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What is a quantum eraser? · Daniel and Kelly’s Extraordinary Universe

0:01
Speaker 1: Hey, Jorhan, Daniel here, and we want to tell you
0:03
Speaker 1: about our new book. It's called Frequently Asked Questions about
0:07
Speaker 1: the Universe because you have questions about the universe, and
0:09
Speaker 1: so we decided to write a book all about them.
0:12
Speaker 1: We talk about your questions, we give some answers, we
0:14
Speaker 1: make a bunch of silly jokes as usual, and we
0:17
Speaker 1: tackle all kinds of questions, including what happens if I
0:20
Speaker 1: fall into a black hole? Or is there another version
0:22
Speaker 1: of you out there that's right? Like usual, we tackle
0:25
Speaker 1: the deepest, darkest, biggest, craziest questions about this incredible cosmos.
0:29
Speaker 1: If you want to support the podcast, please get the
0:31
Speaker 1: book and get a copy, not just for yourself, but
0:34
Speaker 1: you know, for your nieces and nephews, cousins, friends, parents, dogs, hamsters,
0:39
Speaker 1: and for the aliens. So get your copy of Frequently
0:42
Speaker 1: Asked Questions about the Universe is available for pre order now,
0:46
Speaker 1: coming out November two. You can find more details at
0:49
Speaker 1: the book's website, Universe f a Q dot com. Thanks
0:53
Speaker 1: for your support, and if you have a hamster that
0:55
Speaker 1: can read, please let us know. We'd love to have
0:57
Speaker 1: them on the podcast. Daniel, do you ever feel like
1:08
Speaker 1: you really understand quantum mechanics. No, you know, I think
1:11
Speaker 1: it's probably just too alien for us to really ever
1:14
Speaker 1: feel comfortable with. I guess it's too bad there aren't
1:17
Speaker 1: any macroscopic big quantum optics we can really like poke
1:21
Speaker 1: and play with, I know, like a big fat electron.
1:23
Speaker 1: But actually I'm working on a theory that children are
1:26
Speaker 1: governed by the rules of quantum mechanics. Oh really, like
1:29
Speaker 1: there's uncertainty about where they are. Have you lost your kids? No,
1:33
Speaker 1: but I've noticed that you can't like observe your children
1:36
Speaker 1: without sort of perturbing the system, right, I think I
1:39
Speaker 1: know what you mean. Like they won't do their homework
1:41
Speaker 1: unless you're there watching exactly. And when I walk into
1:44
Speaker 1: the room somehow all their conversations collapse suddenly into silence. Yeah,
1:48
Speaker 1: it's like sure the anger as children, they're both excited
1:51
Speaker 1: and sad to see you. Hi am for Handmad cartoonists
2:08
Speaker 1: and the creator of PhD comics. Hi. I'm Daniel. I'm
2:11
Speaker 1: a particle physicist and a professor UC Irvine, and I'm
2:14
Speaker 1: always in many quantum states at once. Really, so that
2:17
Speaker 1: does that mean you're not real? It means I don't
2:20
Speaker 1: even know if I'm real man. Well, I hope you are,
2:23
Speaker 1: because that would mean that I'm talking to myself right now,
2:25
Speaker 1: and that would be a little concerning. Maybe you are
2:28
Speaker 1: the only brain in the universe and the rest of
2:30
Speaker 1: the universe is just part of your mind. That would
2:32
Speaker 1: make a lot of sense why I'm so successful and
2:35
Speaker 1: good looking. But anyways, welcome to our podcast. Daniel and
2:38
Speaker 1: Jorge explain the university production of I Heart Radio, in
2:41
Speaker 1: which we do try to explode our minds out to
2:44
Speaker 1: capture the entire universe. We want to take this vast, glittering, crazy, violent,
2:49
Speaker 1: wild and white cosmos and wrap it all up inside
2:53
Speaker 1: our brains. It's not enough for us to just live
2:55
Speaker 1: in this universe to experience it and to see it.
2:58
Speaker 1: We want to understand it. We want download the whole
3:01
Speaker 1: thing into our minds, and that means understanding the basic
3:05
Speaker 1: rules about how it works, what's really going on with
3:08
Speaker 1: tiny little particles or whatever is happening at the smallest scale.
3:13
Speaker 1: On this podcast, we dive the deepest you can into
3:16
Speaker 1: the hardest and trickiest of questions and we try to
3:19
Speaker 1: explain all of them to you. Yeah, because it is
3:21
Speaker 1: a pretty tricky universe. It's full of interesting rules and
3:24
Speaker 1: interesting phenomena that happens at the smallest of levels and
3:27
Speaker 1: at the largest of scales, and a lot of it
3:30
Speaker 1: is understandable, even if it's not very um intuitive to
3:34
Speaker 1: think about. That's right. And I look at the universe
3:36
Speaker 1: like a big puzzle. It's like a detective novel or
3:39
Speaker 1: a murder mystery, and I want to figure out who
3:41
Speaker 1: did it. I want to understand how it works. It's
3:44
Speaker 1: amazing to me, sort of philosophically, that the universe is
3:47
Speaker 1: presented to us that way, like this big puzzle that
3:49
Speaker 1: isn't obvious to figure out, but yet can somehow be
3:52
Speaker 1: understood if you push hard enough. Do you think the
3:54
Speaker 1: universe is understandable? Daniel? That's a big question in physics,
3:57
Speaker 1: isn't it. It's a big question in the philosophy of
3:59
Speaker 1: physics x and And my answer is it doesn't make
4:02
Speaker 1: sense for it to be understandable, Like how could it
4:05
Speaker 1: be possible that the complexities of this maybe infinite universe
4:09
Speaker 1: could be stored in the minds of a human. On
4:11
Speaker 1: the other hand, we have all these theories that work
4:13
Speaker 1: really well, like surprisingly well, and so I don't know
4:17
Speaker 1: how to hold those two ideas. In my mind, they're
4:19
Speaker 1: in quantum conflict. You're both confused and feeling smart at
4:23
Speaker 1: the same time. That's what this podcast is all about.
4:25
Speaker 1: I'm the confused quantum state and you are the feeling smart.
4:28
Speaker 1: That's right. And all scientists sort of hold those two
4:30
Speaker 1: feelings in their mind at once, like, look at all
4:33
Speaker 1: we have understood, and yet look at all that we
4:36
Speaker 1: do not, And that's both exciting and terrifying. Yeah, and
4:39
Speaker 1: we have a lot of questions about the universe. And
4:42
Speaker 1: by the way, speaking of questions, we have a new
4:44
Speaker 1: book coming out pretty soon in on November two. That's right.
4:48
Speaker 1: Jorge and I celebrate asking questions about the universe, and
4:51
Speaker 1: we love thinking about these questions. We love hearing about
4:54
Speaker 1: your questions about the universe. So we wrote a book
4:56
Speaker 1: that wraps up the most frequently asked questions that we
5:00
Speaker 1: get about the universe. Yeah. So, if you like this
5:02
Speaker 1: podcast and you want to support us, please check it out.
5:05
Speaker 1: It's called Frequently Ask Questions about the Universe and it's
5:08
Speaker 1: on a pre order right now. You can order it
5:10
Speaker 1: now and get it as soon as it comes out.
5:12
Speaker 1: And I think you know what's important is that we
5:14
Speaker 1: kind of didn't quite write it for our listeners, right, Daniel.
5:17
Speaker 1: We kind of wrote it for the people that know
5:19
Speaker 1: our listeners. You know, if you ever have like a
5:21
Speaker 1: nephew or a cousin or an uncle who you want
5:23
Speaker 1: to share this amazing information about the universe, I think
5:26
Speaker 1: this is the book for you or for them, that's right.
5:28
Speaker 1: So every one of you out there, you should buy
5:30
Speaker 1: five copies and give them to all your friends and
5:32
Speaker 1: family so that they can understand the answers to questions
5:35
Speaker 1: like where did the universe come from? Or how can
5:39
Speaker 1: we travel to the stars. Yeah, we tackle all kinds
5:41
Speaker 1: of pretty cool questions in it, and we try to
5:43
Speaker 1: answer it for people like your relatives and friends with
5:47
Speaker 1: all kinds of interesting and clear answers. And also cartoons,
5:50
Speaker 1: which is I think something you don't see in every
5:53
Speaker 1: day in physics books. That's right. All these awesome fun
5:55
Speaker 1: drawings that help clarify the topic and amuse you along
5:59
Speaker 1: the way. That or added to this book. So if
6:02
Speaker 1: you like this blend of physics and silly jokes, then
6:05
Speaker 1: I think you'll enjoy this book. So go out and
6:07
Speaker 1: get your copy. You can find it at universe f
6:10
Speaker 1: a Q dot com all right, Well, speaking of questions,
6:13
Speaker 1: we are tackling a question today and it has something
6:15
Speaker 1: to do with quantum mechanics, which is, I guess, for
6:19
Speaker 1: lack of a better technical term, bonkers. It is my
6:24
Speaker 1: favorite kind of bonkers. It's the kind of bonkers that
6:27
Speaker 1: doesn't make sense to your mind. But the math works
6:30
Speaker 1: perfectly and it keeps predicting absurd experimental conclusions that experimentalists
6:36
Speaker 1: keep verifying. Yeah, because I guess you started with a
6:39
Speaker 1: little nugget of an experiment, and then you worked out
6:41
Speaker 1: some math, and then you find out that the crazy
6:44
Speaker 1: math that it suggests is actually also true. Yeah, we
6:46
Speaker 1: had to change the basic concept of what we thought
6:49
Speaker 1: was going on at the very heart of the universe,
6:52
Speaker 1: at the core of everything that's around me and you.
