We Were WRONG About the Quantum Eraser! ft. @LookingGlassUniverse​

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0:02 Does quantum mechanics allow the future to retroactively influence the past,

0:06 as in the infamous delayed choice quantum eraser experiment?

0:10 Well, how about we get an actual quantum physicist

0:14 who many of you already know to show us

0:17 how to do this experiment at home and hopefully

0:21 put the matter to rest once and for all.

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1:13 Richard Feineman said of the double slit experiment that it has in it

1:17 the heart of quantum mechanics in reality it contains the only mystery.

1:23 Well, the mystery he was talking about is superp position.

1:27 In the double slit experiment that means a superp position

1:31 of a single particle traveling through both of the double slits at once.

1:37 The experiment hints that prior to detection at a screen,

1:41 a quantum particle plays out all possible realities

1:44 that may lead to that one measurement result.

1:48 And the shadow of these overlapping realities, these interacting trajectories,

1:52 is cast on the screen in the form of an interference pattern.

1:57 But if you prune the reality tree early, say by measurement at one of the slits,

2:04 the final pattern of particle locations reflects that.

2:07 As if the standard double slit experiment wasn't strange enough,

2:12 then we have the delayed choice quantum eraser

2:15 experiments in which the measurement of the particles

2:19 path to the screen is made after the pattern is recorded on the screen.

2:25 The most famous example is the one by Kim Yu Kulik Shei and Scully in 1999.

2:33 Such experiments have been interpreted as saying that a future

2:38 measurement can influence a past measurement retrocausal influence.

2:43 We did an episode on the delayed choice quantum arrays a long time ago.

2:50 Now, we didn't quite promote the retrocausal interpretation,

2:53 but we certainly failed to give any alternative.

2:57 Well, there are now some really solid

3:00 counter explanations to the idea of retrocausality.

3:03 A few papers have been published which get at aspects of it

3:08 and various YouTube videos also explain some

3:10 of the errors in the retrocausal interpretation.

3:12 So, it's time for us to weigh in.

3:16 Today we're actually going to do a real experiment.

3:19 And by we, I mean Mithina from Looking Glass Universe,

3:24 who's figured out a way to do the quantum eraser in a homemade setup.

3:29 She's going to show you how this experiment

3:33 gives the illusion of information traveling backwards in time.

3:36 I'm really excited to show you this experiment because 12 years ago I did

3:40 a video on the delayed choice quantum eraser where I got it completely wrong.

3:45 And it was only doing this experiment that made it finally click for me.

3:51 The delay choice quantum eraser is just

3:53 a variation on the usual double slit experiment.

3:56 So for that experiment, all you'll need is some light and two very

4:01 narrow beams that you can shine that laser at.

4:04 What do we expect to see?

4:05 Well, it depends on how we think about light.

4:08 On the one hand, we know that light is a wave in the electromagnetic field.

4:12 And like any wave, it should produce interference patterns

4:15 as components of the wave passing through the different slits overlap,

4:20 stacking up or canceling out.

4:22 At the wall, we get the classic double slit interference pattern.

4:26 On the other hand, we also think of light as particles,

4:30 which is fair because the classical electromagnetic wave is

4:33 made up of indivisible quantum particles that we call photons.

4:37 An intuitive but naive picture for particles is as a bunch of tiny balls,

4:42 spray balls at our pair of slits, and they should end up forming two

4:47 piles corresponding to passage through each slit.

4:50 Photons are really some weird in between thing between waves and particles.

4:54 If you fire individual photons, they do look like they make single hits

4:59 on the wall as though they were tiny balls.

5:02 But after many firings, the double slit pattern becomes visible.

5:05 The interpretation is that each photon really passes

5:08 through both slits as a wave and then

5:11 the final destination of the particle-like spot is

5:13 determined by the interference pattern made by that wave.

5:17 In quantum mechanics, we say that the position wave function of the photon

5:22 passes through both slits or that it exists

5:24 in a superp position of both trajectories and in fact

5:28 of all possible trajectories that reach the wall.

5:31 But when the wall measures the photon location,

5:35 that superp position collapses and a single position is chosen.

5:40 Even then, the path the photon took to get there remains undefined.

5:45 If you want to know the path taken, you have to measure it before the wall.

5:48 For example, you could place a little detector at the slits

5:52 that clocks the passing photon without actually stopping it.

5:55 Essentially, we're collapsing the position wave function

5:58 of the slits, not at the wall.

6:00 Now that we know which slit the photon passed through,

6:02 everything we see after that has to be

6:04 consistent with it only passing through that slit,

6:07 which means no double slit interference pattern.

6:09 One common misconception is that if a photon passes through just one slit,

6:15 it lands ultimately just like a classical particle in a single pile of points.

6:20 But actually, single slit interference looks like this.

