00:00:00.000 Welcome to Topcast and Episode 89 of the series.
00:00:23.280 This one is chapter 4 of The Science of Canon Can't by Chiara Marletta.
00:00:29.080 And the title of the chapter today and the title of this episode is Quantum Information.
00:00:34.960 Now I know that many people who watch and listen are not necessarily people with a physics
00:00:45.760 Indeed, physics itself is becoming increasingly a minority among school students.
00:00:51.040 I'm not exactly sure what the distant historical trends were, but I know that today in Australia
00:00:57.560 and globally broadly speaking in the Western tradition, fewer and fewer students are
00:01:04.000 electing to take physics in these senior years as compared to taking things like biology
00:01:09.280 or chemistry or even sports science, much less to say people taking on physics at university
00:01:15.520 as a proportion of all people who attend university.
00:01:19.480 Now I'm not saying if this trend is neither good nor bad, neither here nor there.
00:01:23.880 The fact is that fewer and fewer people as a proportion of the entire population are comfortable
00:01:34.800 But even if people do choose to go off and do some physics at university after having
00:01:39.160 done it at school, the overall majority of people who take physics at university at the lower
00:01:43.920 or undergraduate level, let's say, engineers and chemists and maybe people who go into
00:01:50.640 medical fields like radiology and so on, they are not predominantly physicists in the
00:01:57.000 And even those people who take physics, the engineers and the radiographers and so on, those
00:02:01.520 people can tend to find areas of physics quite esoteric themselves.
00:02:06.160 So for example, quantum theory is a typical case in points.
00:02:10.080 Some people who do some early physics at undergraduate level at university might never even
00:02:15.040 come across quantum theory to any great deep extent anyway.
00:02:20.360 They regarded as an esoteric curiosity that only comes up at dinner parties or at discussions
00:02:28.000 But even if you're someone like me who was determined to do something like astrophysics
00:02:31.680 and who had to take a lot of units of quantum physics along the way, there are still parts
00:02:37.200 of the physics cannons her to speak that are esoteric.
00:02:40.920 And one of them is what we're about to talk about today, which is quantum information
00:02:47.960 Now, I can't remember exactly when I first heard about quantum information theory.
00:02:52.720 I think it was when I was beginning to do quantum physics and I thought, this is strange
00:02:59.800 And then you hear about this thing called quantum information theory and you think how
00:03:03.400 much more wild can this get? And I remember picking up a few texts that were around,
00:03:08.160 this was decades ago, just the beginnings of quantum information theory and I struggled
00:03:14.200 to understand what was being said and I never really pursued it.
00:03:17.720 And ever since when I've tried to investigate it further, I've always found it dry,
00:03:23.000 which is a hint that I'm finding it difficult because I'm finding it uninteresting.
00:03:27.080 And that's typically because of the way it is explained.
00:03:30.800 And theory itself is rarely well explained, except in the work of people like David Deutsch
00:03:36.280 and David Wallace and so on who actually get to the heart of the matter about what's going
00:03:42.720 Well, quantum information theory doesn't even bother with many of the experiments.
00:03:46.160 It's just, it's often just presented as a dry mathematical tool that's used by people
00:03:51.320 who work in the really rarefied areas of the theoretical part of physics.
00:03:56.240 So it was never really relevant to me, training to do astronomy.
00:04:00.720 But the reason why I'm saying all of this is because I want to give a plug to the book.
00:04:04.720 I really think you should get hold of the book because this chapter in particular does
00:04:08.840 an excellent job on two fronts related to fronts.
00:04:13.560 First is that it manages to make quantum information interesting.
00:04:19.280 It made me want to keep reading to see what Kiaro is going to say next because she's presenting
00:04:28.080 But because of this problem being solved of how do you make this weird subject interesting,
00:04:33.760 it became easy as well and that's always a lesson for all of this.
00:04:37.200 If something is interesting, then it can tend to become more easy and I've said this
00:04:41.240 before, if you had trouble with something at school, it's not because you were ever incapable
00:04:47.680 It's because you never found it interesting and you probably never found it interesting
00:04:50.400 because the person presenting it wasn't making it interesting.
00:05:00.640 If you are not captured by a lecture about how stellar evolution works and what stars
00:05:08.600 can ultimately end up as, then that lecture is really working hard to make things boring
00:05:18.080 And I think everything ultimately must be like that in some way when you get into really
00:05:25.480 They're just really fascinating, inherently fascinating and Kiara's managed to find some
00:05:30.200 of the inherently fascinating parts of quantum information.
00:05:34.720 And I say you really have to get the book because my podcast here is no substitute for
00:05:40.800 reading the book at all and to make that point, I'm skipping the first few pages of this
00:05:47.880 And remember, I'm also skipping whole sections of the book, whole chapters, so to speak.
00:05:52.880 There are these little interstitial bits in between the chapter proper parts.
00:05:58.200 There are little fictional stories which are absolutely worth getting.
00:06:02.040 They're parables of a kind, they have a lesson and the book is worth getting, if only
00:06:08.320 So after skipping these first few pages, I'm going to pick it up where Kiara begins
00:06:14.160 to describe this ball and cups game, which many people will be familiar with, the most
00:06:20.520 basic form, which is the one that she is describing, is where there are two cups and there
00:06:27.000 And the person who is in charge of the game hides eyeball under one of the cups and then
00:06:33.200 the person playing the game has to guess which of the cups the ball is under.
00:06:39.080 And this leads to this curious distinction between people who subjectively experience unpredictability
00:06:47.280 and people who subjectively experience certainty.
00:06:50.760 So in that particular game, of course, the person playing the game, who does not know which
00:06:54.960 cup the ball is under, has a 50-50 chance of guessing which cup the ball is under.
00:07:00.160 But the person who is in charge of the game, of setting up where the ball is going to go knows
00:07:05.920 with certainty, probability equal to one, if you like, that the ball is under that particular
00:07:15.840 And this is a kind of parable of a sort that is going to lead into to the idea of objective
00:07:26.560 So let me pick it up where Keira begins to refine our understanding of some otherwise mundane
00:07:34.200 So as I say, Keira has just been explaining how this ball and cups game works and not picking
00:07:44.800 But the risk in games of chance is due to a counterfactual property.
00:07:50.320 It is the impossibility of correctly predicting something with certainty.
00:07:57.640 In the shell game of my childhood, just by the way, Keira first of this ball and cups game
00:08:04.920 In the shell game of my childhood, what is unpredictable is the position of the marble or
00:08:13.040 Interestingly, in this case, the unpredictability is not objective.
00:08:20.120 From my point of view, the probability of correctly predicting the marble's location
00:08:25.840 But someone who had the full details perfectly knew its position.
00:08:30.480 The unpredictability in this case is therefore apparent to the player only because he or
00:08:38.240 The person who sets up the game, by contrast, sees an entirely certain, predictable and
00:08:45.560 It seems that just like in this game with marbles and cups, most unpredictability in
00:08:53.640 When the weather forecast is uncertain, and the weather unpredictable as a result, it is
00:08:58.240 because the information about the initial condition of all the particles in a given region
00:09:06.320 Same tossers are unpredictable because the initial conditions of the coin and environment
00:09:12.160 The degree of unpredictability is then quantified with probabilities.
00:09:16.160 The probability of some unpredictable event happening expresses the extent to which one expects
00:09:23.400 When asked, what will the weather be like tomorrow?
00:09:25.840 You can reply, for instance, I don't know for sure, it's unpredictable.
00:09:29.960 But there is a 90% probability that is going to be sunny and so on.
00:09:34.280 All unpredictable behaviours were once supposed to be the same as in this game.
00:09:39.720 Not objective, but tied to a specific viewpoint.
00:09:43.360 If given complete information about the actual state of affairs, there is no unpredictability.
00:09:48.400 The latter appears, only if one has incomplete information.
00:09:52.840 Though this belief might seem intuitively true, it was wiped out by the discovery of
00:09:57.240 quantum theory in the first half of the 20th century.
