, we can be sure that they couldn't have been paired with "D1"-idlers, or that they couldn't have been paired with the "D2"-idlers, because the interference pattern has a vanishing probability for them to land in the minima. Only when the probabilities are 0% or 100%, we can be certain.

James clearly believes that there has to be some "retrocausality" in the predictions of quantum mechanics but there is absolutely no retrocausality. If you decide to ignore any conceivable (future) measurements done with the idler photons, and you only look at the pattern created by the signal photons, this pattern (or any measurements of the signal photons) will be completely independent of any torture that the idler photons may be exposed to! In this particular experiment, there will be no trace of interference on the photographic plate.

This is a very important point but it is not specific to this experiment, either. It could have already appeared in the ordinary EPR entanglement experiment, too. If you only measure the left-moving photon, it will be left-handed and right-handed in 50%:50% of cases, and if you measure its linear polarization, it will also be 50%:50% x-polarized or y-polarized, respectively. Whatever dramatic or complicated torture the other guy decides to do to the right-moving photon has absolutely no impact on the separately measured properties of the left-moving photon. The "other experimenter" has no tool to influence what you measure, not even statistically. He's just correlated with you, the "first experimenter", but his correlation with you doesn't mean his control over your lab.

However, if you measure both photons, you will also find the correlations. In the EPR case, the correlation encoded in the two-photon state will guarantee that the two photons either have the same circular polarization or the opposite linear polarizations - in the cases (in the pairs of measurements) when the same "category" of polarization is measured with both photons.

In the complicated case of the eraser, it's analogous. What will happen in our chronological story of the wave function after we have measured the location of the signal photon? It's simple and you must surely know what's the answer by now.

We have already measured the location of the signal photon to be "L". As rule 3 says, Nature never forgets about any correlations. So all the probabilities for states that contradict the assumption that the signal photon landed at location "L" - which is already a historical fact at this moment - become irrelevant. If we want to predict the fate of the idler photon, we may simply imagine that the wave function has "collapsed" to

psi[AFTER] (signal photon location K, idler basis vector "h") =I have simply multiplied the wave function by the Kronecker delta that is only non-vanishing for locations "K" that are consistent with - e.g. (in this case) equal to - the measured location "L". (The new wave function should be normalized so that its norm is 1 again, but I hope you understand similar trivial points.) The rest of the wave function becomes irrelevant for any further predictions because Nature doesn't forget about any correlations and we already know the location of the signal photon.

psi[BEFORE] (signal location L, idler vector "h")

times delta(K,L)

So after you have measured the signal photon's location, you have a simple wave function that effectively depends on the degrees of freedom "h" of the idler photon only. The whole system is now again reduced to quantum mechanics of a single photon. Whatever you do with the erasers is just choosing a basis of the Hilbert space of the idler photon. A single-particle quantum mechanics predicts the probabilities of different outcomes for the idler photon in the simplest way that we know for one-particle quantum mechanics. The signal photon that has already been fully measured becomes totally inconsequential for the predictions of the idler photon's fate.

It's useful to appreciate the rule 4 - which means that the reduction of the wave function is not real. And it doesn't matter at all when and in which order we "reduce" the wave function. The measurements of the two photons may be spatially separated - like in the EPR experiment - but no real signal has to propagate in between the two photons so there is no violation of relativity whatsoever.

There is one extra subtlety that leads people to incorrectly think that there is some retrocausality in the experiment: the measurements of the idler photons are discrete (D1/D2/D3/D4) while the measurement of the signal photon is continuous (location on the would-be interference pattern). People often think that discrete data can "cause" or "select" continuous pictures and interference patterns, but not the other way around. However, that's a wrong assumption: there is absolutely nothing wrong about "collapsing" the wave function of the signal photon and determining the likelihoods of its idler partner's discrete results out of the reduced wave function.

Quantum mechanics simply predicts the probability for any combination of properties of the two (or more) photons (or any other objects) we decide to measure. A psychological issue could be that some people find it harder to "imagine" a probabilistic distribution for many variables instead of just one variable but if you don't find it hard, you should have no problem with quantum mechanics in these experiments.

There is absolutely no retrocausality or superluminality of the influences in similar experimental setups. Quantum mechanics makes predictions that make complete sense, that are fully self-consistent, causal, local (in the case of quantum field theory) and that agree and will always agree with any experiment with as many slits, prisms, mirrors, beam splitters, photographic plates, detectors, and counters as many you want.

And that's the memo.

## snail feedback (4) :

Hello Lubos,

consider Lab A and Lab B at a distance of 10 lightminutes from each other. Consider furthermore an entangled pair of electrons in the singlet state of their z-axis spin. Assume measurement 1 measures the z-axis spin of electron 1 in Lab A and finds it to be +1/2. The wavefunction of the system collapses accordingly as a result of this measurement. Measurement 2 of the z-axis spin of electron 2 occurs 5 seconds later in Lab B and of course respects the collapsed wavefunction to measure -1/2. All this naturally doesn't violate any physical principles, but isn't it hard to "swallow" how an action 5 seconds ago and 10 lightminutes away (measurement 1) directly affects the measurable probability distribution here and now at lab B?

Regards, Dimitris

PS. I by no way mean to defend any hidden-variables theory or the like - I am perfectly quantum-mechanically minded - but I find it intriguing how the wavefunction manages to instantly collapse everywhere in the universe. (And entanglement is obviously not a precondition for instant collapse, it just makes this effect experimentally measurable ...)