6:55
Speaker 1: Even though it seems intuitive and like it follows rules
6:58
Speaker 1: that we're familiar with from growing up, it turn turns
7:00
Speaker 1: out that the tiniest little parts inside are following totally
7:02
Speaker 1: different rules, which mean that the nature of reality is
7:06
Speaker 1: quite different from the one that we thought it was.
7:09
Speaker 1: And people work that out and they thought that's crazy.
7:11
Speaker 1: And if it's true, it would mean you could do
7:14
Speaker 1: this bonkers experiment, which would have this nonsense result, so
7:18
Speaker 1: obviously it can't be true. And then business went out
7:20
Speaker 1: and did the experiment and got the nonsense results, which
7:23
Speaker 1: turns out to be the truth of our reality. Yeah,
7:26
Speaker 1: because I guess at the core of it, it's kind
7:27
Speaker 1: of weird for us humans to think of things that
7:29
Speaker 1: are like two things at the same time, right, I mean,
7:32
Speaker 1: not just in a conceptual ot but like actually in reality,
7:35
Speaker 1: in quantum objects, things can be multiple things at the
7:38
Speaker 1: same time. That's right, And that phrase in reality is
7:41
Speaker 1: the key there, because we imagine at the very basic
7:44
Speaker 1: level that there is a reality out there, that there's
7:47
Speaker 1: a truth that even if we're not looking, the universe
7:50
Speaker 1: is there and it's operating and it's following some rules
7:53
Speaker 1: and it doesn't really matter if we are looking or not.
7:56
Speaker 1: But the reality suggests that the universe is quite different
7:59
Speaker 1: from that, that it does matter if you interact with it,
8:03
Speaker 1: and that what is happening in the universe is not
8:05
Speaker 1: exactly well determined until you interact with it. Yeah, you
8:09
Speaker 1: might say that in reality, the way the world works
8:11
Speaker 1: and the universe works, it's kind of fuzzy, kind of
8:14
Speaker 1: not quite as solid as we might think it is.
8:16
Speaker 1: From our everyday lives. That's right. We have a weird
8:19
Speaker 1: and particular view of this quantum universe. We are only
8:22
Speaker 1: used to interacting with enormous quantum objects like baseballs and
8:26
Speaker 1: rocks and trees, which are quantum objects. But they contain
8:29
Speaker 1: like ten to the twenty six quantum objects. And when
8:33
Speaker 1: you have that many, they do things differently than when
8:35
Speaker 1: you have one or two of them isolated to reveal
8:38
Speaker 1: their sort of true fundamental nature. So today we're gonna
8:41
Speaker 1: be talking about some really crazy experiments that try to
8:44
Speaker 1: reveal exactly what the rules are of how these particles
8:48
Speaker 1: work at the smallest scale when they're left alone. Yeah,
8:50
Speaker 1: and so it turns out that also reality is not
8:53
Speaker 1: just a little bit fuzzy, but it may not even
8:56
Speaker 1: be as permanent as we think it is. Quantum in
8:59
Speaker 1: for meation and quantum things. We think they're there for real,
9:03
Speaker 1: but it turns out that maybe things can be taken
9:06
Speaker 1: away from the universe. That's right. This fuzzy question of
9:09
Speaker 1: what things are doing when you're not looking at them,
9:11
Speaker 1: and if you look at them and then look away,
9:14
Speaker 1: and it doesn't matter who's looking at them and how
9:16
Speaker 1: they look at them, and whether they store the information
9:19
Speaker 1: and look at it later. All these fun thought experiments
9:22
Speaker 1: can help us try to understand what's really going on
9:24
Speaker 1: at the smallest scales. So to be on the podcast,
9:27
Speaker 1: we'll be talking about what is a quantum eraser? Now, Daniel,
9:36
Speaker 1: is this a rubber eraser or what is it made
9:39
Speaker 1: out of? It's something which will erase your mind if
9:41
Speaker 1: you think about it too much. It's kind of stretch
9:44
Speaker 1: it out like a piece of rubber exactly, and push
9:46
Speaker 1: it too hard and it might just snap. No. Quantum
9:49
Speaker 1: eraser refers to the concept of quantum information and what
9:53
Speaker 1: happens if you create information and then erase that information
9:57
Speaker 1: from the universe. So it's an extension of some really
10:00
Speaker 1: on experiments that listeners on this podcast have heard us
10:02
Speaker 1: talk about, the double slit experiment, which reveals how particles
10:06
Speaker 1: can interfere with themselves and have the chance to be
10:08
Speaker 1: in multiple places at once. We dog into that experiment
10:11
Speaker 1: with a fun conversation with Adam Becker, the author of
10:14
Speaker 1: What Is Real, and today we're gonna go double down
10:17
Speaker 1: on those experiments and think about even crazier versions. Yeah, so,
10:21
Speaker 1: what is a quantum erasor now, Daniels this a thing
10:23
Speaker 1: or like a concept. Yeah, it's both. It was first
10:26
Speaker 1: a concept and then people made it a thing. It's
10:29
Speaker 1: like that in quantum mechanics, a lot of people think, well,
10:31
Speaker 1: if the universe really is that way, here's a ridiculous
10:34
Speaker 1: scenario that should lead to a silly result. And then
10:37
Speaker 1: physicists go out and they do the experiment. They make
10:40
Speaker 1: it real. They figure out a way to like build
10:42
Speaker 1: it in their lab to test that crazy property universe,
10:45
Speaker 1: and they get these ridiculous results, which in the end
10:48
Speaker 1: you have to accept because that's what the experiment says.
10:51
Speaker 1: They say, the universe really works that way. All right,
10:54
Speaker 1: Well we'll dig into it, we'll rub that quantum erazor
10:58
Speaker 1: all of our brains and see there is anything left
11:00
Speaker 1: at the end. But first we were wondering if how
11:03
Speaker 1: many people out there had thought about this question or
11:06
Speaker 1: even heard the term quantum eraser. So Daniel went out
11:09
Speaker 1: there and ask people on the internet what they thought
11:12
Speaker 1: a quantum eraser is. That's right. So if you'd like
11:15
Speaker 1: to be a participant in our virtual person on the
11:17
Speaker 1: street interviews and love to speculate about physics without looking
11:21
Speaker 1: anything up, then please write to us two questions at
11:24
Speaker 1: Daniel and Jorge dot com. All right, so think about
11:26
Speaker 1: it for a second. If a random physicist came up
11:28
Speaker 1: to you on the street and you didn't run away first,
11:31
Speaker 1: and I actually listened to them, and they asked you,
11:33
Speaker 1: what is a quantum eraser? What would you say? You
11:36
Speaker 1: are people's answers something an angry physics grad student uses.
11:41
Speaker 1: I have no idea. Maybe something that erases things at
11:45
Speaker 1: random and like gets rid of things, because that's what
11:48
Speaker 1: quantum generally is, is the randomness. Quantum araser could be
11:52
Speaker 1: something that we invent in the future to erase quantum
11:57
Speaker 1: mechanics and quantum physics just because pretty hard to understand.