00:10:00.480 In quantum theory, unpredictability does not arise just from a lack of information.
00:10:05.080 It is inherent to the physical world, even when everyone has all the relevant information.
00:10:14.200 All right, so just my comments here, the thing here, and if you're having your listeners
00:10:19.880 of my series on the multiverse, chapter 10 of the beginning of infinity, you will know that
00:10:25.760 subjective randomness is in fact all we have, because objectively there is no randomness.
00:10:33.600 So we have this subjective feeling that what is going to happen in the future is uncertain,
00:10:38.680 but God's eye view of the multiverse, what's happening is unfolding according to the laws
00:10:43.280 of physics, which are not probabilistic, they're deterministic, they are determining precisely
00:10:48.160 what is happening at every point and every moment in the universe.
00:10:51.680 And just on the side, of course, that is not always the best explanation of anything that
00:10:58.840 Knowledge creation is the most important exception to this rule.
00:11:02.680 When you want to explain what's going on, simply saying that things are determined according
00:11:07.280 to the laws of physics is not the best relevant explanation.
00:11:12.520 Throwing that aside, the point here is that subjectively, we lack knowledge of everything
00:11:18.600 that's going on that just necessarily is the case.
00:11:21.640 Things are uncertain from our perspective, and when I say our mean individually and collectively
00:11:29.760 And worstness as Keira has hinted at there and as many listeners will already be aware,
00:11:34.600 the laws of physics mandate we cannot know simultaneously to infinite precision all of
00:11:40.600 the factors that will come to bear on whatever outcome we're about to see in the future.
00:11:46.280 We'll come back to this when we speak about observing, copying and measuring, which is
00:11:49.800 part of this chapter, and is a new window into viewing these particular things, and we'll
00:11:55.280 see how those things observing, copying and measuring are actually related.
00:12:02.400 It is rather unfortunate that quantum theory has acquired, in the collective imagination,
00:12:07.360 the status of a quirky beast that is incomprehensible, but worthy of attention, because
00:12:15.120 You may recall the words, spooky action at a distance, used by Einstein to describe quantum
00:12:19.800 entanglement, or the creepy idea of locking a cat inside a box with poison, the notorious
00:12:24.840 thought experiment that Schrodinger envisaged to illustrate quantum superpositions.
00:12:29.440 With catchy phrases, the press has promoted the view that quantum theory is destined to remain
00:12:36.680 Leaving the good old days of Newtonian physics behind when the world used to make sense,
00:12:40.680 we now have to resign ourselves to a new and alien picture of physical reality, which
00:12:45.740 accords with the experimental evidence, but whose explanation of the universe is baffling,
00:12:52.440 I think Keira is being polite, polite to her colleagues, not to her close personal
00:12:59.040 colleagues, but to the wider physics community.
00:13:02.400 I don't think we can blame only the press here.
00:13:05.120 I think we can absolutely blame a generation or two of physicists and science communicators
00:13:12.280 for deliberately at times mystifying and obscuring what quantum theory is all about.
00:13:19.240 And I think they have done it consciously or not, for the same reason that priests, to some
00:13:25.640 extent, used to do this with holy books, it ensures that they maintain their authority
00:13:32.880 as the experts you need to go to, whose opinion you're going to seek, whose feet you're
00:13:37.520 going to sit cross leg that and listen to the deep wisdom because they are the ones in possession
00:13:43.760 Very few physicists are like, for example, David and Keira and associates who speak very
00:13:50.240 clearly about these issues, some like to revel in the mystery.
00:13:55.720 And so long as they revel in the mystery, people will keep looking at them as kind of
00:14:03.400 We need to demystify all areas of science that we possibly can, in particular quantum theory
00:14:09.200 because it's been mired in this bad philosophy, this bad way of trying to explain what is
00:14:14.280 really going on and this fear to some extent of trying to grapple with what it's actually
00:14:22.920 Now, the interesting thing here is that Keira is not actually going to even mention the multiverse
00:14:29.480 In fact, she doesn't even mention it in the entire book, except in the acknowledgements.
00:14:33.760 Now, there is an important reason for this that I'll probably come back to later.
00:14:39.360 And I think the reason is I haven't talked to Keira about this, I cannot possibly speak
00:14:44.680 But my guess is that she's using an important heuristic now, it's a heuristic that I heard
00:14:50.880 from David Deutsch many years ago, I think just after the publication of the fabric of
00:14:55.640 reality, I think someone was interviewing him about the fabric of reality.
00:14:59.440 And he said at the time, words of the effect that he thought it far more important to just
00:15:06.160 explain the best explanations that we had and to make progress by taking those explanations
00:15:13.000 seriously, taking them for granted in a certain sense and just building on top of them,
00:15:18.200 standing on the shoulders of the giants that were those explanations and going further
00:15:22.760 and seeing further, rather than simply fighting the old fights, debating the old debates, trying
00:15:32.480 If you want to make progress in genetics and biology, there's no point standing up on stage
00:15:38.680 spending most of your career and your working life debating with creationists.
00:15:43.480 That's not a good way to make progress in science.
00:15:46.960 But similarly, if we're quantum physicists like Yara, we just take it for granted that what
00:15:53.680 quantum physics is saying about the world is the multiverse is true, okay, that we really
00:16:00.000 So we don't have to worry about trying to re-explain what the multiverse is to defend
00:16:04.880 the multiverse at every single point if we're going further and building on what quantum
00:16:10.400 theory is, it's for the same reason that anyone who writes a book about genetics or evolutionary
00:16:16.080 biology does not have to spend multiple chapters saying why evolution by natural selection
00:16:23.000 is actually true and why creationism is actually wrong, you just take it for granted.
00:16:28.920 And so my guess is that Kara is just taking for granted the reality of the multiverse,
00:16:36.040 which I think is great, but just to say that this stuff about the weird catchy phrases
00:16:43.520 have not only been instilled into the scientific conversational zeitgeist, so to speak,
00:16:51.680 by the press, by people reporting on science, and even today some of the physicists themselves,
00:16:57.520 the way in which they talk and public group promote, especially quantum theory, has overtones
00:17:04.480 of weird, mystical, almost spiritual kind of overtones and it has a lot of baggage associated
00:17:12.440 It can do away with all that, certainly by taking it realistically, and accepting the fact
00:17:21.540 None of this nonsense, none of this is even remotely fair to quantum theory.
00:17:26.440 Yes the quantum world seems bizarre and counterintuitive at first, but in reality it is
00:17:31.440 fascinating, subtle, surprising, and above all not mysterious.
00:17:37.960 The more it is understood, the more exciting it becomes.
00:17:40.680 It is true, though, that quantum phenomena cannot be expressed entirely in terms of familiar
00:17:47.860 As you were about to discover, there is a genuinely new, dazzling set of properties that
00:17:52.780 quantum systems have, which are conceptually far from our everyday worldview.
00:17:57.980 And they are all based on a set of simple counterfactuals, which I shall now explore.
00:18:03.260 Incidentally, quantum phenomena are important for the progress of our civilization because
00:18:07.140 they allow for the enhanced information processing capabilities of quantum systems, which
00:18:12.000 our class non-quantum information media, such as those deployed by the classical computers
00:18:17.960 The quantum counterfactuals are the fuel for the next technological revolution, the universal
00:18:23.600 A universal quantum computer is a universal computer, that is, a computer that is capable
00:18:29.680 of performing every computation that is allowed by given laws of physics, as I explained
00:18:35.240 That relies entirely on quantum theory for its information processing.
00:18:40.360 The computers we currently use are classical, because they rely not on quantum phenomena
00:18:45.080 to perform computations, but on entirely classical mechanisms.
