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Does quantum mechanics allow the future to retroactively influence the past, as in the infamous delayed choice quantum eraser experiment?

Well, how about we get an actual quantum physicist who many of you already know to show us how to do this experiment at home and hopefully put the matter to rest once and for all.

Richard Feynman said of the double slit experiment that it has in it the heart of quantum mechanics in reality it contains the only mystery.

Well, the mystery he was talking about is superposition.

In the double slit experiment that means a superposition of a single particle traveling through both of the double slits at once.

The experiment hints that prior to detection at a screen, a quantum particle plays out all possible realities that may lead to that one measurement result.

And the shadow of these overlapping realities, these interacting trajectories, is cast on the screen in the form of an interference pattern.

But if you prune the reality tree early, say by measurement at one of the slits, the final pattern of particle locations reflects that.

As if the standard double slit experiment wasn't strange enough, then we have the delayed choice quantum eraser experiments in which the measurement of the particles path to the screen is made after the pattern is recorded on the screen.

The most famous example is the one by Kim Yu Kulik Shei and Scully in 1999.

Such experiments have been interpreted as saying that a future measurement can influence a past measurement retrocausal influence.

We did an episode on the delayed choice quantum eraser a long time ago.

Now, we didn't quite promote the retrocausal interpretation, but we certainly failed to give any alternative.

Well, there are now some really solid counter explanations to the idea of retrocausality.

A few papers have been published which get at aspects of it and various YouTube videos also explain some of the errors in the retrocausal interpretation.

So, it's time for us to weigh in.

Today we're actually going to do a real experiment.

And by we, I mean Mithina from Looking Glass Universe, who's figured out a way to do the quantum eraser in a homemade setup.

She's going to show you how this experiment gives the illusion of information traveling backwards in time.

I'm really excited to show you this experiment because 12 years ago I did a video on the delayed choice quantum eraser where I got it completely wrong.

And it was only doing this experiment that made it finally click for me.

The delay choice quantum eraser is just a variation on the usual double slit experiment.

So for that experiment, all you'll need is some light and two very narrow beams that you can shine that laser at.

What do we expect to see?

Well, it depends on how we think about light.

On the one hand, we know that light is a wave in the electromagnetic field.

And like any wave, it should produce interference patterns as components of the wave passing through the different slits overlap, stacking up or canceling out.

At the wall, we get the classic double slit interference pattern.

On the other hand, we also think of light as particles, which is fair because the classical electromagnetic wave is made up of indivisible quantum particles that we call photons.

An intuitive but naive picture for particles is as a bunch of tiny balls, spray balls at our pair of slits, and they should end up forming two piles corresponding to passage through each slit.

Photons are really some weird in between thing between waves and particles.

If you fire individual photons, they do look like they make single hits on the wall as though they were tiny balls.

But after many firings, the double slit pattern becomes visible.

The interpretation is that each photon really passes through both slits as a wave and then the final destination of the particle-like spot is determined by the interference pattern made by that wave.

In quantum mechanics, we say that the position wave function of the photon passes through both slits or that it exists in a superposition of both trajectories and in fact of all possible trajectories that reach the wall.

But when the wall measures the photon location, that superposition collapses and a single position is chosen.

Even then, the path the photon took to get there remains undefined.

If you want to know the path taken, you have to measure it before the wall.

For example, you could place a little detector at the slits that clocks the passing photon without actually stopping it.

Essentially, we're collapsing the position wave function of the slits, not at the wall.

Now that we know which slit the photon passed through, everything we see after that has to be consistent with it only passing through that slit, which means no double slit interference pattern.

One common misconception is that if a photon passes through just one slit, it lands ultimately just like a classical particle in a single pile of points.

But actually, single slit interference looks like this.

It looks a lot like double slit interference, but if you compare them side by side, you can see the difference.

Single slit interference still has dark fringes, but just not as many as double slit.

This is a form of frownhoffer defraction with the edges of the slits causing the interference in this case.

So photons keep behaving like waves even if we measure which slit they traversed.

If you want to understand single slit defraction better, Mina did a video explaining why this is and what it means.

The transition from double to single slit interference occurs when we measure which slit the photon passed through.

We sometimes call this a whichway measurement.

That's weird enough, but at least in this case, we have a sensible causal ordering.

You make the measurement, it collapses the wave function, stopping it from forming a double slit pattern further down the line.

Things get a lot weirder in the case of the delayed choice quantum eraser.

In the classic experiment by Kim and collaborators, the photons passing through the slits are actually cloned using a beta barium borate or BBO crystal which converts each incoming photon into a pair of near identical outgoing photons that are quantum entangled.

The key is that both photons in this pair carry the which way information.

They both know which slit their progenitor photon passed through before the BBO crystal duplicated it.

So now we send one of the new pair, call it photon A, to our usual double slit detector to be observed by Alice.

The other photon B goes to Bob who can extract the whichway information or alternatively can choose to erase that info.

The device is simple enough.

The path of photon B depends on which slit it came through.

So we can place a detector on both paths and see which one is triggered.