12:01
Speaker 1: If we don't have it around anymore, we don't have
12:02
Speaker 1: to deal with it. We can stick with general relativity
12:05
Speaker 1: and regular gravity. So yeah, just get rid of that stuff. Really,
12:11
Speaker 1: I don't know either if I were to compare a
12:14
Speaker 1: quantum or race or to a regular racer, which essentially
12:17
Speaker 1: just kind of distorts the little graphite particles and absorbs
12:23
Speaker 1: them in in a way that you know, it gets
12:25
Speaker 1: rid of the material on a piece of paper or something,
12:29
Speaker 1: so that you know you can reuse that material to
12:31
Speaker 1: write on something. Maybe a quantum race, or some sort
12:34
Speaker 1: of force or phenomenon that causes sometime of particles to
12:38
Speaker 1: break up from a given area or concentration, so that
12:43
Speaker 1: the information is very very difficult to understand, or to
12:47
Speaker 1: receive or to observe. Maybe well, I'm a teacher, and
12:50
Speaker 1: I know that my students use a razors to hide
12:53
Speaker 1: their mistakes, So I think a quantum e razor is
12:58
Speaker 1: something that quantum physicists used to hide their errors from
13:02
Speaker 1: everyone else. All I can come up with for that is,
13:06
Speaker 1: sometimes you make a mistake and it's for the better,
13:08
Speaker 1: it's a better idea than what you originally planned. And
13:10
Speaker 1: sometimes it's a big catastrophe. But since you don't know
13:14
Speaker 1: ahead of time, you use your quantum eraser two either
13:18
Speaker 1: undo your mistake or make it permanent, and you just
13:21
Speaker 1: roll the dice and let fate decide. No idea. Never
13:26
Speaker 1: heard of a quantum eracer, but the image of a really,
13:29
Speaker 1: really kindy eraser pops into my mind. So let's say
13:33
Speaker 1: a tool that allows you to change the sub potomic
13:39
Speaker 1: structure of stuff. I would guess that a quantum eracer
13:45
Speaker 1: is having to do with erasing particles that have certain
13:51
Speaker 1: quantum states. Thereby leaving behind particles that are in the
13:55
Speaker 1: state you want, uh that, or it's the weapon that
13:58
Speaker 1: they used in the movie Racer with Arnold Schwarzenegger. All right,
14:02
Speaker 1: a lot of fun answers here. Everyone's a comedian on
14:04
Speaker 1: the internet. Well, especially if I have no idea what
14:06
Speaker 1: we're talking about, then they got to go to that
14:08
Speaker 1: joking place. I like the joke about the really tiny eraser, right, Yeah,
14:14
Speaker 1: like if you have a quantum pencil, I guess it
14:16
Speaker 1: would have a quantum eraser on the one end of it. Yeah,
14:19
Speaker 1: or e raises quantum particles or something like that. I'm
14:22
Speaker 1: imagining a super tiny little vacuum cleaner that like slurps
14:24
Speaker 1: up electrons. Yeah, and and those one with them passes
14:27
Speaker 1: them through a wormhole into another universe. I guess, tantalizing.
14:31
Speaker 1: All right, well, let's get into this concept of a
14:33
Speaker 1: quantum erasor Daniel, you're saying it has something to do
14:36
Speaker 1: with the double slit experiment. Now, this is going to
14:39
Speaker 1: be kind of hard because I feel like this is
14:41
Speaker 1: a podcast and it's an audio only and this is
14:43
Speaker 1: a very kind of visual experiment. But I guess we
14:47
Speaker 1: can try our best to describe what it is. Yeah,
14:49
Speaker 1: maybe we can add a dance element to it. Do
14:50
Speaker 1: you think that will help? Yeah? Or I could draw
14:52
Speaker 1: our tunes. I'm doing watercolor painting at the same time
14:55
Speaker 1: as we do our podcast. By the way, Oh really,
14:57
Speaker 1: you're in a quantum artistic s am. Know. This quantum
15:01
Speaker 1: er racer is an experiment that's like a permutation or
15:04
Speaker 1: an add on to the basic double slit experiment. So
15:08
Speaker 1: to understand why the quantum racer is so weird and crazy,
15:11
Speaker 1: you definitely have to understand what's going on in the
15:13
Speaker 1: double slit experiment. And so I think we're gonna have
15:15
Speaker 1: to use our words to describe the wiggly crazy nature
15:18
Speaker 1: of that experiment, and then we can build on it
15:20
Speaker 1: to get to the quantumer racer. All right, So I
15:22
Speaker 1: guess the double slicks experiment starts with a single slit first.
15:26
Speaker 1: I guess we'll explain that one, and then we'll multiply
15:28
Speaker 1: by two. So the basic experiment is like you have
15:31
Speaker 1: a wall, like a barrier, like a plate of metal,
15:35
Speaker 1: and you cut a little slit on it, like a
15:36
Speaker 1: little opening that's long in one direction, and then you
15:40
Speaker 1: shoot like a laser or like just a regular beam
15:42
Speaker 1: of light through it and then onto a wall behind
15:45
Speaker 1: the first wall. Yeah, exactly. So imagine your mind some
15:48
Speaker 1: source of light. A laser is good because then you
15:50
Speaker 1: have photons of all the same wavelength in the same direction,
15:53
Speaker 1: and then a screen on the other side with the
15:55
Speaker 1: laser hits. What do you get. You get a laser spot. Now,
15:57
Speaker 1: as you say, put something in between, like a barrier
16:01
Speaker 1: that has a very thin slit in it. Then what
16:03
Speaker 1: do you see on the screen Instead of the full
16:05
Speaker 1: laser spot. Now you see like a slice of that spot.
16:08
Speaker 1: You get a smooth pattern on the screen, but it's
16:11
Speaker 1: sort of cut by the slit in the barrier, and
16:13
Speaker 1: it has smooth edges, not a sharp edge, because that's
16:16
Speaker 1: what happens when light goes through a slit, intends to
16:18
Speaker 1: like spread out a little bit and smooth out. So
16:21
Speaker 1: the thing you start with is this single slit where
16:23
Speaker 1: the light goes through and hits the barrier on the
16:25
Speaker 1: other side, right, and you get a smooth light pattern
16:28
Speaker 1: on the other side. Now, the weird thing is in
16:30
Speaker 1: what happens if you put a second slit next to
16:33
Speaker 1: the first slip, right, that's right, So now you put
16:35
Speaker 1: two slits really close together so that the beam could
16:38
Speaker 1: pass through one slit or the other slit, and once
16:41
Speaker 1: you get on the other side, instead of having like
16:44
Speaker 1: two smooth patterns or you know, the simple addition of
16:47
Speaker 1: two patterns like you saw before, now you get an
16:50
Speaker 1: interference pattern, which means that you get these patterns of
16:53
Speaker 1: light and dark and light and dark and light and dark.
16:56
Speaker 1: And what's happening there is interference. Just like if you
16:59
Speaker 1: have waves when you add them, if one wave is
17:01
Speaker 1: going up while the other wave is going down, then
17:04
Speaker 1: they cancel each other out. Whereas if one wave is
17:07
Speaker 1: going up and the other one is going up at
17:08
Speaker 1: the same time, then they add up on top of
17:10
Speaker 1: each other. They get twice as strong. So the interference
17:13
Speaker 1: pattern has these slices that are twice as bright as
17:16
Speaker 1: the previous pattern, and these dark slices as well. And
17:19
Speaker 1: that's because you have two sources of light. Now each
17:21
Speaker 1: of the slits is giving you photons and they can
17:23
Speaker 1: either constructively or destructively interfere on the screen. Right because
17:28
Speaker 1: I guess you're shooting a laser at both slits at
17:30
Speaker 1: the same time, or you're like shooting a laser and
17:32
Speaker 1: it and the beam of the laser kind of goes
17:34
Speaker 1: through both slits at the same time, right, Yeah, the
17:36
Speaker 1: slits are very narrow and very very close together. This
17:39
Speaker 1: only works if the scale we're talking about here is
17:42
Speaker 1: sort of related to the wavelength of light that we're
17:44
Speaker 1: shooting at it. So this needs to be very microscopic. Right,
17:47
Speaker 1: So if you have one slid, you get a fuzzy,
17:49
Speaker 1: like a plain fuzzy image on the other side. But
17:52
Speaker 1: if you have two slids, then suddenly it's not a smooth,
17:55
Speaker 1: fuzzy image. It's like a weird Rippley kind of image,
17:59
Speaker 1: which means at somehow light is interacting with itself. Yeah,
18:02
Speaker 1: in this case, we don't know if the light is
18:04
Speaker 1: interacting with itself. You could say, hey, look, light is
18:06
Speaker 1: a wave, and waves interfere. This happens with waves in
18:10
Speaker 1: the bathtub, it happens with waves in the air. Like
18:12
Speaker 1: noise canceling headphones, right, they generate a second pattern of
18:16
Speaker 1: noise to cancel out the noise that's coming into your ear.
18:20
Speaker 1: So interference in waves is not necessarily a quantum mechanical thing.