00:18:49.240 The theoretical description of the universal quantum computer has been around since the 1980s,
00:18:55.960 It has superior computational abilities compared with classical computers, because
00:19:00.520 its elementary information units, the quantum bits, can explore a much richer set of possibilities
00:19:08.840 The effect of this richness, which is entirely due to quantum physics effects, is that
00:19:13.360 the universal quantum computer can be faster and more efficient than a classical computer
00:19:18.240 when it comes to certain computational tasks, searching a large database, factoring a
00:19:23.560 What is important here for you to have in mind, is that the leap in possibilities that
00:19:27.800 a universal quantum computer would bring about is analogous to that brought about by the
00:19:32.960 introduction of the classical computers in the first place.
00:19:37.480 It will make an entirely new class of technological improvements possible in the realm
00:19:41.960 of information processing, alas, the actual realization of a universal quantum computer
00:19:51.240 This enterprise is engaged some of the finest minds among physicists, engineers, and material
00:19:56.840 Numerous IT companies such as Google, IBM, and Microsoft, and startups as well are now
00:20:01.960 trying to race towards the first viable prototype of this machine, but we are still quite
00:20:07.560 away, even if we are definitely getting closer, pause there, my reflection.
00:20:12.920 Okay, so this thing about being quite a ways away.
00:20:17.040 It's been said for decades now, and we hope that we're getting closer, and it seems
00:20:23.040 as though sometimes we're getting closer, and then it seems like we're not, we often get these
00:20:26.800 announcements, especially by the very companies mentioned there by Kiara.
00:20:31.880 Google has announced more than once over the decades now that they've got a functioning
00:20:40.800 I don't buy it, and the reason I don't buy it, well, yeah, maybe they have some weird
00:20:47.520 I think we'd know, I think that'd be an actual leak from the company, I mean there'd be high
00:20:53.040 financial incentives for anyone within that company working on such a technology to actually
00:20:59.160 take the information somewhere else and sell it at a really high price, but that's not
00:21:04.640 Okay, once we have a quantum computer in one place, what I'm saying is we'll end up having
00:21:08.720 a quantum computer in lots of places simultaneously, or the word will quickly get out.
00:21:14.800 The second thing is, as I've mentioned in previous episodes, I think the multiverse chapter
00:21:20.360 in particular of the beginning of infinity, the engineering difficulties with quantum
00:21:26.480 computation are something I'm a little familiar with.
00:21:32.560 I'm a layperson who takes a special interest, but I'm definitely not an expert, but because
00:21:38.240 of my special interest, I've visited one of my local universities who actually have a
00:21:43.840 center for quantum computation, the university of New South Wales, and I've had some discussions
00:21:49.600 with some of the people there, including the person that runs the entire place.
00:21:53.720 And their quantum computer, the best they can do at the moment, is one or two qubits.
00:22:01.160 You hear, you know, when places like Google make these announcements, like, oh, we've
00:22:05.160 got heaps and heaps of qubits, hundreds of qubits or something or other, and now and again,
00:22:10.480 I think it's all a scam because of the way in which I understand at least some of the hardware
00:22:17.160 I've heard tell that there are alternatives to the way in which you could have these quantum
00:22:24.320 I don't fully get these alternatives, but the way in which I've heard these quantum computers
00:22:28.680 work, at least the university of New South Wales, I think they use ion trapping.
00:22:33.360 They use phosphorus type atoms, and they have to keep these things very, very cold.
00:22:37.320 I have to keep it cold so that the information doesn't leak out, so to speak, so that
00:22:42.240 the, and we're going to come to this in the chapter, actually, that we're reading right
00:22:45.160 now, so that the, the atoms remain entangled so they can do the job of computation.
00:22:51.680 Entanglement says technical term where the two atoms are kind of behaving like the one
00:23:01.240 And the only way to ensure this entanglement remains in place, at least in this particular
00:23:06.800 setup, is to have them not moving around very much.
00:23:10.080 Once they start to move around, they deco here, they go here fancy name for lose the information.
00:23:15.320 They become entangled with the rest of the environment, but you don't want them entangled
00:23:20.560 You want these two atoms, these two particles, to only be entangled with each other in order
00:23:27.080 We can rather think of it like, this is a poor analogy, but you can rather think of it
00:23:30.400 like, the electricity inside of the computer is something that you want to keep inside of
00:23:35.800 Once the electricity starts to get into the computer from the outside, you've got serious
00:23:41.160 So once the electricity that's in your computer starts leaking to the outside world, you've
00:23:47.720 You don't want additional electricity, additional electrons wandering through the circuits
00:23:54.720 So in order to maintain the entanglement of the particles in the quantum computer, at least
00:24:00.160 at the University of New South Wales, and I think this is the way many different kinds
00:24:04.480 of races towards an actual quantum computer are working is you have to get these things
00:24:09.440 very cold, and you need to get down to near absolute zero, and I've told this story
00:24:14.120 before, and at UNSW, what they do is they use liquid helium, which gets you down to about
00:24:19.880 minus 270, something like that, but that's still not cold enough.
00:24:23.840 And so what they do is they use evaporative cooling, and so they have this isotope of helium,
00:24:29.040 helium 3, the light isotope of helium, and they use that to evaporate away from the
00:24:33.840 helium 4, in which the atoms of phosphorus are being bathed, and as that evaporative cooling
00:24:41.000 happens, you can actually get down to minus 272 points, something or other.
00:24:45.600 So it's very, very close to absolute zero, and they need to get down to that so that
00:24:52.160 Now, an interesting question was said to them, where do you get helium 3 from?
00:24:58.640 And the curious thing was, I don't know if they were supposed to tell me this or not,
00:25:01.480 but they did tell me, maybe it's public knowledge, they said that, well, the American
00:25:05.880 army of all people sponsors their project, sponsors their race towards building this first
00:25:13.760 Of all the people, the American army, you know, I said, American army, like, why, why
00:25:20.760 The project at UNSW, the University of New South Wales is running, requires the helium 3.
00:25:26.880 The American army, being the military, would like to ensure that they have access to the
00:25:34.040 I'm sure they're sponsoring a lot of such projects, okay?
00:25:36.560 America, for defense reasons, wants to ensure they're not left behind.
00:25:40.640 And so they are supporting lots and lots of people financially in order to build a quantum
00:25:47.520 But the way in which the American army gets the helium 3 is from the radioactive decomposition
00:25:54.400 of uranium, I think it is, might be plutonium, whatever, one of the radioactive materials
00:26:01.600 And so their nuclear bombs are irradiating this stuff and they collect it in big balloons
00:26:06.720 and then they send it to people like the University of New South Wales to do research
00:26:11.560 So that's a curious way in which nuclear bombs are assisting with research towards quantum
00:26:19.640 Anyways, for even more details about that, again, my multiverse series has some more details.
00:26:26.040 And I'm picking it up where Cara says of herself, and I would also agree with this, quote,
00:26:32.040 I belong to the cohort of people who look at technological developments in awe with optimism
00:26:37.640 and high expectations, but are ultimately more interested in the foundations of the theories
00:26:45.400 What is it precisely about quantum media that makes them capable of supporting such
00:26:50.520 super-efficient quantum information processing?
00:26:53.400 What can one learn by looking at the foundations if the technology is already pushing ahead?
00:26:58.920 In fact, by digging into the foundations of quantum theory, we stumble upon a surprising
00:27:03.840 All properties of quantum systems, which are crucial for the universal quantum computer
00:27:08.200 and the related quantum technologies, rest on a few elementary counterfactual properties.
00:27:13.800 In chapter 3, I explain that information media assistance with attributes that can be
00:27:19.880 Quantum systems have these counterfactual properties, and therefore are information media
00:27:25.640 But they have more counterfactual properties, making them so much more powerful.
00:27:29.960 To see what these properties are and understand why quantum systems are a more powerful
00:27:33.840 kind of information medium, I shall invite you to look again at the game of chance with
00:27:38.400 cups, now through the lens of information theory.
00:27:42.160 The two cups together with the marble are using the terminology of chapter 3 and information
00:27:47.840 They can contain a bit of information encoded in the position of the marble, as you can
00:27:51.840 see in the figurine screen from page 111 of the book.
00:27:55.720 When the marble is under the cup on the right, it encodes the value 0.
00:28:00.280 When it is under the cup on the left, it encodes the value 1.