6:23 It looks a lot like double slit interference,

6:27 but if you compare them side by side, you can see the difference.

6:31 Single slit interference still has dark fringes,

6:34 but just not as many as double slit.

6:36 This is a form of frownhoffer defraction with the edges

6:40 of the slits causing the interference in this case.

6:43 So photons keep behaving like waves even

6:46 if we measure which slit they traversed.

6:48 If you want to understand single slit defraction better,

6:52 Mina did a video explaining why this is and what it means.

6:56 The transition from double to single slit interference occurs

6:59 when we measure which slit the photon passed through.

7:02 We sometimes call this a whichway measurement.

7:04 That's weird enough, but at least in this case,

7:07 we have a sensible causal ordering.

7:09 You make the measurement, it collapses the wave function,

7:13 stopping it from forming a double slit pattern further down the line.

7:17 Things get a lot weirder in the case of the delayed choice quantum eraser.

7:21 In the classic experiment by Kim and collaborators,

7:25 the photons passing through the slits are actually

7:28 cloned using a beta barium borate or BBO crystal

7:32 which converts each incoming photon into a pair

7:36 of near identical outgoing photons that are quantum entangled.

7:40 The key is that both photons in this pair carry the which way information.

7:46 They both know which slit their progenitor photon

7:49 passed through before the BBO crystal duplicated it.

7:53 So now we send one of the new pair,

7:57 call it photon A, to our usual double slit detector to be observed by Alice.

8:02 The other photon B goes to Bob who can extract

8:06 the whichway information or alternatively can choose to erase that info.

8:10 The device is simple enough.

8:11 The path of photon B depends on which slit it came through.

8:15 So we can place a detector on both paths and see which one is triggered.

8:20 We can also choose to scramble

8:22 the whichway information by recombining those two paths.

8:25 So there's no way to tell them apart anymore.

8:28 The bizarre result of the Kim paper and similar experiments

8:32 is that if the whichway information is measured from photon B,

8:37 then photon A acts as though we also measured which slit it came from.

8:44 Alice sees that those photon A's have their whichway

8:49 info measured and fall according to a single slit pattern.

8:53 But if the whichway information in photon B is scrambled,

8:58 Alice sees the corresponding photon A's landing

9:01 according to the classic double slit pattern.

9:04 It acts as though it did pass through both slits.

9:08 It's like we change photon A's behavior without actually touching it,

9:12 only by measuring its twin.

9:13 And if that wasn't weird enough, in the Kim experiment,

9:16 the choice of whether to keep or discard the whichway information in photon

9:21 B is made after its entangled partner photon A reached its detector.

9:26 That makes it seem like photon A landed

9:29 according to a choice that hasn't been made yet.

9:32 And this has been referred to as retrocausal influence,

9:35 and it's what makes the delayed choice quantum eraser so befuddling.

9:39 To explain how we can get around this paradox,

9:43 I'm going to do an analogous experiment that you can do very easily at home.

9:47 And when I say it's analogous,

9:49 what I mean is that it's mathematically equivalent.

9:53 Like if you looked at the maths

9:55 of the original experiment versus this experiment,

9:58 it's exactly the same except for how you label a few things.

10:02 That's not to say though that this experiment is as cool as the other one,

10:07 because the other one involves actual entanglement,

10:10 whereas this uses a little trick to kind

10:14 of mimic entanglement without actually making an entangled photon,

10:18 which I wish I was able to do, but BBO crystals are very expensive.

10:23 So, we've got our laser here, and we've also got a double slit.

10:27 In the original experiment, for every photon A that goes through the slits,

10:32 they make a photon B which knows the slit that photon A took.

10:37 So photon B is what stores the information about which way photon A went.

10:42 So we need in our experiment some other way to store that information.

10:47 Well, we're going to do it in a really simple way.

10:51 Okay, so here's our double slit and we've put some filters in front of it.

10:56 two different filters in front of each of the slits.

10:59 In front of slit one, we have a horizontal polarization filter.

11:03 And in front of slit two, we have a vertical polarization filter.

11:07 Well, now we know that any light

11:10 that makes it through this double slit is marked.

11:13 If it's horizontally polarized, it must have come through slit one.

11:17 Whereas, if it's vertically polarized, it must have come through slit two.

11:21 In other words, the polarization is what's now marking

11:25 whether the light went through slit one or slit two,

11:28 which is convenient for us because we can

11:31 use this crystal to split the light accordingly.

11:35 So, this is called calite.

11:37 It's not as cool as BBO, which is what they used in the original experiment,

11:42 but it does have one very nice property.

11:45 If I put it here at this angle, it splits the light into two.

11:49 These two bits of light represent the two slits of the double slit experiment.

11:55 If the light is vertical, it came from slit one.

11:59 Whereas, if it's horizontal, it came from slit two.

12:03 The light in the top pattern only went through slit one.

