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Quantum Mechanics Isn’t Weird, We’re Just Too Big | Qiskit Seminar Series with Phillip Ball

Jun 05, 2021
with these strange looking parentheses to emphasize that this is

just

a formal way of writing an overlay. It is not an image of reality. You see, although it is often said that a spin qubit placed in a

quantum

superposition is in two states at once, simultaneously a one and a zero, let's say that is not really correct, because remember that the wave function only tells us what we can expect when we make a measurement. And so in this case, it says that a measurement on the qubit could return a one or a zero; both, in this case, are possible outcomes and, in fact, are the only possible outcomes.
quantum mechanics isn t weird we re just too big qiskit seminar series with phillip ball
But what is the qubit like before the measurement is made? Well,

quantum

mechanics

doesn't really tell us that. Well, you see, I'm not talking about blurring particles or collapsing waves, but about how information can be encoded in quantum systems and how it can be read. And this is the foundation that quantum information technology offers, which is not

just

a foundation, which is not just, you know, the foundation for these amazing new technologies, like quantum computing or quantum cryptography. It is also a new way of talking about quantum

mechanics

itself. And potentially, talking about quantum information could allow us to see past all this paraphernalia of wave functions, Schrodinger equations and quantum leaps, and get to the core of what the theory really seems to tell us.
quantum mechanics isn t weird we re just too big qiskit seminar series with phillip ball

More Interesting Facts About,

quantum mechanics isn t weird we re just too big qiskit seminar series with phillip ball...

And there's an example of what I mean by that, and one of the most puzzling features of quantum mechanics, which is that the results of measurements or observations seem to depend on how much information we gather about the system. And this is something that is commonly illustrated by the quantum double slit experiment. So if we send water waves or light waves through two closely spaced slits, the waves emerging from each will spread out and interfere with each other and create a pattern in which they alternately reinforce or cancel each other out. So for light, if you do this, you'll find a