We can also choose to scramble the whichway information by recombining those two paths.

So there's no way to tell them apart anymore.

The bizarre result of the Kim paper and similar experiments is that if the whichway information is measured from photon B, then photon A acts as though we also measured which slit it came from.

Alice sees that those photon A's have their whichway info measured and fall according to a single slit pattern.

But if the whichway information in photon B is scrambled, Alice sees the corresponding photon A's landing according to the classic double slit pattern.

It acts as though it did pass through both slits.

It's like we change photon A's behavior without actually touching it, only by measuring its twin.

And if that wasn't weird enough, in the Kim experiment, the choice of whether to keep or discard the whichway information in photon B is made after its entangled partner photon A reached its detector.

That makes it seem like photon A landed according to a choice that hasn't been made yet.

And this has been referred to as retrocausal influence, and it's what makes the delayed choice quantum eraser so befuddling.

To explain how we can get around this paradox, I'm going to do an analogous experiment that you can do very easily at home.

And when I say it's analogous, what I mean is that it's mathematically equivalent.

Like if you looked at the maths of the original experiment versus this experiment, it's exactly the same except for how you label a few things.

That's not to say though that this experiment is as cool as the other one, because the other one involves actual entanglement, whereas this uses a little trick to kind of mimic entanglement without actually making an entangled photon, which I wish I was able to do, but BBO crystals are very expensive.

So, we've got our laser here, and we've also got a double slit.

In the original experiment, for every photon A that goes through the slits, they make a photon B which knows the slit that photon A took.

So photon B is what stores the information about which way photon A went.

So we need in our experiment some other way to store that information.

Well, we're going to do it in a really simple way.

Okay, so here's our double slit and we've put some filters in front of it.

Two different filters in front of each of the slits.

In front of slit one, we have a horizontal polarization filter.

And in front of slit two, we have a vertical polarization filter.

Well, now we know that any light that makes it through this double slit is marked.

If it's horizontally polarized, it must have come through slit one.

Whereas, if it's vertically polarized, it must have come through slit two.

In other words, the polarization is what's now marking whether the light went through slit one or slit two, which is convenient for us because we can use this crystal to split the light accordingly.

So, this is called calite.

It's not as cool as BBO, which is what they used in the original experiment, but it does have one very nice property.

If I put it here at this angle, it splits the light into two.

These two bits of light represent the two slits of the double slit experiment.

If the light is vertical, it came from slit one.

Whereas, if it's horizontal, it came from slit two.

The light in the top pattern only went through slit one.

And the light from the bottom pattern only went through slit two.

So, we'd expect that each of these patterns are going to be single slit patterns.

And if you look closely at them, you can see that they are a double slit pattern would have a lot more dark fringes in it.

So the calsite oriented this way is acting just like Bob's which way measurement and collapsing the light into a single slit.

But if we rotate the calsite by 45, we'll see why this new measurement is now going to look like Bob's erasure measurement, and we're going to get the double slit pattern back.

Okay, so that took me a while, but I managed to rotate this by 45.

And I haven't yet put the double slit in there, but I just want to show you what rotating the calite actually does.

So, if you look over there, you can see that there are two dots.

Well, there are roughly two dots.

Again, it's a little bit noisy because my cow site here is nowhere near perfect, but roughly it's two dots.

And we're going to give the two dots names.

So, we're going to call the top one plus and we're going to call the bottom one minus.

And it's annoying, but it's an inevitability of this geometry that they're not aligned with each other anymore.

So, they're not one on top of each other.

So, with the magic of post-prouction, we'll stack these so that they are one on top of each other.

That's going to make everything a little bit easier in a second.

To figure out why calcite oriented like this is going to be an erasure measurement.

Let's see what happens when we put in vertically oriented light into this calite.

You can see that the total amount of light decreases but both dots are still there.

It's not like last time where horizontal light all goes to the same dot and vertical light all goes to the other dot.

With this plus and minus measurement, half of the vertical light will go to plus and half will go to minus.

And same for horizontal.

But then what does that tell us about light that's going to go through this marked double slit?

So light that goes through the first slit is horizontally polarized whereas light that goes through the second slit is vertically polarized.

And previously our calite was letting us measure the polarization and therefore measure whether it went through slit one or slit two.

But now think about what this plus minus calcite is doing.

It's going to take light from the first slit and it's going to have an equal probability of ending up at plus or at minus.

And same for the second slit.

So now just looking at the light after it goes through the calcite isn't enough to tell us anything at all about which way the light went.

In fact, it scrambles up the light from both paths.

So it basically erases all of that information that we had gotten by marking the paths.

So what do we expect to happen then?

Well, let's find out.

Oh man, that looks so good.

Okay.

Um, that kind of worked out better than I expected.

Um, okay.

So, have a look and hopefully you can see two double slit patterns.

So, if we use the calite in its previous orientation, then we're using it to measure which way the light went and that collapses the light into going into single slit interference.

But if we use the calite in this orientation, the plus minus orientation, we're using it as an eraser.

It erases the information about which slit the light went through.