00:28:03.400 You can imagine a standard procedure to set up the game, first toss a coin.
00:28:07.760 If the coin shows heads, put the marble under the cup on the left.
00:28:11.280 If it shows tails, put it under the one on the right.
00:28:14.520 At this point for the player, the bit has perfectly randomised and maximally unpredictable
00:28:22.400 The marble could be either under the cup on the right or that on the left, each with
00:28:28.440 For the person who sets up the game, however, the bit has a definite value, either
00:28:36.440 Now imagine setting up the game with systems that behave according to quantum theory.
00:28:40.880 Not simple information media, but quantum information media.
00:28:47.640 Our quantum game involves a photon, a quantum particle of light, instead of the marble,
00:28:59.680 As you might guess, again, if you've read the beginning of infinity, if you've listened
00:29:05.440 to my podcasts on the beginning of infinity, what we are about to describe is the mark zender
00:29:13.320 What I like to remind people of, as we're discussing something like this, is that these
00:29:22.640 They're not abstract ideals, and because they're really existing physical objects,
00:29:28.080 you just have to accept the fact they're obeying the laws of quantum theory.
00:29:33.240 They might violate your common sense, but so much for your common sense.
00:29:38.320 You might think you know how a mirror works, or a crystal works, or how light bouncing
00:29:46.040 And you could be completely wrong about all of it.
00:29:48.040 So we have to take at face value to a certain extent.
00:29:54.640 But to another extent, you can always go and check yourself.
00:29:58.840 You can always do the experiment yourself if you were willing to try hard enough.
00:30:06.160 You can see for yourself, you can check for yourself.
00:30:11.400 It simply is the case that there are some strange and unusual results that we're about
00:30:18.920 So we're discussing this quantum game involving a photon.
00:30:23.760 In chiarants, a source emits the photon, then the photon can travel straight along the horizontal
00:30:31.360 The photon and its path constitute an information medium.
00:30:37.000 If the photon travels on the horizontal path, it encodes a zero.
00:30:40.840 If it goes on the vertical path, it encodes a one.
00:30:43.840 It is possible to set up the game by following the randomization procedure I explained earlier,
00:30:51.720 For example, someone sets the photon to travel vertically or horizontally according to the
00:30:56.000 output of a coin toss, again the chances are a half for the player to guess correctly,
00:31:01.800 As you can see, this version of the game is not different from the marble because it does
00:31:05.360 not use quantum properties of the photon in any way.
00:31:09.120 We need to explore some other kind of setup, using the quantum properties over the photon.
00:31:14.360 What is the quantum stuff that a photon can do while a marble cannot?
00:31:18.120 It can be prepared in states that are exclusively quantum, in the sense that they do not
00:31:22.440 exist according to classical physics, but only under quantum physics.
00:31:26.640 An example of these chiefly quantum states, which is relevant for the photon in this example,
00:31:32.080 is what I shall call a superposition of the horizontal and vertical path.
00:31:37.200 In order to understand what kind of state this is, how it is related to the state, where
00:31:42.600 the photon is on a definite path, and what counterfactuals have to do with all this, we
00:31:48.080 need to look at a definite experiment where the photon is prepared in a superposition
00:31:53.280 of different states, and then certain measurements are performed on the photon.
00:31:57.880 This will tell us how the superposition is in a certain sense similar to the state of
00:32:02.160 the marble, but is fundamentally very different.
00:32:04.920 The photon, after having been emitted by a source, can be put into a superposition of
00:32:08.960 paths by guiding it through a special kind of crystal, which when interacting with the photon
00:32:14.200 causes it to split along two paths, horizontal and vertical, as in the figure.
00:32:19.680 If you were to guide a beam of light, made of lots of photons through this crystal, you
00:32:24.640 would see that the beam is split across the two paths, horizontal and vertical, which is
00:32:29.920 why sometimes this crystal is called a beam splitter, but here I am talking about a single
00:32:35.200 photon, not a beam of light, and what it means for it to be split can be understood only
00:32:42.000 So what do we mean by the photon being split across two different paths in a quantum
00:32:47.200 Well, one key aspect of the superposition is that after passing through the crystal,
00:32:52.160 the photon could be found with probability a half on the horizontal path, and with probability
00:32:59.320 When the photon is in a superposition of different paths, it is impossible to predict
00:33:09.800 Experiments happening daily around the globe in the quantum laboratories confirm this
00:33:14.640 behavior to the highest precision, pausing their myreflection.
00:33:18.600 Okay, so on this point about split, now, Chiara here is avoiding the terminology of broader
00:33:28.360 The way I would phrase this if I was explaining this to someone is that, of course, the
00:33:34.080 photon is a quantum object, and as a quantum object, it is made up of fungible instances.
00:33:41.040 And so what a photon is is a multiverse object, such that when it collides with a physical
00:33:50.040 crystal of the kind that Chiara is describing, or it could be a half-silvered mirror, whatever
00:33:55.240 you want, half of the instances bounce off, and go vertically, and half of the instances
00:34:01.320 are transmitted through the crystal and go horizontally.
00:34:06.440 The splitting is the differentiation of the instances.
00:34:14.080 The laws of physics say that this is what happens.
00:34:16.640 It's almost kind of like when people ask, if they do ask, you know, why does an object
00:34:22.760 in motion stay in motion unless acted on by a force?
00:34:27.000 There's no force acting, and there's no reason for it to change its course of motion.
00:34:32.040 This is one of Wittgenstein's lines, you know, my spade is turned, you can't explain
00:34:38.880 And so if you do have this possibility, this physical possibility of the photon taking one
00:34:44.200 path or the other, then it can indeed take both paths, and it doesn't indeed take both
00:34:48.560 paths in this particular situation, and it's testable.
00:34:52.720 You can test the claim that it only takes one path by doing the experiment.
00:34:57.960 And if you have the hypothesis, it must only take one path, well, you will find repeating
00:35:03.960 But in fact, it takes both paths, repeating the experiment often enough.
00:35:09.120 So it approximates what happens in the multiverse, okay, back to the book, okay, all right.
00:35:16.360 Does this mean then that when the photon is in a path superposition, its properties are
00:35:21.200 exhausted by saying with what probability the photon is on one path and with what probability
00:35:26.560 is on the other, just like for the marble in our classical example with two cups?
00:35:31.280 No, in reality, the story is much more subtle, quantum superpositions are not about probabilities.
00:35:37.800 The photon is not a randomized bit, even if, in some instances, it can look like one.
00:35:43.360 The first difference from a randomized bit, such as the marble, is that no one can predict
00:35:51.080 Even the person who prepared the photon superposition cannot predict that.
00:35:54.280 The unpredictability of the photon path is absolute, unlike in the case of randomization,
00:36:02.320 Now, the reason for this, by the way, is because, well, as we're going to get to, everything
00:36:05.400 about what the experimenter is doing and what a person who tries to make a measurement here
00:36:10.800 And the reason that it's subjective is because you're in a particular universe with a
00:36:17.120 Copies of you are in the other universes with instances traveling along the other path.
00:36:21.680 And so when you make the measurement, you will always only ever detect if the photon
00:36:24.640 traveling along one of the two paths, you can't possibly detect it traveling upon both
00:36:29.680 paths, even though it does, even though it does travel along both paths.
00:36:33.400 It's in this superposition of traveling along these paths, but you will only detect it
00:36:37.880 along one of the paths because of the differentiation of the universes.
00:36:42.560 And Kiara is just saying, here, this is the way it is.
00:36:48.560 She goes on to say, quote, the person who prepared the photon knows all the details about
00:36:52.760 the situation, yet cannot predict where the photon is.
00:36:56.400 The photon is in a quantum superposition of two different paths.
00:37:00.720 When in that state, it does not have a definite position.
00:37:04.160 And if you were to measure its position, that measurement would have an unpredictable outcome.