18:23
Speaker 1: It's just a wave thing. So in this version of
18:26
Speaker 1: the experiment so far, you could just say, look, light's
18:28
Speaker 1: a wave. It's interfering, No big whoop. It gets quantum
18:31
Speaker 1: when you remember that the beam is actually made not
18:34
Speaker 1: of waves but of photons, little individual packets, and so
18:38
Speaker 1: you can take it to the next step by slowing
18:40
Speaker 1: down the experiment and dimming the laser. So that's shooting
18:44
Speaker 1: like one photon through the experiment at a time. Yeah,
18:47
Speaker 1: you shoot one photon at a time, and then you
18:49
Speaker 1: would think that just throwing like one ft doon at
18:52
Speaker 1: a time, this photon would pick like the right or
18:55
Speaker 1: the left slid and then end up on the other
18:56
Speaker 1: side of the wall and you would get the same
18:58
Speaker 1: fuzzy smooth pattern. But the weird part I guess is
19:01
Speaker 1: that you're shooting one ft at a time, but you
19:03
Speaker 1: still get the ripley kind of interference pattern on the
19:05
Speaker 1: other side. Exactly. You expect that if you shoot one
19:08
Speaker 1: photon at a time that it can't interfere because you
19:11
Speaker 1: were thinking, well, the interference comes from two photons going
19:14
Speaker 1: through both slits at the same time. Now you have
19:17
Speaker 1: just one photon in the experiment, so what's it interfering with?
19:20
Speaker 1: Because you still see the interference pattern on the other
19:23
Speaker 1: side of the screen and you shoot one photon through
19:25
Speaker 1: at a time. It's just that it takes longer to
19:27
Speaker 1: build up if you watch it for an hour or so.
19:29
Speaker 1: As those photons go through, one lands here, one lands there,
19:33
Speaker 1: one lands this other spot, it gradually builds up that
19:36
Speaker 1: interference pattern. So what's it interfering with. It's interfering with itself.
19:40
Speaker 1: It has the probability to go through both slits, and
19:43
Speaker 1: that wave function, which controls where a quantum particle goes,
19:47
Speaker 1: interferes with itself and creates this probability distribution on the
19:51
Speaker 1: screen for where it might land, and that probability distribution
19:55
Speaker 1: has the interference effects inside of it, and that's why
19:58
Speaker 1: you get this interference pattern. Every photon that goes through
20:01
Speaker 1: like randomly pulls a number from the probability distribution on
20:05
Speaker 1: the screen which has the interference pattern built in, and
20:07
Speaker 1: lands there, and gradually it builds up that distribution. It's
20:11
Speaker 1: almost like, you know, if you were to shoot a
20:13
Speaker 1: photon as a little ball, it would go through one
20:15
Speaker 1: of the slids, but because it's quantum, it's almost like
20:17
Speaker 1: it's going through both slits at the same time. Right.
20:20
Speaker 1: That's that's kind of the quantum thing. It's going through
20:21
Speaker 1: both slits. At the same time, and then it's sort
20:24
Speaker 1: of going through both slits and then interacting with itself
20:27
Speaker 1: in a quantum way so that when it gets to
20:29
Speaker 1: the screen it's not a smooth pattern. Yeah, it's tempting
20:32
Speaker 1: to say that it's in two places at once, or
20:35
Speaker 1: that goes through both slits at the same time, and
20:38
Speaker 1: that's our tendency to try to like tell a story
20:40
Speaker 1: for what happens. But I'm not sure that's the right
20:42
Speaker 1: way to think about it. The way I think about
20:44
Speaker 1: it is that it has a probability to go through
20:46
Speaker 1: both at once. What it actually does is not determined,
20:50
Speaker 1: you know, until it gets to the other side. So
20:53
Speaker 1: what happened when it went through the slits we don't know.
20:55
Speaker 1: We might never know. There isn't necessarily a story there.
20:59
Speaker 1: So it's a all change in wording, but an important
21:01
Speaker 1: change in meaning for me to say that it had
21:03
Speaker 1: probability to go through both slits rather than it actually
21:06
Speaker 1: went through both. Right, it's like saying that it's not
21:08
Speaker 1: that the cat is alive and dead. It's just said
21:10
Speaker 1: it has the same probability, where it has a certain
21:13
Speaker 1: probability of being alive and a certain probability of being dead.
21:16
Speaker 1: All right, Well, then now the weird part here. Now
21:18
Speaker 1: it's going to come when we try observing this photon,
21:21
Speaker 1: and that's when we get into this idea of quantum razors.
21:25
Speaker 1: So let's talk about that. But first let's take a
21:27
Speaker 1: quick break. Alright, we're asking the question what is a
21:42
Speaker 1: quantum erasor? And now Daniel feel like half of my
21:44
Speaker 1: brain is already erased trying to talk about quantum objects.
21:49
Speaker 1: And now we were explaining the double slit experiment, and
21:52
Speaker 1: so I think we were done with that. Like if
21:54
Speaker 1: you shoot a laser at a two small slits on
21:56
Speaker 1: a screen, then on the other side you're gonna get
21:58
Speaker 1: an interference parent because of the way the quantum probabilities
22:02
Speaker 1: kind of affect each other. And so now the weird
22:04
Speaker 1: thing happens when you try to like add a detector, right,
22:07
Speaker 1: when you try to see which slid it actually went through.
22:09
Speaker 1: That's right, because our tendency is to want to know, like, well,
22:12
Speaker 1: what happened, Right, did it go through one slit or
22:15
Speaker 1: did it go through the other. We feel like it
22:17
Speaker 1: must have gone through one or the other, right, because
22:19
Speaker 1: you know it was over here and then it's over there.
22:21
Speaker 1: So it must go from here to there is our sense,
22:25
Speaker 1: and so trying to get it like a more accurate
22:27
Speaker 1: understanding of what happened. You can add a little detector
22:30
Speaker 1: when that gives you a signal if a photon goes
22:33
Speaker 1: through slit A, for example, instead of slit B. So
22:36
Speaker 1: that way you can know, hey, did it go through
22:37
Speaker 1: slit A or slip B. Because you know, photons are observable, right,
22:41
Speaker 1: you can interact with them. They make splashes of light,
22:43
Speaker 1: you can measure them. They are quantum objects, but they
22:45
Speaker 1: are also physical. And so what happens when you do that,
22:47
Speaker 1: when you ask, when you insist on knowing which slid
22:50
Speaker 1: it went through, is that the interference pattern disappears. When
22:54
Speaker 1: you add that detector that just tries to understand which
22:56
Speaker 1: slid it went through, then the interference pattern is gone.
22:59
Speaker 1: So if you try to like measure at the photon
23:02
Speaker 1: as it goes through slid how would you even do that?
23:04
Speaker 1: Don't you have to stop the photon to do that.
23:06
Speaker 1: That's the crux of the matter right there. To measure
23:08
Speaker 1: a photon, you have to interact with it somehow. You
23:12
Speaker 1: can't just like observe a photon without interacting with it.
23:16
Speaker 1: You know, a photon that like passes in front of you,
23:18
Speaker 1: you can't see it. It's a piece of light, but
23:21
Speaker 1: unless it hits your eyes, you can't see it, or
23:24
Speaker 1: unless you put something in front of it to stop it,
23:27
Speaker 1: measure it and then re emit it. Right, So, for example,
23:30
Speaker 1: the reason you see something in front of you as
23:31
Speaker 1: red is because the photons hit that object and then
23:34
Speaker 1: emitted red photons. So you can't see things without interacting
23:39
Speaker 1: with them, even light, right, you need to interact with
23:41
Speaker 1: it somehow. So you can make up lots of different
23:44
Speaker 1: physical systems that could do this, but the simplest one is,
23:47
Speaker 1: you know, just like a simple photon detector, a photo
23:49
Speaker 1: multiplier tube for example, or a scintilator screen that indicates
23:53
Speaker 1: when a photon went through and then re emits it
23:55
Speaker 1: on the other side. Oh, I see what you're saying,
23:57
Speaker 1: Like if you catch it and then really sit back
24:00
Speaker 1: on the other side, that's one way you can measure
24:02
Speaker 1: the photon. And you'd like to think, oh, can't I
24:05
Speaker 1: just take a peek? Can I just look and see
24:07
Speaker 1: where it went without touching it, without interfering with it,
24:11
Speaker 1: without messing with it in any way. But you can't
24:13
Speaker 1: do that. Quantum mechanics tells us that the only way
24:15
Speaker 1: to get information about an object is to interact with it.
24:19
Speaker 1: You can like bounce a photon off of it, or
24:21
Speaker 1: you can bounce an electron off of it, or you
24:23
Speaker 1: can put another little screen, but somehow you have to
24:25
Speaker 1: interact with it. You can't get information from that particle
24:29
Speaker 1: without somehow interfering with its path. Yeah, because I guess
24:32
Speaker 1: in our everyday lives were used to this idea of
24:35
Speaker 1: being able to like see things but not touch them,
24:37
Speaker 1: and so we think we can tell where something is
24:40
Speaker 1: without actually like influencing it. But when you get down
24:43
Speaker 1: to the smallest of levels, like all seeing is interacting
24:47
Speaker 1: in a way, that's right, and it's also true with
24:48
Speaker 1: the macroscopic scales, just that you don't notice it. Like
24:51
Speaker 1: if you are walking outside at night and you want
24:53
Speaker 1: to know, hey, is there a rock in my path?