series

of bright and dark bands on a screen that sits behind the slits.
quantum mechanics isn t weird we re just too big qiskit seminar series with phillip ball
If we do it with electrons, which according to quantum mechanics can also show these light-wave properties, we will find the same thing. You don't just get two images of the slits like you would if you were shooting sand grains through a mask with a sandblaster, i.e. like you would do in the upper case here. You get wavy interference, you get this interference pattern, lots of bands of more or less electrons hitting the screen and this interference persists even if the intensity of the beam is so low that you know the electrons will pass through the slits one at a time. one moment.
quantum mechanics isn t weird we re just too big qiskit seminar series with phillip ball
It is as if each electron passes through both slits at the same time and interferes with itself. But now, if you put a detector near a slit, that can detect the electron and see which slit it went through, then you will find that the detector will tell you that it goes through one and not the other. The detector may or may not click. But then if you discover that information, what you find is that the electron, this interference pattern disappears and the electron behaves like a normal particle. And more importantly, it's not the detection, it's not that the detection itself is somehow perturbing the motion of the electron and eliminating this wavy behavior.
It seems to be the very fact of knowing which slit it passes through that destroys the quantum ripple. And you can see that's true in what's called a delayed choice experiment where you set up a detection scheme in a clever way. Therefore, the trajectory of the particle is only detected at some point after it is certain that it has passed through the slits. This way you can be sure that the detection itself does not physically influence the passage through the slits. However, discovering that information will, again, destroy the interference pattern. The particle will behave like a particle.
But if you leave the detector there and just turn it off so you don't know which slit it went through, then the interference comes back. It's as if the particles eerily know if you're eavesdropping on the path they took. It is behavior like this that led Niels Bohr to claim that there is something special about measuring quantum systems that forces a quantum object to make choices about the properties it has. And until we do that, or unless we do that, the property that the object has is undefined and we simply can't meaningfully talk about it. It is well known that this idea was at the center of the dispute that arose between Bohr and Einstein.
For Einstein, this notion that the world is somehow indeterminate until we look at it seemed absurd. It seemed the antithesis of science itself. And in 1935, Einstein and two younger colleagues, Boris Podolsky and Nathan Rosen, devised a thought experiment that they believed demonstrated why Bohr's view had to be wrong. Now this experiment was later explained a little more clearly by the physicist David Bohm, and that is how I will describe it here. That's how it goes. You have a box that spits out two quantum particles that are in an entangled state, and this means that their properties are interdependent.
So let's say they have spins, which can only have two values ​​when measured. Like I said, turn up or down, okay? So if they are entangled, this means that the measurement results must be correlated between the two particles. Normally, if one has a swivel that points up, the other has to point down. We can't specify which is which, but we know for sure that they must be correlated because of the way we produce them. So if we measure the spin of a particle and find that it is a spin-up particle, then we know that the other particle must be spin-down.
Well, now this correlation in itself might seem unremarkable because the same could be said for two gloves, say, if someone sent them separately, one glove for me and one for you in the mail. If I open my package and see that I have the right hand glove, I know instantly, without even having to ask, that you have the left hand glove. But here's the crucial thing, because according to Bohr's interpretation of quantum mechanics, the direction of the spins, unlike the direction of those two gloves, is not determined in advance. It is not determined until one of us observes what he has.
Now, if that's so, then the so-called EPR experiment seems to be saying that making a measurement on one particle instantly fixes the direction of the spin arrow of the other. It's as if there is an eerie communication between them. And this is what Einstein called spooky action at a distance. It was Erwin Schrodinger who shared Einstein's skepticism about the Copenhagen view of quantum mechanics. It was Schrodinger who gave it the name entanglement. This type of instantaneous communication or action between two objects separated in space was known to be impossible. It was prohibited by Einstein's own special theory of relativity.
So Einstein argued that quantum mechanics couldn't be the whole story here. There must be some missing ingredient, which he said was some property, which we now call a hidden variable that we couldn't measure, but which somehow gives definite values ​​to these properties, spins say, to the particles from the beginning. The problem is that simply measuring particles in an experiment like this, measuring to entangle particles, won't tell you who is right, Bohr or Einstein, because that won't reveal anything about what the particles were like before you measured them. So no one knew how to address this apparent EPR paradox, as it was called.
And for decades, it was simply swept under the rug. But all that changed in 1964 when Irish physicist John Bell, whose day job was as a particle physicist at CERN in Geneva, reformulated the EPR experiment in a way that he suggested could actually be carried out. In short, Bell's experiment involved a clever way of combining measurements made by two observers on pairs of entangled particles where the observers themselves decided how they would make the measurement. And the setup is actually what's drawn here on the board with the bell, but here's maybe a clearer description of it. So what you need to do is look at how strong the correlations are between pairs of entangled particles for different options of how to make the measurement.
And Bell showed that if the particles were classical or governed by Einstein's hidden variables, so that in any case their properties were really fixed at all times, then the strength of the correlations obtained in many experiments repeated here summed up certain way could not exceed a certain value. But if they were governed by quantum mechanics, as Bohr said, and Bohr was right that we couldn't say anything about the properties before we measured them, then this correlation would be greater than that threshold. So these experiments were first done in the 1970s by John Clauser and Stuart Freedman at the University of California at Berkeley using laser photons as the entangled object.
And they have been repeated many, many times since then, and then they have become increasingly rigorous and increasingly able to rule out possible holes in Bell's argument. Every time they have given the same very clear result, which is that quantum mechanics alone is correct, Bohr's argument seems to be correct, and there is no need, there is no argument for Einstein's hidden variables. So how can this be? What's wrong with the EPR argument? Well, Einstein and his colleagues made the perfectly reasonable assumption of locality, which actually means that the properties of a particle are localized to that particle itself and that what happens to this particle here cannot affect what happens to another particle. there somewhere. different part of space without some way of transmitting an effect through the intervening space between them.
But it seems that nature doesn't work like that at the quantum level. We cannot consider the two particles in the EPR experiment as separate entities, even though they are separated in space. When quantum entangled, both must be considered, in some sense, a single, non-local entity. And in fact, only if we accept Einstein's locality assumption can we tell the story in terms of how particle one somehow influences the spin of particle two when we make the measurement, the spooky action at a distance. The alternative view is that we have to invoke quantum nonlocality, this idea that somehow quantum properties can be delocalized between two or more objects.
So here again we seem to run into this question of how measurement and data and the information we get from them seem to affect the results themselves. As with the double slit experiment, it can be shown in a Bell test that it is not the physical act of making the measurement on one particle that somehow appears to fix the property of the other. The question is whether that measurement tells you unambiguously what the state of your particle is. So what matters is knowledge. Measurement has become a bit of a dirty word in quantum mechanics, because it seems to bring out all these peculiarities.
In fact, John Bell included it in a list of bad words, which incidentally also included information that, in his opinion, should be prohibited from talking about quantum mechanics. But today measurement is no longer the mystery it once was, although it is also not fully understood yet. You see, in Bohr's view, a measurement was necessarily something governed by classical physics, because ultimately it needed things of a classical scale, you know, an apparatus and us to see it. And for him, that was the end of the mystery, because he said that quantum and classical were simply different regimes and that we shouldn't expect to be able to pronounce on what was happening in the quantum world from the perspective of the classical world. .
In fact, he declared that there was a fundamental division between them. Now, not surprisingly, some researchers, including Einstein, found that a deeply unsatisfactory answer, but today we can do a little better than that. We know, for example, that a measurement that involves the interaction of a quantum system with some classical environment, such as a measurement device that aims to extract information about it, any measurement like that involves entanglement. Entanglement occurs every time one particle encounters another. In fact, quantum mechanics says that if they interact, that's the only thing that can happen. They have to get tangled.
And if a measurement is made so that more and more of these interactions with the environment accumulate, the quantum system itself becomes more and more entangled with its environment. You could say that what this means is that the process extends the quantity of the original object to its surroundings. And in that process the amount is diluted in the environment. And this is what I mean by that. Let's say you have a quantum bit, a qubit in a super position, and then you make a measurement on it and it becomes entangled with its environment. So, theSuperposition also extends to those particles.
And if you want to show that you still have a superposition there, then you have to look for it in all those entangled particles. And that quickly becomes impossible. It will be a bit like trying to keep track of every molecule of gas you just exhaled as it collides and mixes with all the ones in the air around it. So this process is a real physical process. It is called decoherence and it leads to the apparent disappearance of quantum properties such as superposition in the original quantum object when it is observed and when its properties are measured.
And it is possible to calculate how quickly decoherence should occur when a quantum system interacts with its environment. And we found that generally it's so fast that it's almost instantaneous. So, for example, if you could somehow put a grain of dust in the air in a quantum superposition of states, you can calculate that it would decohere in this room in about 10 to minus 31 seconds, faster than anything we can measure. And what's left when that happens is classical behavior, properties defined with fixed values, like a well-defined position in space. In fact, according to a theory that describes how this decoherence changes the quantum to the classical, it is a theory called quantum Darwinism.
This process not only gives objects fixed definite values ​​of their properties, but actually selects from all quantum characteristics precisely those properties that have meaning in the classical world. Properties such as position, speed and load. So only certain quantum features, the theory goes, have the right characteristics to survive the decoherence filtering process. And they do, they have those properties, because they are particularly good at leaving many traces or copies of themselves in their environment. That's where the Darwinian part of the theory comes into play. You see, when we make an observation of an object, what we are really doing is detecting the traces it leaves in its environment.
Now you can only see me thanks to the photons that have scattered from my particles and reached the camera. (laughs) This all gets a little more complicated when we talk about virtual talks, but it kind of works. And you can only hear me because of the imprint that my particles have left on the air molecules, the movements of the air molecules. In fact, you could reconstruct everything you can say about an object like a grain of dust, for example, from those traces like the photons of light that bounce here. You could reconstruct all of that, even if just after those photons had bounced off, the object itself magically disappeared from space, you would still be able to see it by the trace it left, for a short time.
And everyone can agree on my properties, my position, the color of my shirt, what I'm saying, classic properties like this, you can agree on them, because there are a lot of footprints that I'm leaving on the environment. . There are very redundant traces. There are plenty of scattered photons of mine, enough for all of you to share. If one of you were able to somehow collect every photon he scattered, no one else would be able to see me and, you know, make any observations about me. And it is this creation of many copies or traces through interactions and entanglements with the environment that quantum Darwinism describes.
And at least in this theory, that's how quantum becomes classical. Now it seems that decoherence cannot be avoided in objects the size of a grain of dust. Even in a vacuum, interactions with thermal radiation and infrared photons from the heat of the room would cause it to decohere in about 10 to minus 18 seconds. Even if we put that grain of dust in the cold vacuum of interstellar space, it would still decohere in about a second due to microwave photons from the cosmic background radiation. In other words, the reason we generally cannot observe quantum effects, such as superposition in everyday-scale objects, is not because there is some law of nature that prohibits quantum at those scales, but because decoherence will destroy it almost instantly. , and he does it.
This is due to quantum entanglement itself. If we could figure out how to put a tennis