00:37:09.080 So the quantum unpredictability associated with superpositions does not come from the lack
00:37:15.040 of information about the preparation of the photon, just pausing it yet, just so just to
00:37:18.960 emphasize that point, the real-life version of this experiment requires preparation of the
00:37:25.560 Preparation of the photon requires something like the energy of the photon to be tuned.
00:37:34.520 And given a particular set up of the apparatus, these things again are physical, then
00:37:41.200 upon encountering the crystal, which has a certain thickness, which has certain properties,
00:37:46.400 and it will cause phase changes in the photon, the way in which the photon reacts to striking
00:37:54.480 the crystal, to passing through or bouncing off the crystal.
00:37:57.600 And even that passing through and bouncing off is not a simple process at all.
00:38:04.520 When a photon passes through something like a crystal, what's actually going on is the
00:38:08.880 photon is absorbed by the atoms of the crystal and then re-emitted, absorbed and
00:38:12.920 re-emitted, absorbed and re-emitted, this process goes on.
00:38:15.560 It actually slows down the travel time of any beam of light or ray of light travelling
00:38:20.520 through it, because the absorption and re-emission of these photons takes time, even though
00:38:25.480 the photons always only have a travel at the speed of light, the absorption and re-emission
00:38:30.400 So it slows the photon down as it goes through the crystal.
00:38:33.760 And this is, by the way, why you need this second crystal as we're about to get to in
00:38:41.760 But the two photons that travel along these two different paths actually travel along exactly
00:38:48.160 Almost exactly the same length, they're out of phase by a certain amount, and because
00:38:52.560 they're out of phase you get a certain amount of interference, which always causes the
00:38:56.360 photon to travelling one direction and not the other.
00:38:58.760 Again, same by multi-verse series for the entire explanation of this.
00:39:02.240 Okay, so I'm skipping a little bit and I'm picking it up where Chiara writes, quite.
00:39:07.000 The second significant difference can be seen by repeating the game after having played
00:39:10.560 it once, for the marble you randomly put the marble under one of the two cups by tossing
00:39:16.280 Then you repeat the coin toss, and according to the coin value, reposition the marble
00:39:20.400 under one of the cups, the player has still the same chance of guessing correctly where
00:39:26.840 Indeed, adding uncertainty to an already uncertain situation can only make things equally
00:39:31.200 or more uncertain, randomising once or twice, or a hundred times, leaves the unpredictability
00:39:36.560 the same, repeating the steps of the preparation does not change the odds of guessing correctly.
00:39:44.960 To repeat the preparation of the quantum superposition twice, you need to let the photon
00:39:52.120 In a real experiment, this can be done by arranging a second crystal after the first
00:39:56.080 and by setting up a system of mirrors, so that the photon would go through the first crystal
00:40:01.160 bounce off the mirror and go through the second crystal when travelling on both the vertical
00:40:05.000 and horizontal path, which path will the photon be on after encountering the second crystal?
00:40:10.920 If you think of each crystal, as randomising the photon path, judging from the marble example,
00:40:16.800 you would expect the photon to be found with probability of half on one path and one
00:40:25.800 The photon, after encountering the second crystal, will invariably end up on the same
00:40:34.000 If you believe that the crystal is simply randomising the photon path, the fact we have uncovered
00:40:41.040 That applying the same randomising procedure, the crystal twice, produces something certain
00:40:49.200 If it were, you could go to a casino and always win simply by waiting for the dice to
00:40:53.000 be rolled twice, or the cards to be shuffled twice.
00:40:58.400 As always happens with contradictions, something in the assumptions has to give.
00:41:03.680 The revelation here is that the crystal is not a randomising operation even though it looks
00:41:09.720 The quantum superposition, created by the beam splitter, unlike coin tossers, dice throws
00:41:15.680 and the like, cannot be described with probability only.
00:41:21.440 I first probably learnt about this surprising fact during my doctoral studies from
00:41:25.560 Arthur Eckert, who in the early days of his pioneering work on quantum cryptography, had
00:41:30.400 to think hard about how to explain to an incredulous scientific community what was so special
00:41:35.280 about quantum systems as opposed to simply randomised phenomena.
00:41:39.560 The explanation for this counterintuitive phenomenon resides at the heart of quantum theory.
00:41:44.800 So much so that this fact alone can be taken as a signature of the photon being a genuine
00:41:51.560 quantum mechanical particle, just pausing their myreflexion just to emphasise this.
00:41:55.920 We can see on the screen the picture here of what is in Kiara's book actually, of the
00:42:04.840 And the way in which she explains what's going on is that it seems as though, it seems
00:42:09.640 as though, what the crystal is doing is perfectly randomising things, after all, after all,
00:42:16.360 if you perform the experiment one photon at a time, then you can't predict whether it's
00:42:21.960 going to go straight horizontally or up vertically, you just don't know.
00:42:26.320 You can put detectors after the crystal in both of those places.
00:42:30.400 And sometimes it will go straight through and sometimes it will go up, sometimes it's
00:42:33.640 horizontal, sometimes it's vertical, you don't know.
00:42:35.920 It just appears to be completely and utterly random.
00:42:39.920 And so putting a second crystal there should be, seemingly, completely and utterly random.
00:42:45.000 Now, for an explanation of this, and I don't want to go into it now because it took me
00:42:48.360 some 15 minutes to explain it in the multiverse series.
00:42:51.600 If you go to talkcast episode 23 on YouTube in particular, it's, I've been calling it
00:42:58.720 It's chapter 11, chapter 11, the multiverse, chapter 11, the multiverse, part zero, part zero.
00:43:05.120 And in that part zero there, if you skip to like the 42 minute mark and the 42 minute
00:43:11.640 mark, I go, I spend a long time going into excruciating detail about the Markzendi interferometer.
00:43:19.200 And the mechanics of what's going on from one perspective that might give you an insight into
00:43:27.840 Now we're about to have another perspective as well.
00:43:31.600 And that other perspective is as Keira says, the assumption that it's simply randomizing
00:43:36.960 things, that it simply is purely random, whether it goes horizontal or goes vertical, is
00:43:45.640 As Keira said, and I'll say it again, and she said that she, she credits Artie Eckert
00:43:50.680 for this quote, what makes the photon so different from the randomized marble in the original
00:43:57.000 game is that once it has gone through the first crystal, once it has, another one of its
00:44:02.720 physical properties, not the position or the path is perfectly definite.
00:44:10.560 The property is that of being in that particular superposition of paths.
00:44:15.440 What's more, there is another counterintuitive fact, letting the photon through the second
00:44:19.240 beam splitter, and then measuring where it is, is one way of measuring that other property.
00:44:24.760 That is, measuring which superposition the photon is in.
00:44:27.800 Okay, just say that again, because that's such a subtle point.
00:44:33.160 So once the photon goes through the first beam splitter, in other words, the first half
00:44:38.320 silver mirror, the first crystal, everything is not equal.
00:44:50.840 There are physical properties here that you can play with.
00:44:54.000 Such that once that photon has gone through that first part of the apparatus, once the
00:44:58.700 photon has passed through the crystal, there is something about it that is perfectly definite.
00:45:07.520 Well, as Chiara is calling it there, it is this superposition property.
00:45:17.640 The reason we say this is because, well, in my understanding anyway, it has a certain kind
00:45:23.280 of phase, and the way I describe it in that other episode, is that, well, in the wave
00:45:30.240 theory of light, it's sort of vibrating upwards.
00:45:38.840 The more precise way of explaining what this is in quantum terms, but you can still speak
00:45:43.640 of individual photons as having a certain phase, such that whether it's gone horizontally
00:45:49.520 or whether it's gone vertically, it's got this particular one thing.
00:45:56.520 Or the better way of saying it, of constructing this explanation, is as Chiara has said there.
00:46:02.480 The property being, in that particular superposition of paths, okay, so it's in a superposition
00:46:10.360 There could be many different ways and many different kinds of superpositions of paths you
00:46:15.880 If you change the energy of the photon, you would have a different superposition of paths.
00:46:20.080 If you change the thickness of the crystal, a different superposition of paths.