12:06 And the light from the bottom pattern only went through slit two.

12:10 So, we'd expect that each of these patterns

12:13 are going to be single slit patterns.

12:15 And if you look closely at them, you can see that they are a double slit

12:20 pattern would have a lot more dark fringes in it.

12:23 So the calsite oriented this way is acting just like Bob's

12:27 which way measurement and collapsing the light into a single slit.

12:32 But if we rotate the calsite by 45°, we'll see why this new measurement is

12:38 now going to look like Bob's arasia measurement,

12:40 and we're going to get the double slit pattern back.

12:43 Okay, so that took me a while, but I managed to rotate this by 45°.

12:47 And I haven't yet put the double slit in there,

12:50 but I just want to show you what rotating the calite actually does.

12:54 So, if you look over there, you can see that there are two dots.

12:59 Well, there are roughly two dots.

13:01 Again, it's a little bit noisy because my cow site here is nowhere near perfect,

13:07 but roughly it's two dots.

13:09 And we're going to give the two dots names.

13:12 So, we're going to call the top one plus

13:14 and we're going to call the bottom one minus.

13:17 And it's annoying, but it's an inevitability of this geometry

13:20 that they're not aligned with each other anymore.

13:22 So, they're not one on top of each other.

13:25 So, with the magic of post-prouction,

13:27 we'll stack these so that they are one on top of each other.

13:30 That's going to make everything a little bit easier in a second.

13:33 To figure out why calsite oriented like

13:35 this is going to be an erasia measurement.

13:38 Let's see what happens when we put

13:42 in vertically oriented light into this calite.

13:45 You can see that the total amount

13:48 of light decreases but both dots are still there.

13:51 It's not like last time where horizontal light all goes

13:55 to the same dot and vertical light all goes to the other dot.

13:59 With this plus and minus measurement,

14:01 half of the vertical light will go to plus and half will go to minus.

14:06 And same for horizontal.

14:07 But then what does that tell us about light

14:10 that's going to go through this marked double slit?

14:13 So light that goes through the first slit is horizontally polarized

14:17 whereas light that goes through the second slit is vertically polarized.

14:21 And previously our calite was letting us measure the polarization

14:25 and therefore measure whether it went through slit one or slit two.

14:30 But now think about what this plus minus capsite is doing.

14:35 It's going to take light from the first slit and it's going

14:39 to have an equal probability of ending up at plus or at minus.

14:43 And same for the second slit.

14:45 So now just looking at the light after it goes through the calite

14:49 isn't enough to tell us anything at all about which way the light went.

14:54 In fact, it scramles up the light from both paths.

14:58 So it basically erases all of that information

15:01 that we had gotten by marking the paths.

15:04 So what do we expect to happen then?

15:08 Well, let's find out.

15:09 Oh man, that looks so good.

15:12 Okay.

15:13 Um, that kind of worked out better than I expected.

15:18 Um, okay.

15:19 So, have a look and hopefully you can see two double slit patterns.

15:26 So, if we use the calite in its previous orientation,

15:30 then we're using it to measure which way the light went

15:35 and that collapses the light into going into single slit interference.

15:38 But if we use the calite in this orientation,

15:42 the plus minus orientation, we're using it as an eraser.

15:46 It erases the information about which slit the light went through.

15:50 And so we're back to the double slit.

15:52 If you look really closely at these two double slits,

15:56 you'll see that they don't exactly line up.

15:59 And in fact, they seem to be offset from each

16:03 other the exact right amount that if you superimpose them,

16:07 it looks like the single slit interference pattern.

16:10 So what's going on?

16:12 Well, it turns out that this is the solution to the entire paradox.

16:17 Let's go back to the original setup to see why.

16:20 In the Kim experiment, photons are cloned by the BBO crystal and the whichway

16:25 analysis is done with only one of those.

16:28 We called it photon B.

16:31 Just as our experiment sorts the photons into two interference patterns,

16:35 the original experiment sorts the B photons into two bins.

16:40 For the measure case,

16:42 those bins tell you which slit the corresponding photon A also went through.

16:47 For the eraser case, there are still two bins,

16:50 but there is no whichway information in how photons are sorted into them.

16:54 Alice never sees a double slit

16:57 interference pattern until something very specific occurs.

17:00 At first, she sees just a blob of positions for all photon A's.

17:05 Then Bob comes and tells her which of those had a twin photon

17:10 B for which the way info was measured and for which it was erased.

17:15 The measured cases are still a blob.

17:18 Basically a blurry single slit interference.

17:20 Now, the sum of all of the cases in which the whichway

17:25 info was erased are actually a very similar mess, a blob.

17:30 So, Bob also needs to tell Alice which of those erased

17:34 photon B's ended up in each of the separate eraser detectors.

17:39 Only then can Alice disentangle the featureless blob

17:42 of photon A positions and see interference bands.