24:55
Speaker 1: You turn on your flashlight. You are shooting photons at
24:58
Speaker 1: that rock. Those photons are hitting the rock, they're warming
25:01
Speaker 1: up the rock. Then the rock is reflecting photons back
25:04
Speaker 1: at you. So yeah, you're not touching the rock, but
25:06
Speaker 1: you're definitely interacting with the rock and you're changing its
25:09
Speaker 1: quantum state. It's just like it doesn't like really heat
25:11
Speaker 1: up the rock or push the rock far away. When
25:14
Speaker 1: these are quantum particles that they can have significant effects
25:17
Speaker 1: if you interact with them by shooting beams of light
25:19
Speaker 1: at them or other particles. All right, So then in
25:21
Speaker 1: the double slit experiment, meant if you try to measure
25:24
Speaker 1: these photons before they hit the wall in the bag,
25:26
Speaker 1: then what you're doing is you're collapsing the quantum wave, right,
25:31
Speaker 1: You're messing up the quantum information. Yeah, so here's where
25:34
Speaker 1: the different interpretations of quantum mechanics. I'll tell you different
25:37
Speaker 1: stories for what happens the experiments. Save you measure the
25:40
Speaker 1: photon before or after the screen, that the interference pattern
25:43
Speaker 1: goes away. The classical interpretation of quantum mechanics, the Copenhagen interpretation,
25:48
Speaker 1: is what you just described. It says that the probability
25:50
Speaker 1: to go through both slits only exists if you haven't
25:53
Speaker 1: made a measurement. But then if you interact with it,
25:56
Speaker 1: it collapses the wave function. Now it can only go
25:58
Speaker 1: through one slit or the there so there's no interference
26:01
Speaker 1: because the interference came from the ambiguity came from the
26:04
Speaker 1: probabilities to go through both. The many worlds interpretation tells
26:07
Speaker 1: a different story. It says collapse is nonsense, that doesn't happen,
26:11
Speaker 1: it's ridiculous. It says that the universe splits into two,
26:14
Speaker 1: one where the photon went through one slit and one
26:16
Speaker 1: where the photon went through the other, and you're in
26:17
Speaker 1: one of those universes and not the other. Right, So
26:20
Speaker 1: that's those are the two ways in which you can
26:22
Speaker 1: interpret what happens. So how does that relate to, like
26:25
Speaker 1: the information and the quantum information. So the idea here
26:28
Speaker 1: is that you have the information about whether it went
26:31
Speaker 1: through slit A or slit B, and that's what destroys
26:34
Speaker 1: the interference because you've made this measurement. Somehow it changes
26:38
Speaker 1: the experiment. And you know, this is not something that
26:40
Speaker 1: we understand very well, this whole concept of measurement and
26:43
Speaker 1: quantum mechanics. And that was the topic of our episode
26:46
Speaker 1: with Adam Becker, and then recently we did an episode
26:48
Speaker 1: with Carlo Rovelli and he's got a whole new theory
26:51
Speaker 1: for how to understand measurement and quantum mechanics. It's not
26:54
Speaker 1: something that physics understands very well, how interacting with something
26:57
Speaker 1: changes its way? Function, does it collapse it as it
27:00
Speaker 1: split the universe? All of this kind of stuff. But
27:02
Speaker 1: the key idea here is if you extract information about
27:05
Speaker 1: which way the photon went, then there's no interference. Right,
27:09
Speaker 1: you somehow get rid of the quantumness of it, Like
27:12
Speaker 1: when when I poke something, it's no longer quantum if
27:14
Speaker 1: you poke something with a classical object like a big detector, right,
27:18
Speaker 1: that big detector can't be in two stays at once.
27:21
Speaker 1: It can't be like well, yes I saw it and
27:23
Speaker 1: no I didn't. It has to make a decision, and
27:25
Speaker 1: so it decoheres, and you get this weird thing where
27:28
Speaker 1: quantum object is interacting with the classical objects and so
27:31
Speaker 1: now it has to like follow the classical rules. So
27:34
Speaker 1: the quantum or racer is an attempt to get around that,
27:37
Speaker 1: is to say, what if instead of poking it with
27:39
Speaker 1: a big finger or a big classical detector, what if
27:42
Speaker 1: we got this information but we somehow kept it quantum
27:45
Speaker 1: at the same time. I see. So it's almost like
27:47
Speaker 1: the cat in show Dingerous Box, Like before you open
27:49
Speaker 1: the box, it's both alive and dead. The probability of
27:52
Speaker 1: it being one or the other. And if you open
27:54
Speaker 1: the box, that's the classical way of checking it out,
27:56
Speaker 1: Like you open it and it's either alive or dead,
27:58
Speaker 1: and and then you get it of the quantum probabilities.
28:01
Speaker 1: You're saying, can I like somehow, you know, poke my
28:05
Speaker 1: quantum finger into the box and measure but not kind
28:10
Speaker 1: of destroy that superpocisition of being both alive and dead exactly?
28:14
Speaker 1: The quantum or racer experiment tries to do that as well.
28:16
Speaker 1: Let's try to get this information out, but not look
28:19
Speaker 1: at it directly, not use like our classical objects, our eyeballs,
28:23
Speaker 1: our brains, even our computers to access that information, so
28:27
Speaker 1: we can stay in a quantum superposition, so we can
28:30
Speaker 1: make a decision later about whether we want that information.
28:34
Speaker 1: And here's where the mind bending stuff comes in. If
28:36
Speaker 1: you can like extract that information about which way the
28:39
Speaker 1: photon went, keep it in a quantum state by storing
28:41
Speaker 1: it in some other entangled particles, then you can decide
28:45
Speaker 1: after the photons I hit the screen whether or not
28:48
Speaker 1: you want to know which way it went wait to
28:50
Speaker 1: see it again. So the idea is you want to
28:52
Speaker 1: know which way the photons went, right? Did they go
28:54
Speaker 1: through slit air slip B. You know, if you add
28:56
Speaker 1: a classical object like a big detector, you're going to
28:59
Speaker 1: collapse the a function. So instead you add a quantum
29:02
Speaker 1: detector one that can record this information, but maybe without
29:06
Speaker 1: collapsing the state of the wave function. Somehow it gets
29:09
Speaker 1: this information. But because it's a quantum object, it doesn't
29:12
Speaker 1: trigger the collapse, right, So it can be like entangled
29:15
Speaker 1: with the photon without like forcing the photon to deco
29:18
Speaker 1: here completely. And so it's different from interacting with like
29:20
Speaker 1: your big body or something. You interact with it like
29:22
Speaker 1: with a single particle, and that stores the information about
29:25
Speaker 1: which way the photon went, but you haven't looked at it.
29:27
Speaker 1: You haven't collapsed that wave function yet. You let the
29:30
Speaker 1: photon then go hit the screen, and then after the
29:33
Speaker 1: photon has already hit the screen and decide where it's
29:35
Speaker 1: going to land, then you access that quantum information. It's
29:39
Speaker 1: called the delayed choice version, where you decide after the
29:42
Speaker 1: photon has hit the screen whether or not you want
29:44
Speaker 1: to know the information about which way it went. You're
29:46
Speaker 1: saying that the photon did go through one of the
29:48
Speaker 1: two slits, like once it hits the screen in the
29:50
Speaker 1: bag then it sort of chooses a history of having
29:53
Speaker 1: gone through the left or the right slit. That's right.
29:55
Speaker 1: This is trying to like force the photon to make
29:57
Speaker 1: its decision about whether or not to make an interfere
30:00
Speaker 1: it's pattern before you decide whether you want to know
30:03
Speaker 1: which that it went through. So it's sort of like,
30:06
Speaker 1: you know, trying to play quantum bluff with the photon.
30:09
Speaker 1: And that's when we get into these really funny questions
30:11
Speaker 1: of like, how does the photon know whether it's going
30:14
Speaker 1: to be measured? How does the photon know whether you're
30:16
Speaker 1: going to have information about it. It's almost like you wanna,
30:20
Speaker 1: you know, peek inside of the short Inger's box, but
30:23
Speaker 1: not look at the answer, so that it's still alive
30:26
Speaker 1: and dead inside the box. But you sort of have
30:27
Speaker 1: the answer in your pocket, but you don't. You haven't
30:29
Speaker 1: looked at the answer yet exactly. And so why is
30:32
Speaker 1: that call it a quantum eraser? Right, So this is
30:34
Speaker 1: not yet a quantum eraser. This is the delayed choice version,
30:38
Speaker 1: the delayed measurement, Yes, exactly, delayed measurement choosing whether or
30:41
Speaker 1: not you have the information, that's the delay. So you
30:43
Speaker 1: might wonder, like, well, what happens on the screen. What
30:45
Speaker 1: does the screen look like? What does the experiment look like?