00:46:24.520 But because you have a particular energy and a particular thickness of a crystal, amongst
00:46:29.240 other things, amongst other things, the distance between, for example, the mirrors and the
00:46:35.880 crystals would also affect this superposition of paths as far as I'm aware.
00:46:40.480 One thing that's different, and I keep coming back to this, is that the apparatus is
00:46:45.320 physical, so the crystal is physical, and bouncing off the front surface is not symmetrical
00:46:55.720 Although you end up 50-50, in terms of the proportion of instances of the photon either
00:47:01.720 going vertically or horizontally, the ones that end up going horizontally have passed through
00:47:07.600 that crystal, and that it's changed something about the photon.
00:47:11.760 Call it the phase if you like, whereas that kind of process has not happened with the
00:47:21.160 These two situations are not symmetrical, and therefore that comes to bear on what happens
00:47:27.640 to the two instances when they recombine at the second crystal, causing them to always
00:47:35.800 And the reason for this is that on recombination at the second crystal, the interference
00:47:43.240 could be constructive, could be destructive, there's different kinds of interference,
00:47:47.160 and it happens to be the case that the interference is such that these two instances recombine
00:47:53.240 such that the whole thing goes horizontally again.
00:47:56.520 But the point here is, if you're interested in the physical properties of what's going
00:48:00.880 on here, the actual physics of what's occurring, passing through end reflection, refraction
00:48:07.720 and reflection if you like, are not perfectly the same kind of thing.
00:48:12.840 And that's why it's not completely randomized, that they're not the same kind of groups
00:48:19.280 of photons, whatever the case, it has this perfectly definite property.
00:48:28.960 That is why we see a definite outcome after the second crystal.
00:48:33.520 A crystal creates a definite path superposition, then another crystal and a subsequent
00:48:38.480 measurement of where the photon is constitute jointly, a measurement of which superposition
00:48:45.560 The outcome at the end is definite because after the first crystal, the photon is in a
00:48:50.040 definite path superposition, but it does not have a definite path, okay, pausing their
00:48:56.720 So once you've passed through that first crystal, you have a definite path superposition,
00:49:05.960 Not a definite path because it's 50-50, it's half of the instances are going vertically
00:49:10.920 and half of the instances are being transmitted horizontally, so not a definite path, but
00:49:16.320 For more on this by the way, even more than what I've said, David Wallace himself, the
00:49:23.280 emergent multiverse guy actually has a lecture online about the Mark Zender interferometer
00:49:30.880 from the multiverse perspective, and so there is yet more information from this way of
00:49:39.680 It's hard to find, but it is out there, okay, so Kiara goes on to write, quote, so there
00:49:44.800 are two properties of the photon that play a role in the experiment with beam splitters,
00:49:49.240 the property which path, call that P, and the property which path superposition, call
00:49:55.360 that PS, counterfactuals come in at this important point to explain the relation between
00:50:00.640 these two properties in the case of quantum systems such as the photon.
00:50:04.840 If it is possible to predict the value of P, or a zontal or vertical, with certainty,
00:50:11.120 it is impossible to predict which path superposition the photon is in and vice versa.
00:50:16.960 When the photon is predictably travelling along a definite path and P is sharp, in the sense
00:50:22.320 that the photon has a definite value of its position, the other property, PS, is not sharp.
00:50:28.120 A measurement of PS yields an unpredictable outcome.
00:50:32.080 But when the photon has gone through the crystal once, the outcome of a path measurement
00:50:37.320 Its position P is not sharp anymore, whereas the other property PS has become sharp.
00:50:45.080 This relation between the two properties P and PS is based on counterfactuals, and as I
00:50:50.160 shall now illustrate, is that the heart of the notorious bore complementarity displayed
00:50:55.080 by quantum systems, the fact that different properties of quantum systems, such as energy
00:50:59.840 and position, or P and PS, cannot be simultaneously measured to arbitrarily high accuracy,
00:51:09.160 This is one of the most challenging parts of quantum theory, whether or many challenging
00:51:16.000 And some people just balk at it and just violate common sense, so they refuse to believe
00:51:22.320 I think this is something I've explained a few times.
00:51:25.760 You simply have to give up certain assumptions.
00:51:28.400 Like, for example, we know that a car going down the street can, in a common sense why,
00:51:37.760 It's travelling at 60 kilometres per hour, and it also has a position.
00:51:45.320 So we can say these two things without contradiction, without violating our common sense
00:51:49.600 nations, without seemingly violating any law of physics.
00:51:54.600 The problem is we are violating a law of physics if we're going to take things really,
00:52:01.520 Newtonian physics, the way in which I just described things, or even Galileo's physics,
00:52:06.600 allows us to say that the car has got this particular velocity, 60 kilometres per hour,
00:52:13.720 And we are just so very used to speaking in that way that we think, how could it possibly
00:52:20.520 be otherwise, but in quantum physics, it can be otherwise.
00:52:25.520 Not only can it be otherwise, it simply is otherwise.
00:52:30.040 The observables, the things like the velocity, the things like the position of the car cannot
00:52:36.640 be known with arbitrarily high accuracy simultaneously.
00:52:42.880 We can't both know exactly what the position is, and exactly what the velocity is.
00:52:46.920 Now it's unproblematic for a car, because the car is so big.
00:52:51.840 But when we get down to individual particles, it becomes significant.
00:52:56.960 The effect is significant, measurable, noticeable, important.
00:53:02.120 So we simply cannot say of something like an electron.
00:53:06.200 It is precisely there and travelling at this particular speed.
00:53:10.360 Now, there are many ways of getting to this truth.
00:53:13.440 One is due to the fact that, well, if you were to try and find out what the position
00:53:19.720 of an electron is at any particular point, think about what finding out really means.
00:53:35.080 Now you have to interact with the car when you look at it.
00:53:39.040 But looking at a car means shining a light on it, seeing where it is, and the light doesn't
00:53:42.880 affect the car, but in the case of the electron, if you want to know where that electron
00:53:49.280 The photon is going to collide with that electron, so you know where it is.
00:53:56.000 And that photon can only give you so much information once that collision between the
00:54:00.280 photon and the electron has happened, among other things.
00:54:04.880 This is just the way the universe happens to be.
00:54:07.280 So you simply have to just get over the sense that you can know these things similar
00:54:16.040 That's the way in which we understand the observations that we're making.
00:54:20.640 The way in which we understand these strange quantum effects is precisely by speaking in
00:54:28.560 We cannot have perfect precision simultaneously of these two things.
00:54:32.440 So you have to give up on this simultaneously knowing these two things at the same time.
00:54:37.880 The rest of quantum physics only makes sense if you do give that up.
00:54:42.080 But if you refuse to give them up, well, you're kind of in the position of rejecting
00:54:46.520 that the explanation of the apparent motion of the Sun across the sky is, in fact, not
00:54:52.200 the motion of the Sun across the sky at all, but rather the rotation of the Earth.
00:54:57.680 The Earth revolving is the actual motion that's going on, causing the appearance of the
00:55:05.680 So making the mental shift to a rotating Earth, a revolving Earth, to a situation where
00:55:11.520 quantum physics says of systems, they do not have A-speed and A-position.
00:55:18.040 They have both, and they're intimately related.
00:55:24.640 That's the kind of shift in your mind you need to make.
00:55:26.960 And this is all related to the fact that in quantum theory, remember, an object, let's
00:55:31.840 be simple and just call it a particle, or a particle is actually a kind of complex thing.
00:55:37.320 Well, that was fundamental, that's one way I've talking about it.
00:55:41.440 It's not the case that it's just one single photon, as we've said before.
00:55:45.040 And so when this thing we call a photon, which is not one single thing, encounters something
00:55:49.560 like a crystal, as Chiara has explained, it ends up being in this superposition of states.
00:55:54.560 The superposition of the design could be one of any number of possible superpositions
00:55:59.600 it depends upon the physical properties of the crystal, as we've said.
00:56:08.040 And indeed, someone asked me recently about, well, do particles really exist?