Dear bmk,

let me first correct you in one of your misconception implicitly written in your comment. When two electrons' spins are in the singlet state, they're in the singlet state relatively to *any* axis. This is a trivial consequence of the singlet's being SO(3) invariant as a quantum state.

Such a correlation of a bit "with respect to any direction" may be counterintuitive if we think in terms of classical physics, but this is the first insight of EPR about the entanglement.

"The wavefunction of the system collapses accordingly as a result of this measurement."

The wave functions never "collapse" as they're not "material objects" or "observables". You wanted to say that "After the first measurement, we switch from the overall probabilistic predictions for the second electron to the conditional ones."

"swallow... that ... directly affects the measurable probability distribution"

No, nothing is directly affecting anything else faster than light. The two electrons are just entangled - they're correlated with each other. They're correlated because these two electrons were in contact in the past when they were produces at a single place of space. There's nothing mysterious about finding a perfect correlation between two particles that were created in a perfectly correlated state, namely the singlet state.

The right analogy is Bertelmann's socks: he always takes different colors of 2 socks in the morning so if you see one of them as red, you may be sure that the other won't be read. The only difference in QM is that it allows far richer correlations in many quantities, something that can't be represented by classical correlations assuming that the electrons have some well-defined properties at each point. But once you accept they don't have such properties before the measurement, the correlation of their final properties is fully analogous to correlations in classical physics and may be predicted probabilistically in QM just like in statistical classical physics.

"wavefunction manages to instantly collapse everywhere"

Again, the wave function never collapses. The wave function isn't an actual observable wave. It's a (template for) a probability distribution.

Cheers

LM

Thank you for clarifying the point about the nature of the singlet. Despite any apparent counter-intuitiveness, I perfectly well understand its quantum-mechanical origin. It's a Hilbert space state vector that would deliver the same probability distribution when expanded on any chosen axis.

I will also adopt your wording concerning what happens to probabilistic predictions after a measurement, just to ease your worry that faster-than-light actions are my concern. I am not worried about causality, violations of special relativity or the like.

I understand what a correlation is and the difference between a correlation and a cause/effect-dependency.

However: the difference, as you yourself point out, between Bertelmann's socks and the electron's spin is that the sock is green or red all the time before and after the "measurement", while the electron's spin is perfectly random and assumes its value at the moment of measurement. Metaphorically speaking, the electrons's sock is yellow, until randomly, exactly at the moment of measurement, it becomes green or red.

This is not puzzling me. What puzzles me is that exactly at the moment that one electron's sock, triggered by our measurement, becomes green, the other electron's sock becomes instantly red. The wave function or the state vector is not a physical object, so we don't violate relativity. But the probabilistic distribution is something measurable. It abruptly changes at a specific moment in time (unlike Bertelmann's sock) triggered by a measurement event far away. How does nature achieve this? I am aware of he fact that it doesn't violate any fundamental principles by doing so, and I do not want to disprove or challenge quantum mechanics, since it's predictions are undisputably in accordance with experiment, but how the hell does nature go about to abruptly change the probability distribution of a quantum system everywhere in the universe instantly, when a measurement occurs somewhere in it? How does the second sock become red exactly at the moment we measure the first sock to be green, not a moment earlier and not am moment later? What mechanism does nature employ to achieve this?

This question certainly doesn't affect the predictive or explanatory power of quantum mechanics and is furthermore irrelevant for the practioner. But it does puzzle me.

Regards, Dimitris

"What puzzles me is that exactly at the moment that one electron's sock, triggered by our measurement, becomes green, the other electron's sock becomes instantly red. The wave function or the state vector is not a physical object, so we don't violate relativity. But the probabilistic distribution is something measurable."

It's not true that the electron "became" spinning-up or the sock "became" green when we looked. When we looked, we just learned the answer to a question. The question couldn't have any well-defined answer before the measurement, but this also means that you can't say that the sock "wasn't" green before the measurement or the electron "wasn't" spinning up before the measurement. It had some probabilities for any possibility: the measurement, whether it's in classical statistical physics or quantum physics, just showed the answer.

Also, the wave function isn't measurable in a single measurement. That's why we say (in physics) that a wave function isn't an observable. This statement is true both "technically" as well as "colloquially". Its predicted distributions may only be "measured" by repeating the same experimental situation many times. But when you do so, you can't talk about "events that happened" during a particular copy of an experiment.

According to quantum mechanics, every observable quantity in the world must be encoded in a linear Hermitian operator on the Hilbert space. The wave function isn't one, so it can't be and it isn't an observable.

"It abruptly changes at a specific moment in time (unlike Bertelmann's sock) triggered by a measurement event far away."

Just a few sentences before this one, you agreed that there wasn't any instantaneous effect and that you would avoid this class of invalid statements. So why did you make this invalid statement, anyway? There isn't any physical change of the other sock or electron occurring when you meausure its first cousin.

This fact is true both in the classical sock case as well as the quantum electro case. The only difference is that in classical physics, you may imagine/assume that there exists some "objective answer" to the question about the colors even if you don't look at it. In the quantum case, it's not the case: this very assumption would lead to errors in your predictions. Quantum mechanics implies that there isn't any "certain state" of any objects or degrees of freedom prior to the measurement. Only the probabilities of various observations make a physical sense. But that doesn't mean that something is changing at a distance to "achieve" the correlation. Nothing physical is changing whatsoever. The nonzero probabilities existed for spins "up-down" and "down-up", or "red-green" and "green-red" socks. These options have always been there and they had some probabilities. The observation just means that one of the options was confirmed and became a known fact to the observer. But the observer's learning something doesn't mean that something changed about the socks or the electrons themselves.

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