30:47
Speaker 1: If you do this, If you capture this information in
30:50
Speaker 1: a quantum state, but you don't look at it yet, well,
30:52
Speaker 1: what happens is you don't see interference on the screen
30:55
Speaker 1: because by doing this, by slurping this information out of
30:58
Speaker 1: the photons, you have stored the interference. But people think, well,
31:01
Speaker 1: that's interesting. But I haven't yet looked at that information, right,
31:05
Speaker 1: So what happens if I then erase that information? This
31:08
Speaker 1: is where the quantum eracer comes in. If I take
31:11
Speaker 1: that quantum information which is stored in these quantum objects,
31:14
Speaker 1: but I haven't looked at it yet, if I erase
31:16
Speaker 1: that information somehow, can I then recover the interference? Can
31:20
Speaker 1: I make the interference pattern reappear on the screen by
31:23
Speaker 1: adding this quantum eracer, which like deletes that information from
31:26
Speaker 1: the universe because I never peeked at it. What are
31:28
Speaker 1: you saying that this is an actual experiment, Like we've
31:31
Speaker 1: sort of intercepted the photon before it goes into the slids,
31:34
Speaker 1: and we've stored that information and we see that it
31:38
Speaker 1: now it doesn't generate an interference pattern. A weekly pattern
31:42
Speaker 1: on the screen, even though nobody really knows which slid
31:45
Speaker 1: it went through. That's right, nobody knows which slid it
31:47
Speaker 1: went through, though it is in principle stored in this
31:49
Speaker 1: quantum object. Although that quantum object can be in a superposition,
31:52
Speaker 1: it doesn't have to be in a definite state. Right.
31:55
Speaker 1: You can say there's a probability of one and a
31:57
Speaker 1: probability of the other, and we don't see that interference patterns.
32:00
Speaker 1: And then people thought, well, what if we delete that information?
32:02
Speaker 1: What would the universe do if we measure the photon
32:05
Speaker 1: but don't look at it, keep in the quantum state,
32:07
Speaker 1: and then erase that information. Can it somehow go back
32:10
Speaker 1: and recover the interference pattern? I see you're saying that
32:13
Speaker 1: maybe it's not the fact that it's interactive with your
32:16
Speaker 1: little secret finger poking quantum poking that destroyed the interference pattern.
32:21
Speaker 1: Maybe if I poke it with my quantum finger and
32:23
Speaker 1: then I destroy my finger, will it go back to
32:27
Speaker 1: being a quantum object? Does that what you mean? Like,
32:29
Speaker 1: we know that if I poke it, even with a
32:31
Speaker 1: quantum finger and not look at the answer, it destroys
32:34
Speaker 1: the quantum information. But now what happens if I poke
32:36
Speaker 1: it with a band of finger and then the story
32:37
Speaker 1: to finger will go back to being a quantum object.
32:40
Speaker 1: Is that kind of the idea, that's the quantum eraser
32:42
Speaker 1: is destroying that quantum information you've extracted from the experiment
32:45
Speaker 1: but haven't yet looked at, so it's still quantum. So
32:48
Speaker 1: that's the crazy experiment. And I can hear you react
32:50
Speaker 1: and say, what is that real experiment? Did we really
32:52
Speaker 1: do that? And yes, we have really done this experiment.
32:55
Speaker 1: We have done it with photons, and you can go
32:58
Speaker 1: up and google and learn all about the details of
33:00
Speaker 1: this experiment. I think there's a slightly simpler version that's
33:02
Speaker 1: easier to talk about, where you use electrons. But the
33:05
Speaker 1: principles are all the same. And so how can you
33:08
Speaker 1: destroy quantum information? Well, for example, if when the photon
33:11
Speaker 1: is passing through the slit, you have some detector, and
33:14
Speaker 1: that detector takes an electron and puts it in like
33:17
Speaker 1: a spin up state. If the photon went through one
33:19
Speaker 1: slit and it's been downstate, if the photon went through
33:22
Speaker 1: another slit. This is just a way to like store
33:24
Speaker 1: that information about which way the photon went and keep
33:28
Speaker 1: it quantum. Right, we don't want to mark it on
33:30
Speaker 1: a piece of paper, or put it in a computer
33:32
Speaker 1: or some big classical object. We want to keep it
33:34
Speaker 1: as quantum information. So here the electron is just like
33:36
Speaker 1: a single cube bit. It contains some quantum information. But
33:40
Speaker 1: it can be in a super position. It can be
33:41
Speaker 1: a little spin up and a little spin down. We
33:43
Speaker 1: don't know yet, right, but I feel like you bumped
33:45
Speaker 1: the photon right, Like the photon was going through the slip,
33:48
Speaker 1: but you made a bump into this electron. And now
33:51
Speaker 1: it feels like it's in now an impure experiment because
33:54
Speaker 1: you bumped it right, not just in a quantum way,
33:56
Speaker 1: but you did sort of. It's not maybe the same photon,
33:58
Speaker 1: or it's not the same path as a photon who
34:00
Speaker 1: didn't bump into an electron. That's right, And that's why
34:02
Speaker 1: you no longer see the interference. Right, you add this
34:05
Speaker 1: experiment where you're bumping it into the electron, it destroys
34:08
Speaker 1: the interference pattern because that information is now stored in
34:11
Speaker 1: the electron, so it can be extracted. The knowledge of
34:14
Speaker 1: which way the photon went can be measured in the universe,
34:16
Speaker 1: and so that destroys this interference pattern on the screen.
34:19
Speaker 1: So you're right, it's a different experiment, right, Like the
34:22
Speaker 1: quantumness went from being on the wall behind the screen
34:25
Speaker 1: to now being in this electron you poked it with,
34:28
Speaker 1: And now I guess the question is if I destroy
34:30
Speaker 1: the information in that electron, do I get back my
34:34
Speaker 1: weekly pattern on the screen. That's the idea. It's totally
34:37
Speaker 1: mind bending and crazy what actually happens. I love it.
34:40
Speaker 1: And so you take this electron and you might wonder like, well,
34:43
Speaker 1: how do you destroy the information? How do you erase
34:45
Speaker 1: the information? Well, it's actually not that hard to erase
34:49
Speaker 1: quantum information. It happens all the time. Like if you,
34:52
Speaker 1: for example, measure particles momentum, then that scrambles your knowledge
34:57
Speaker 1: of the particle's position because the Heisberg uncertainty prints will
35:00
Speaker 1: says you can't know both very very precisely. If you
35:02
Speaker 1: have a particle, for example, you measure its position really precisely,
35:05
Speaker 1: and then you measure its momentum, then you've erased the
35:08
Speaker 1: quantum information about its position because you can't have both simultaneously.
35:13
Speaker 1: So you can do certain things sort of similar to
35:14
Speaker 1: this electron. You can't know an electron spin in one
35:18
Speaker 1: direction and in another direction at the same time. So
35:21
Speaker 1: if you want to erase the spin up and down information,
35:24
Speaker 1: all you need to do is measure the spin of
35:26
Speaker 1: the electron sort of left right, and that will scramble
35:30
Speaker 1: the information about the electron spin up down. I plucked
35:33
Speaker 1: the box with the cat with my quantum finger, and
35:35
Speaker 1: now instead of running my finger through a filter, that
35:39
Speaker 1: then kind of scrambles or filters out the information from
35:43
Speaker 1: the cat. That's right. So now it's no longer possible
35:45
Speaker 1: to know which state it was in. Was it's been
35:48
Speaker 1: up or was it's been down. We don't know anymore.
35:50
Speaker 1: And it's scrambled. It's not like the information existed and
35:53
Speaker 1: we've overridden it. It was in a quantum superposition. It
35:56
Speaker 1: was undetermined, and now the information about those probability is lost.
36:01
Speaker 1: So that's the quantum eraser. It says, destroy the information
36:04
Speaker 1: that you've extracted from this experiment. All right, So we
36:07
Speaker 1: did the experiment, actually we poked it with something and
36:09
Speaker 1: then we erased the information. And did they actually somebody
36:13
Speaker 1: actually built this. Somebody actually built this, and they did it.
36:15
Speaker 1: They did this experiment. And so there's a lot of
36:18
Speaker 1: discussion of this kind of experiment online, and I find
36:21
Speaker 1: a lot of these to be sort of misleading because
36:23
Speaker 1: they suggest that what happens when you apply the quantum racer,
36:26
Speaker 1: when you erase this experiment, is that the interference pattern
36:28
Speaker 1: like reappears on the screen, which is impossible because you
36:32
Speaker 1: could do this like quantum eraser experiment like years later,
36:36
Speaker 1: after you've already done the original experiment. You know, you
36:38
Speaker 1: could like store these electrons somehow and then five years
36:41
Speaker 1: later decided to erase the information. You can't go back
36:43
Speaker 1: in time and then change the interference pattern on the screen.