00:56:16.200 And in that theory, the field is the most fundamental thing, not the particles.
00:56:20.720 And well, my response to that is, particles still exist.
00:56:24.040 It's just that we can understand them through these fields.
00:56:26.640 We can always get a better, deeper understanding of what these things are.
00:56:31.080 It's not like the Greeks came up with what atoms are, these indivisible things.
00:56:35.960 And then once they had decided what the theory of atoms were, we could never have a deeper
00:56:40.600 Not turns out we still have atoms, it's just that we understand them more deeply.
00:56:44.920 We know that they have nuclei and electrons, and those nuclei contain particles of neutrons
00:56:51.320 And indeed, those protons and neutrons contain particles within them as well.
00:56:54.160 So atoms become this rich area of delving of a deeper into our understanding of what this
00:57:04.240 We can call them vibrations of the quantum field.
00:57:09.360 We can call them an ensemble of fungible instances.
00:57:13.360 These are all different ways of coming out the same idea, our best understanding of what
00:57:21.440 These entities that exist within quantum theory are at the moment.
00:57:25.040 And so, because we get to these ever deeper understandings, our intuitions, not surprisingly,
00:57:34.600 What does all this tell us of the information theoretic properties of the photon?
00:57:38.320 The counterfactual property I've just uncovered provides the key to answering this question.
00:57:43.400 In chapter 3, copyability emerged as one of the characteristics of information media.
00:57:49.720 Now you are about to discover that the copying task is much more ubiquitous than it may seem
00:57:57.360 It does not pertain to the world of computers and digital machines only.
00:58:00.560 The task of copying and that of measuring anything physical are fundamentally the same.
00:58:07.440 An apparatus that measures a given property is a system that, when given in input some system,
00:58:13.720 provides in output the value of the relevant property of the system as it really is.
00:58:18.720 A kitchen scale is a familiar example of a measuring apparatus that measures the mass of
00:58:23.640 When given some amount of flour, for instance, it gives an indication of the mass of the
00:58:27.800 If it is a perfect scale, when given an input of one kilogram of flour, it should give
00:58:30.720 us a reading exactly one kilogram on its display.
00:58:34.400 When provided say 10 kilograms of flour, it should read 10 kilograms exactly, and so on.
00:58:39.840 The transformation that the scale realizes has precisely the same form as a copy operation
00:58:45.320 because it copies the value of the mass given in input into the display of the scale.
00:58:52.520 You have just encountered a new important fact, a fundamental link between different
00:58:57.960 Things that can be copied can also be measured, and vice versa.
00:59:01.840 Another example of a measuring apparatus is the measure of the photon position.
00:59:06.760 It is a device that when given in input a photon traveling on the vertical or horizontal
00:59:11.560 path, as in our earlier example, can display a message saying, respectively, photon on the
00:59:17.560 vertical path or photon on the horizontal path.
00:59:20.720 Once more, this apparatus copies the value of the path, horizontal or vertical, from the
00:59:27.440 Okay, supposing there, just to my reflection on this, very brief reflection, I found
00:59:34.040 This idea that copyability and measureability are the same thing, they're the same property.
00:59:40.960 And Kiara says that properties that can be copied, she's going to call observables.
00:59:46.760 Now I'm going to skip a significant part of the book here, and she talks about how, well,
00:59:52.320 in the example that P and the SP are not simultaneously copyable.
00:59:57.320 And the reason is, because you can't measure them both simultaneously, well, you cannot
1:00:02.000 measure them both to arbitrarily high precision simultaneously.
1:00:05.960 And I'm going to pick it up where she writes.
1:00:07.960 Kiara is an important and fascinating conclusion.
1:00:10.800 Quantum systems have at least two observables, such as P and SP, which are not copyable jointly
1:00:18.200 or not measurable jointly to the same arbitrarily, high accuracy.
1:00:22.640 It is a can of actual property to do with what is impossible to perform on quantum systems.
1:00:28.480 It also constitutes the crucial difference between the classical unpredictability of a coin
1:00:32.920 toss and the quantum unpredictability arising with quantum superpositions as for the photon
1:00:40.080 Is that all there is to quantum systems and their counterfactual properties?
1:00:44.960 You need another counterfactual property to capture quantum information media.
1:00:54.520 Reversibility in physics usually refers to the possibility of reversing a transformation.
1:00:59.440 A transformation is physically reversible if whenever there is a way to perform it, performing
1:01:04.080 it in the reverse direction is also possible.
1:01:07.160 When you cross a bridge from one side of the other, you're performing irreversible transformation.
1:01:12.280 Flipping a bit from zero to one is also reversible.
1:01:15.080 But cooking an egg is not a reversible transformation, nor is splitting the egg on the floor.
1:01:20.720 A photon, when it behaves in a quantum way, must have the counterfactual property that
1:01:25.240 all transformations allowed on it, can be reversed.
1:01:28.640 So if you apply, for example, the crystal on the photon, you should be able to use the
1:01:34.720 So here is the main conclusion of this discussion.
1:01:37.480 A quantum information medium is a system with the following counterfactual properties.
1:01:44.400 It has at least two information variables, such as P and PS, that are impossible to copy
1:01:51.360 simultaneously to arbitrarily high accuracy, non-copiability of information variables, too.
1:01:59.280 It must be possible to reverse all the transformations involving these variables, reversibility.
1:02:05.360 The smallest unit of quantum information is a quantum bit, a qubit.
1:02:09.600 Photons, electrons, and other elementary particles can all be used as qubits.
1:02:13.280 The reason why perfect qubits are hard to combine everyday life, and the universal quantum
1:02:17.440 computer is very hard to realise in practice, is that accurate reversibility is extremely
1:02:22.160 difficult to achieve in practice, while at the same time preserving the other quantum
1:02:26.200 properties of the physical object in question.
1:02:29.520 Quantum theory says that it is possible, but it arises only under carefully controlled
1:02:37.040 Most photons around us, such as those emitted from the sunlight, do not undergo reversible
1:02:40.640 transformations, when left to naturally occurring conditions, pausing their microflection.
1:02:46.240 Yes, this is the problem of decoherence, so we have the issue of trying to ensure that
1:02:53.840 the quantum properties that are going to participate in a particular computation are going
1:02:59.920 to be preserved over time, without leaking into the environment, or without the environment
1:03:05.760 leaking into our quantum computer and becoming entangled with our quantum computer.
1:03:09.600 Now, I'm skipping another substantial part of the chapter here, and I'm picking it up where
1:03:15.840 Cara is about to talk about entanglement and its relevance here.
1:03:20.480 And she writes, quote, entanglement is one of the most exotic and powerful and misunderstood
1:03:28.800 When its properties were fully discovered, it soon became clear that it would revolutionise
1:03:32.800 the way we understand composite quantum systems, that is to say systems made of two or
1:03:39.640 The concept was already known to the pioneers of quantum theory, Schrodinger is usually credited
1:03:44.640 with introducing the idea, but the full potential of entanglement-based ideas was unleashed
1:03:52.080 When that was first considered as a resource for quantum computation.
1:03:56.560 The physicist, Lattco Vedrol, who in the 1990s pioneered our most subtle measures of entanglement,
1:04:03.200 often jokes about the explosive development of the field, remarking that he managed to
1:04:07.560 get an academic job, simply by working on the foundations of this elusive and fascinating
1:04:14.200 Those were times when working on fundamental and adventurous topics were still encouraged
1:04:17.800 in academia in the current academic monoculture, pursuing transformative and risky projects
1:04:28.680 Yes, this is a modern yet perennial problem now in physics in particular, but science more broadly
1:04:41.240 But wrong with having a career in science, absolutely nothing except that in the industry
1:04:48.120 that is science today at a university, for example, at a research institute, what you are
1:04:54.600 rewarded for is doing research, publishing stuff.
1:05:00.280 If you need to demonstrate that you are a competent scientist, according to your boss,
1:05:05.680 then what you might want to do is to publish regularly.