And so we're back to the double slit.

If you look really closely at these two double slits, you'll see that they don't exactly line up.

And in fact, they seem to be offset from each other the exact right amount that if you superimpose them, it looks like the single slit interference pattern.

So what's going on?

Well, it turns out that this is the solution to the entire paradox.

Let's go back to the original setup to see why.

In the Kim experiment, photons are cloned by the BBO crystal and the whichway analysis is done with only one of those.

We called it photon B. Just as our experiment sorts the photons into two interference patterns, the original experiment sorts the B photons into two bins.

For the measure case, those bins tell you which slit the corresponding photon A also went through.

For the eraser case, there are still two bins, but there is no whichway information in how photons are sorted into them.

Alice never sees a double slit interference pattern until something very specific occurs.

At first, she sees just a blob of positions for all photon A's.

Then Bob comes and tells her which of those had a twin photon B for which the way info was measured and for which it was erased.

The measured cases are still a blob.

Basically a blurry single slit interference.

Now, the sum of all of the cases in which the whichway info was erased are actually a very similar mess, a blob.

So, Bob also needs to tell Alice which of those erased photon B's ended up in each of the separate eraser detectors.

Only then can Alice disentangle the featureless blob of photon A positions and see interference bands.

And her patterns look a lot like Mithuna's, clear peaks and valleys that are slightly offset.

And when you merge them, you get a featureless pile.

This is a critical point that's often glossed over in descriptions of the quantum eraser.

So to recap, the solution is Alice always sees the single slip pattern, and that's regardless of what Bob chooses to do on his side.

But a single slip pattern is really just two double slip patterns on top of each other.

The plus version and the minus version that's slightly offset.

Now, if Bob does happen to do the eraser measurement, then he and Alice can reconvene afterwards and sort Alice's photons into the plus and minus groups and they find those two double slit patterns.

In our experiment, we did this sorting via calite, but in the real version of the experiment, it just looks like post-processing the data to make the single slit interference pattern into two double slit ones.

So instead of Bob's erasure measurement instantly uncollapsing the wave function and turning the single slit interference into the double slit, you only see this effect of his measurement much later once he and Alice sift through the data.

The story told by the retrocausal interpretation of the delayed choice quantum eraser is that Bob's choice of measurement retroactively influences where Alice's photon lands.

But the same logic works the other way around.

The position of photon A measured by Alice absolutely influences where photon B can go, where it's measured by Bob.

Just for reference, I've drawn the plus double slit pattern here and the minus one here just so that we can kind of see where they are.

And then let's say that Alice measures her photon and it happens to land right here.

This lines up perfectly with the minus double slip pattern, but not at all with the plus one.

Now after that Bob measures his photon B and he chooses to do an erasure measurement.

So that means that there are two possible outcomes.

He could get a plus or he could get a minus.

But could he get a plus?

I mean that wouldn't be consistent with the result that Alice has already got.

And so actually Alice's result forces Bob's result to also be minus.

I should add that Alice's measurement of the position of photon A only affects one thing for Bob, and that's which of the eraser detectors Photon B will land in, assuming Photon B makes it all the way to the eraser section.

But then one of the eraser detectors will strongly favor photons whose twin landed in one set of bands on Alice's screen, while the other favors the complimentary set of bands.

So, we've had this causality thing the wrong way around this whole time.

We thought that it was Bob's measurement that forced Alice's photon to act in a particular way.

But now we can see it's the other way around.

If Alice's measurement happens first, then her outcome is what determines what happens when Bob does his erasure measurement.

In other words, Alice's outcome is what forces Bob's to happen in the way it does.

But that's only the case if Alice's measurement happens first and then Bob's.

But here's the twist.

Does Alice's measurement always happen first?

Or does that depend on your frame of reference?

It's possible to set up this experiment so that Bob's detections actually happen before Alice's just by changing the path lengths for each section.

If we do that, we expect exactly the same result.

The same interference fringes are revealed when photon A is filtered according to where photon B lands in the eraser.

In that case, it seems that B caused A, but with the same result as when A caused B. This makes it harder to assign a causal power to either case.

It's also hard to even imagine a causal path connecting Alice and Bob's observations beyond the sort of spooky action at a distance that Einstein so hated.

That said, there is a way to explain the causation that connects Alice's and Bob's observations that doesn't need to assign a causal direction.

It all depends on what mysterious property really determines how our photons get sorted into the two detectors in the quantum eraser or how they are sorted into the two interference patterns by the calcite crystal in Mithuna's experiment.

What property is that?

Well, you're going to have to wait for an upcoming episode for that one.

But in the meantime, go check out Mithuna's channel for quantum deep dives and more DIY quantum experiments.

It's looking glass universe, and it's amazing.

I also had a long and really fun chat with Mithuna about quantum mysteries, her work, and a lot more.

It's in our new series of space-time conversations that you can find in our community tab.

The delayed choice quantum eraser.

It's weird, but no weirder than any double slit experiment.

And in case you don't believe me yet, we'll finalize the debunking in an upcoming spacetime.