36:47
Speaker 1: So that's not what happens. That would be crazy in
36:49
Speaker 1: bonkers and awesome. But instead, what happens is that if
36:52
Speaker 1: you do this, if you erase the quantum information, then
36:55
Speaker 1: you are making a measurement of those electrons. You're measuring
36:58
Speaker 1: them like left right instead of up down. If you
37:00
Speaker 1: take those results and you look at only the ones
37:03
Speaker 1: that have like electron that turned out to be left,
37:05
Speaker 1: or only the ones that electron that turned out to
37:06
Speaker 1: be right, then you see the interference pattern. So the
37:09
Speaker 1: photons that had like a right spinning electron, you see
37:13
Speaker 1: an interference pattern in those photons, And there's an interference
37:17
Speaker 1: pattern in the photons that had a left spinning electron.
37:19
Speaker 1: If you put them together, they add up to the
37:21
Speaker 1: same smooth shape. So it's sort of like the interference
37:24
Speaker 1: pattern was hiding inside that smooth shape. And if you
37:28
Speaker 1: scramble the information that you knew about which photon went where,
37:31
Speaker 1: you can recover the interference pattern from within the smooth
37:35
Speaker 1: shape that you saw on the screen. All right, But
37:37
Speaker 1: then that still requires an observation, right, because you're seeing
37:42
Speaker 1: some of that information you thought you destroyed, but you
37:43
Speaker 1: didn't really destroy it in a way, or like, did
37:45
Speaker 1: you destroy the information in one direction and so the
37:48
Speaker 1: quantum objects sort of adjusted into the other direction. Yeah,
37:51
Speaker 1: you destroyed the original information, you can't know which way
37:55
Speaker 1: the photon went right, and so that allows you to
37:58
Speaker 1: have interference, and you can recover that interference if you
38:01
Speaker 1: look at like some of the photons. And the reason
38:04
Speaker 1: is that you know some of these photons are entangled
38:07
Speaker 1: with some of these electrons in this way, and you
38:09
Speaker 1: need to like know how to pull out the subset
38:12
Speaker 1: of photons that have the interference pattern you're looking for.
38:15
Speaker 1: You can only get that if you have erased the
38:18
Speaker 1: quantum information you're looking for. If you access the quantum
38:21
Speaker 1: information directly. If you measure spin up or down so
38:23
Speaker 1: you know which photon went through with slit, then that
38:26
Speaker 1: collapses the way functions essentially and means that you see
38:30
Speaker 1: no interference. You cannot access any interference only if you
38:33
Speaker 1: erase the information in those electrons by measuring left right
38:36
Speaker 1: instead of up down. Can you then go back and
38:38
Speaker 1: split the photons into two categories, each of which shows interference. Interesting?
38:43
Speaker 1: All right, let's get into what this all means and
38:46
Speaker 1: what it can mean about how we see reality. But
38:48
Speaker 1: first let's take another quick break. All right, Daniel, I'm
39:03
Speaker 1: still sort of stuck in the cat in the box
39:05
Speaker 1: experiment because I feel like that's a little easier to grasp.
39:08
Speaker 1: So we had a cat in the box, we puked
39:10
Speaker 1: up with a plantum finger, and then we measured my
39:12
Speaker 1: finger in one direction, and then we sort of destroyed
39:16
Speaker 1: that information by measuring the finger in a different direction,
39:21
Speaker 1: and then we see that the cat is still sort
39:23
Speaker 1: of alive and that but only if we use the
39:25
Speaker 1: information we got from the finger, right, Yeah, exactly. I
39:29
Speaker 1: think if you want to talk about cats and boxes,
39:31
Speaker 1: then you'll need like a hundred cats and a hundred boxes,
39:35
Speaker 1: because in the end, This is a probabilistic effect. Just
39:37
Speaker 1: like with the double slid experiment, these interference patterns are
39:41
Speaker 1: only obvious if you do a lot of photons, so
39:43
Speaker 1: you can see the patterns. Because a single photon could
39:45
Speaker 1: hit the screen wherever, you can't tell if you're seeing
39:47
Speaker 1: an interference pattern or not from one photon. So let's
39:49
Speaker 1: say you have you know you're a cat person. Your
39:52
Speaker 1: house is swarming with cats. Each one you put in
39:54
Speaker 1: a box, right, and then you make this quantum measurement
39:58
Speaker 1: of each one, but you don't look at the result.
40:00
Speaker 1: You poke it with a quantum finger, you don't look
40:03
Speaker 1: at the result, all right, So then you've figured out
40:05
Speaker 1: I guess with this experiment that the objects sort of
40:09
Speaker 1: goes back to being quantum, but not really because maybe
40:12
Speaker 1: you're in a way, you're sort of cheating, right, You're
40:14
Speaker 1: using some of the information you got from poking it
40:17
Speaker 1: to make it look quantum again in a way. Right,
40:20
Speaker 1: It's almost like the photon went back to being quantum,
40:22
Speaker 1: but only half quantum because you were able to measure
40:26
Speaker 1: some of it. Yeah, exactly. It's a bit mind bending
40:28
Speaker 1: because we like to think about what happens to these
40:31
Speaker 1: particles like what are they really doing? And we like
40:34
Speaker 1: to think that things can't go back in time and
40:36
Speaker 1: change their decision. And you know, the core fuzziness of
40:39
Speaker 1: this experiment is that if you think about these things
40:42
Speaker 1: in terms of particles and waves, you like to think
40:44
Speaker 1: that it's a wave and then it gets collapsed into
40:47
Speaker 1: a particle after it goes through the slit if there's
40:49
Speaker 1: a detector there, and then needs to decide like am
40:52
Speaker 1: I a particle or am I a wave before it
40:54
Speaker 1: hits the screen, so that it can either make an
40:56
Speaker 1: interference pattern or not. Right, it doesn't seem like it
40:58
Speaker 1: would make sense for or that to depend on what
41:01
Speaker 1: you do later on, because you can make this like
41:03
Speaker 1: quantum information decision a year later, or ten years later,
41:07
Speaker 1: or a thousand years later. So some people are attempted
41:10
Speaker 1: to say that this means that there's retro causality that
41:14
Speaker 1: based on what you do later, you can go back
41:17
Speaker 1: in time and change the results of the experiment. I
41:20
Speaker 1: think that's kind of nonsense. Really, what you're doing here
41:23
Speaker 1: is just interpreting the experiment in a different way using
41:26
Speaker 1: additional information you've extracted from the experiment. As you said,
41:30
Speaker 1: you're sort of cheating but it's backwards right now. You're
41:32
Speaker 1: making a measurement to know how to separate those photons
41:35
Speaker 1: to see the interference pattern. You're not making a measurement
41:38
Speaker 1: about which way the photon went about which slid it
41:41
Speaker 1: went through. Instead, you're just separating the photons into the
41:44
Speaker 1: ones that were entangled with electrons in one way versus
41:47
Speaker 1: photons that were entangled with the electrons in the other way.
41:49
Speaker 1: And those subsets do have the interference going on, it's
41:52
Speaker 1: just it was masked because when you add up the
41:55
Speaker 1: two kinds of interference, they add up to the smooth pattern.
41:58
Speaker 1: It's like it's still the same sort of get to
42:00
Speaker 1: a quantum that was going on before. Like the photon
42:03
Speaker 1: looked like it had lost this quantum information, but really
42:06
Speaker 1: like if you take that information that you got from
42:09
Speaker 1: the poking with the finger, then you can sort of
42:11
Speaker 1: find its quantumness in the direction that you didn't poke
42:15
Speaker 1: it in. Yeah, exactly. And so it's a really fun
42:18
Speaker 1: experiment to try to think about this nature of decoherence.