1:05:09.840 And if you are not publishing regularly, then your boss might start asking questions,
1:05:14.680 the management, the administrators, what are you doing with all that time you have?
1:05:23.960 We need papers to be published in journals that say the name of the university that you
1:05:30.920 We need to be promoted out there so that students will sign up to our university courses.
1:05:37.680 But it's kind of the issue and so what happens in that kind of culture is you have people
1:05:43.120 working on small things, incremental things.
1:05:47.160 They don't take a risk to really challenge the foundations, let's say, or physics, and
1:05:51.200 to make huge breakthroughs because that takes a lot of time.
1:05:55.240 You've really got to sit down for ages and ages and ages and you can't waste your time
1:05:59.120 publishing on small incremental things if you're aiming at the big discoveries.
1:06:04.680 But how many universities are going to employ someone like that?
1:06:08.680 How many research institutes are going to find interest in supporting someone like that?
1:06:15.360 What they're interested in is public outreach.
1:06:19.080 A way of demonstrating that look at our university, our physics department gets published
1:06:24.360 more often in more journals than anyone else.
1:06:27.160 Look at how good our research department is.
1:06:30.600 This is the problem with having such metrics, ways of determining who's a good researcher
1:06:38.480 How well are you going to objectively, in scare quotes, assess one physicist, for example,
1:06:45.120 against another physicist, if not how often they're publishing.
1:06:49.160 So this is why Cara says, working on adventurous topics.
1:06:53.000 Topics which might very well fail, they're risky because they're risky, they might fail.
1:06:57.920 But if they succeed, there's a huge upside, but universities may not want to take risks.
1:07:05.240 And this can kind of put a dampener on risk-taking and then for progress.
1:07:11.160 That takes us far afield from the chapter, that's just a little hobby horse of mine.
1:07:16.560 Cara writes, quote, entanglement arises when you have two or more quantum entities interacting.
1:07:22.480 For example, two photons or an electron and a photon, the essential feature of entanglement
1:07:27.880 is called quantum systems, is that the information one can gain by jointly observing the
1:07:32.880 systems is more than the information obtained by observing each system separately.
1:07:39.280 This phenomenon is rather counterintuitive and removed from everyday experience, pausing
1:07:45.120 Now, I would encourage you to, of course, refer to the book and Cara will go through an
1:07:49.720 example to try and help you understand what entanglement is.
1:07:54.160 The way I like to think about entanglement myself is that you have something kind of fundamental
1:08:01.080 But if it's entangled with something else, then the two particles together are acting
1:08:06.760 You can't think of them separately anymore.
1:08:15.240 And the overall, the whole is greater than the sum of the parts, as we like to say.
1:08:19.440 An entanglement is a form of that, and as Cara goes on to say later.
1:08:25.400 For example, with qubits of qubits are entangled, quote, it is possible to extract information
1:08:30.640 globally, acting on both qubits, but impossible to do locally, acting on each qubit separately.
1:08:38.120 This fascinating fact is not just curious, it is actually practically useful.
1:08:42.280 For instance, if you wish to hide information in the two qubits like in a safe, it is the
1:08:46.880 base for entanglement-based quantum cryptography, where entanglement is used in order to
1:08:51.600 transmit information securely by exploiting the fact that by looking only at the two
1:08:56.200 qubits individually, it is not possible to guess the joint state of the two qubits
1:09:02.040 Entanglement is often used to try and again, mystify quantum theory in all sorts of unfortunate
1:09:08.280 People will say things like it violates the speed of light restriction from relativity.
1:09:15.000 This is what Einstein was referring to with spooky action at the distance.
1:09:18.800 The fact is, in order to entangle particles, that has to be done locally, and the measurement
1:09:23.320 of particles has to also be done locally, and so then if you end up comparing one particle
1:09:27.960 to another that is separated by a huge distance, and if you think the information is
1:09:31.560 traveling from one particle to another, fasten a bit of light, the only way you can try
1:09:35.840 and establish that this is going on is to bring the two particles close enough together
1:09:40.240 to compare them again, and that always happens locally as well.
1:09:45.120 David Deutscher's written papers on this, that all of quantum physics is local.
1:09:49.760 In other words, when we say local, does not violate special relativity.
1:09:55.720 The prohibition on exceeding the speed of light.
1:09:59.880 There's no transmission of information, fasten the speed of light.
1:10:02.240 Entanglement doesn't show that, bells in a quality doesn't show that.
1:10:05.240 One of these purported ways of trying to get around the speed of light, using quantum
1:10:15.080 I'm skipping some more, and I'll pick it up towards the end of the chapter where Chiarats
1:10:22.600 With the physics of counterfactuals, features of quantum and classical information media
1:10:27.520 can be expressed independently of the details of quantum theory or classical physics.
1:10:32.560 The copyability of specific properties, the impossibility of copying others, and reversibility,
1:10:37.840 are general features, about which we can talk, without committing to quantum theory explicitly.
1:10:44.080 They provide profound connections between intuitively very distant patches of the fabric
1:10:48.040 of reality, such as photons, electron spins, neutrons, and other particles, that would
1:10:52.880 otherwise look very different, and yet they all have the same set of counterfactual properties.
1:10:58.440 The power of the science of canon card is that it expresses the essence of quantum systems
1:11:03.560 without ever committing to quantum theory's full machinery, made of specific laws of motion
1:11:10.000 This is important in view of the fact that, as I mentioned, quantum theory soon may be
1:11:16.360 My bet is that even if quantum theory eventually gets wiped away, the counterfactual information
1:11:21.640 theoretic structure we have explored in this chapter will remain, because it has deeper
1:11:29.120 These are the features that will survive the next revolution in physics.
1:11:32.600 At a glance, all around you, things appear superficially very diverse, but when looking
1:11:37.280 for long enough, and with the right spirit of scientific discovery, while also asking
1:11:41.960 good questions and trying to play around with some bits here and there, sometimes we find
1:11:46.520 a shiny, far-reaching connection between things that seem diverse and unrelated, and this
1:11:51.520 connection is based on a unifying explanation of the distant phenomena in question.
1:11:56.960 For example, physics tells us that a specific relation exists between mass and energy, between
1:12:01.320 the finiteness of the speed of light and the structure of spacetime, and, as you have
1:12:09.000 We have achieved yet another unification with the science of canon card, quantum and classical
1:12:13.680 information turn out to be two aspects of the same set of information theoretic properties.
1:12:19.680 Some information media are a special case of classical information media, with two additional
1:12:24.000 properties, reversibility, and the non-copiability of certain sets of states.
1:12:28.960 Quantum and classical media are different, but perfectly compatible with each other.
1:12:33.560 The fracture between the quantum and the classical world, the former, supposedly loof
1:12:37.760 and incomprehensible, the latter more friendly and intuitive, has been healed.
1:12:43.080 Realizing a unification of this kind goes together, with abstracting away irrelevant details,
1:12:48.520 thanking our understanding more general and the robust than it was previously.
1:12:52.560 These are promising features for a deeper understanding of nature, of which you and I are
1:12:58.840 Once the edifice is built, it will be beautiful in its elegance and simplicity and counterfactuals
1:13:03.400 will be the robust elements of its foundation.
1:13:08.840 So that's where I'll end the reading today, and I hope you agree with what I said at the
1:13:13.800 beginning there, that this chapter really was able to do what I thought was such a difficult
1:13:19.760 task, engaging the reader about quantum information, quantum information theory, a highly
1:13:31.080 And if you've managed to understand this, you know, pat yourself on the back, because
1:13:36.000 even if you understood a small amount of this chapter, you'll be in a rarefied class
1:13:40.960 of people who does understand this, because of the way that it's presented here.
1:13:45.160 I think anyone can understand this stuff, but they need to get a hold of the book.
1:13:50.800 Good gift for someone actually who's showing an interest in science and physics and
1:13:55.920 wants a new way of looking at the world and to have their intuitions challenged about some
1:14:02.920 otherwise dry subjects, especially this particular one, until next time, bye-bye.