17:46 and her patterns look a lot like masers,

17:49 clear peaks and valleys that are slightly offset.

17:53 And when you merge them, you get a featureless pile.

17:57 This is a critical point that's often

18:00 glossed over in descriptions of the quantum eraser.

18:03 So to recap, the solution is Alice always sees the single slip pattern,

18:07 and that's regardless of what Bob chooses to do on his side.

18:12 But a single slip pattern is really just

18:14 two double slip patterns on top of each other.

18:17 The plus version and the minus version that's slightly offset.

18:21 Now, if Bob does happen to do the eraser measurement,

18:25 then he and Alice can reconvene afterwards and sort Alice's photons

18:30 into the plus and minus groups and they find those two double slit patterns.

18:35 In our experiment, we did this sorting via calite,

18:39 but in the real version of the experiment,

18:42 it just looks like post-processing the data to make

18:45 the single slit interference pattern into two double slit ones.

18:49 So instead of Bob's arasia measurement instantly uncolapsing the wave

18:53 function and turning the single slit interference into the double slit,

18:58 you only see this effect of his measurement much

19:01 later once he and Alice sift through the data.

19:05 The story told by the retrocausal interpretation of the delayed choice quantum

19:10 araser is that Bob's choice of measurement

19:14 retroactively influences where Alice's photon lands.

19:16 But the same logic works the other way around.

19:20 The position of photon A measured by Alice

19:23 absolutely influences where photon B can go, where it's measured by Bob.

19:29 Just for reference, I've drawn the plus double slit pattern here and the minus

19:34 one here just so that we can kind of see where they are.

19:39 And then let's say that Alice measures

19:42 her photon and it happens to land right here.

19:47 This lines up perfectly with the minus double slip pattern,

19:52 but not at all with the plus one.

19:56 Now after that Bob measures his photon B

20:00 and he chooses to do an erasia measurement.

20:03 So that means that there are two possible outcomes.

20:06 He could get a plus or he could get a minus.

20:09 But could he get a plus?

20:11 I mean that wouldn't be consistent with the result that Alice has already got.

20:16 And so actually Alice's result forces Bob's result to also be minus.

20:22 I should add that Alice's measurement of the position

20:25 of photon A only affects one thing for Bob,

20:28 and that's which of the eraser detectors Photon B will land

20:32 in, assuming Photon B makes it all the way to the eraser section.

20:37 But then one of the eraser detectors will strongly favor photons

20:43 whose twin landed in one set of bands on Alice's screen,

20:48 while the other favors the complimentary set of bands.

20:52 So, we've had this causality thing the wrong way around this whole time.

20:56 We thought that it was Bob's measurement that forced

20:59 Alice's photon to act in a particular way.

21:02 But now we can see it's the other way around.

21:06 If Alice's measurement happens first, then her outcome is what determines what

21:11 happens when Bob does his arasia measurement.

21:14 In other words, Alice's outcome is what forces

21:18 Bob's to happen in the way it does.

21:21 But that's only the case if Alice's measurement happens first and then Bob's.

21:26 But here's the twist.

21:28 Does Alice's measurement always happen first?

21:32 Or does that depend on your frame of reference?

21:36 It's possible to set up this experiment so that Bob's detections actually

21:40 happen before Alice's just by changing the path lengths for each section.

21:46 If we do that, we expect exactly the same result.

21:50 The same interference fringes are revealed when photon A is

21:54 filtered according to where photon B lands in the eraser.

21:58 In that case, it seems that B caused

22:01 A, but with the same result as when A caused B.

22:05 This makes it harder to assign a causal power to either case.

22:08 It's also hard to even imagine a causal path connecting Alice and Bob's

22:14 observations beyond the sort of spooky action

22:17 at a distance that Einstein so hated.

22:20 That said, there is a way to explain the causation that connects

22:25 Alice's and Bob's observations that doesn't need to assign a causal direction.

22:30 It all depends on what mysterious property really determines

22:33 how our photons get sorted into the two detectors

22:36 in the quantum erasor or how they are sorted into the two interference patterns

22:41 by the calcite crystal in Mithan's experiment.

22:45 What property is that?

22:47 Well, you're going to have to wait for an upcoming episode for that one.

22:51 But in the meantime, go check out Mith's channel for quantum

22:56 deep dives and more DIY quantum experiments.

22:59 It's looking glass universe, and it's amazing.

23:02 I also had a long and really fun chat with Mna about quantum mysteries,

23:07 her work, and a lot more.

23:09 It's in our new series of space-time conversations

23:12 that you can find in our community tab.

23:15 the delayed choice quantum eraser.

23:17 It's weird, but no weirder than any double slit experiment.

23:21 And in case you don't believe me yet,

23:24 we'll finalize the debunking in an upcoming Spacetime.

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