42:21
Speaker 1: Like you were saying before, if you poke something with
42:23
Speaker 1: a big classical object, it decoheres. It gets entangled with
42:26
Speaker 1: the whole environment. Millions and millions of particles, and so
42:29
Speaker 1: all of its quantum properties are essentially lost. Here. What
42:32
Speaker 1: we're doing is we're like kind of cheating, we're deco
42:35
Speaker 1: hearing it only Italian a little bit by interacting with
42:38
Speaker 1: it with a quantum object. So we can play quantum
42:41
Speaker 1: games with that decoherence later and in the end recover
42:45
Speaker 1: some of that interference by erasing that information. And so
42:48
Speaker 1: it's really sort of a great mental exercise to think
42:51
Speaker 1: about whether you understand decoherence. And we had a whole
42:54
Speaker 1: podcast episode about what quantum decoherence is. It's closely connected
42:58
Speaker 1: to this question of what is a quantum measurement and
43:01
Speaker 1: what happens when you measure something, but it's not quite
43:04
Speaker 1: the same thing. It's more about whether quantum properties can
43:07
Speaker 1: be observed because the different quantum states are still coherent,
43:10
Speaker 1: whether they add up and cancel out in just the
43:12
Speaker 1: right ways to make something have a quantum effect. I
43:15
Speaker 1: think the main point is that you know everything is quantum,
43:18
Speaker 1: but quantumness of something can exist kind of in different
43:21
Speaker 1: directions in a way, or in like different aspects that
43:24
Speaker 1: are part of the whole, but it's still you can
43:25
Speaker 1: sort of take out half of the quantumness of an
43:27
Speaker 1: object and still for serve sort the other half of
43:30
Speaker 1: the quantumness that it has in the other direction. Yeah, exactly,
43:34
Speaker 1: And so you know, the trickiness here relies in the
43:37
Speaker 1: fact that, like by becoming entangled with a single electron
43:41
Speaker 1: rather than the whole environment, these photons hit the screen
43:43
Speaker 1: only become kind of decohered, right, and so it's just
43:47
Speaker 1: a single particle to worry about. We're sort of able
43:50
Speaker 1: to think about measuring in different ways, and that's really fun,
43:53
Speaker 1: and it's easier to think about what this experiment means
43:55
Speaker 1: in some interpretations of quantum mechanics than in others. Like
43:59
Speaker 1: in many world it's not that big a deal because
44:01
Speaker 1: the whole universe has a wave function and now we're
44:03
Speaker 1: just talking about the quantum wave function of the photon
44:06
Speaker 1: and the electron and they're kind of entangled and that's
44:08
Speaker 1: no big deal, whereas in like a strict theory where
44:11
Speaker 1: you have collapse, then you have to wonder like, well,
44:14
Speaker 1: did the photon collapse or not, because if I don't
44:18
Speaker 1: destroy the information and I measure it, then the photon
44:21
Speaker 1: has to collapse because I knew which way it went.
44:23
Speaker 1: But if I do destroy the information, then how am
44:26
Speaker 1: I getting an interference pattern later on? Because for that
44:29
Speaker 1: to happen, it has to stay a wave. And so
44:31
Speaker 1: this is sort of troublesome for the collapse theories of
44:34
Speaker 1: quantum mechanics, not so much trouble for other theories like
44:37
Speaker 1: many worlds and relational quantum mechanics. Doesn't it just mean
44:40
Speaker 1: that maybe like the wave collapse in one direction but
44:43
Speaker 1: not the other direction, Like couldn't you still you know,
44:46
Speaker 1: use the coping dating interpretation and just say that it
44:49
Speaker 1: collapse in like one direction and not the other. Well,
44:51
Speaker 1: the electron is the one that has these multiple directions
44:54
Speaker 1: of information that spin up down versus spin left right.
44:57
Speaker 1: The photon is either interfering or it's not, you know,
45:00
Speaker 1: and it either collapses and it's just like a single
45:03
Speaker 1: source which gives you the smooth pattern, or it doesn't
45:06
Speaker 1: collapse and you have the wave function which does interfere.
45:09
Speaker 1: There aren't multiple directions there, and so it's hard to
45:13
Speaker 1: understand how it collapse theory can really work because that
45:16
Speaker 1: does kind of require going back in time and like
45:18
Speaker 1: uncollapsing the way of function. To me, collapsing the way
45:22
Speaker 1: function makes no sense at all it's not even consistent
45:24
Speaker 1: with quantum mechanics because it destroys quantum information in a
45:27
Speaker 1: way that we know violates basic principles and it violates
45:31
Speaker 1: the like time continuity of quantum mechanics that says you
45:33
Speaker 1: should be able to run experiments forward and backwards. So
45:36
Speaker 1: the collapse theory never made any sense to me, really,
45:38
Speaker 1: and I think this experiment really highlights how it's sort
45:41
Speaker 1: of nonsense. But then the only other interpretation that we
45:44
Speaker 1: have is the multi world theory, right, which this multiverse
45:47
Speaker 1: theory that every time a quantum object makes a decision,
45:51
Speaker 1: that the two universes are created. Yeah, that's another interpretation,
45:54
Speaker 1: and that one is pretty happy with this experiment. There
45:57
Speaker 1: are other interpretations that you can use the are consistent
46:00
Speaker 1: with this experiment, Like relational quantum mechanics works well with
46:03
Speaker 1: this because it says that like, hey, everything in the
46:05
Speaker 1: universe has its own measurement of these things, and so
46:07
Speaker 1: it doesn't matter what you measure, there is no reality anyway.
46:11
Speaker 1: And then there are also like other variants of collapse
46:13
Speaker 1: theories that are not as strict. You know, let's say, well,
46:16
Speaker 1: collapse happens in this way or in that way, so
46:18
Speaker 1: there's a whole spectrum of them, but this is troublesome
46:20
Speaker 1: for like the most hardcore collapse theories. All right, well,
46:23
Speaker 1: then I guess to answer the question what is a
46:25
Speaker 1: quantum eraser? I feel like the answer to that is
46:27
Speaker 1: sort of straightforward, but it's sort of the implications of
46:31
Speaker 1: what quantum eraser can do. That's really sort of what
46:34
Speaker 1: we spend in our check on and that's really hard
46:37
Speaker 1: to sort of get your head around. So a quantum
46:39
Speaker 1: razer is just taking quantum information from something and erasing
46:43
Speaker 1: it in a way. Right, Like, if I have quantum
46:45
Speaker 1: information stored in one direction of an electron spin, by
46:48
Speaker 1: measuring it in the other direction, I can destroy that
46:51
Speaker 1: quantum information. Right, that's the idea of a quantum razor.
46:54
Speaker 1: And if that electron happens to be entangled with photons
46:57
Speaker 1: which may or may not be interfering, then whether or
47:00
Speaker 1: not you erase that information or not can determine whether
47:03
Speaker 1: or not you can see interference in those photons. Well,
47:06
Speaker 1: it doesn't determine whether or not you can see it
47:08
Speaker 1: the way it tells you how to look for that interference. Well,
47:10
Speaker 1: if you measure the which way of the photons using
47:13
Speaker 1: those electrons, then you cannot see interference. The only way
47:16
Speaker 1: to see interference is to destroy that information and then
47:19
Speaker 1: use the results of destroying that information to pick out
47:22
Speaker 1: the interference patterns from the photons. You can't do that
47:25
Speaker 1: if you measure which way the photon went right, right,
47:28
Speaker 1: But you're sort of still measuring the electron, and then
47:30
Speaker 1: that's telling you how to look for the interference in
47:33
Speaker 1: the photon parent right. Yes, you're measuring the electron, but
47:35
Speaker 1: you're not measuring which way the original photon went. You're
47:38
Speaker 1: measuring something else about the electron, which destroys that information.
47:42
Speaker 1: All right, it sounds like we erase people's brain and
47:47
Speaker 1: hopefully not their time for the last hour. Thanks very
47:50
Speaker 1: much for going on this journey into the weird quantum world.
47:53
Speaker 1: I love these thought experiments, the ones people think of
47:55
Speaker 1: and say, whoa, what would actually happen? Because that's the
47:59
Speaker 1: fun thing about experien mental physics is confronting the universe
48:02
Speaker 1: and saying, all right, universe, show us what you got.
48:04
Speaker 1: We set up a situation that forced you to reveal
48:07
Speaker 1: what's happening, and the quantum universe always responds with something crazy.
48:11
Speaker 1: And that's why we're here. To uh talk about the
48:13
Speaker 1: craziness and to hopefully get you to wrap your mind
48:16
Speaker 1: around all the different and interesting implications about what it
48:19
Speaker 1: means about the things around you that you see in
48:21
Speaker 1: touch or maybe don't see your touch. And so, if
48:23
Speaker 1: you're a person who likes questions and maybe even answers,
48:26
Speaker 1: check out our book Frequently Asked Questions about the Universe,
48:29
Speaker 1: available now and coming out in just a couple of weeks.
48:32
Speaker 1: You can find the links at Universe f a Q
48:35
Speaker 1: dot com. All right, well, thanks for joining us. We
48:38
Speaker 1: hope you enjoyed that. See you next time. Thanks for listening,
48:49
Speaker 1: and remember that Daniel and Jorge explained. The Universe is
48:51
Speaker 1: a production of I Heart Radio or more podcast from
48:55
Speaker 1: my Heart Radio. Visit the I Heart Radio Apple Apple
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