ball

into some well-defined superposition of states and completely isolate it from the sources of decoherence and still, this is the tricky part, somehow still monitor its state with sufficient precision, quantum mechanics says that There is no reason why such an overlap should be impossible. Now some physicists think there might be other factors in that situation that would come into play that would destroy quantum coherence anyway. Some think gravity will do it. But it's not clear that anything additional is needed to effectively banish quantum phenomena at everyday scales.
In other words, the world is quantum through and through, and classical physics is pretty much what quantum physics looks like when you're six feet tall. And that's really what I mean by the title of my talk. And I should just add for this audience that decoherence is the enemy of quantum computing. Computing with qubits is based on keeping them in a collective state of superposition while the calculation is performed. Decoherence will ruin that. And that's why quantum circuits have to be called at such extremely low temperatures within a range of absolute zero, because this suppresses all the thermal shocks that would otherwise disturb the qubits.
And it is the difficulty of maintaining this coherence and suppressing decoherence in an increasing number of entangled qubits that explains why quantum computers so far have no more than about 70 qubits to play with. But it's a testament to how much power quantum rules give you that quantum computers can already do in seconds with just that many qubits. Calculations that would take current classical supercomputers approximately one billion years. Now, there is something curious about Bell's experiment that is often not discussed. Remember I said that quantum mechanics allows correlations between entangled particles to be stronger than they would be in classical physics or for hidden variables, but quantum mechanics also sets a limit on how strong those correlations can be.
And it's not obvious why they couldn't be stronger. We don't know why this is so, why the world is not, if you will, even more quantum than it is. But a useful way to think about what's happening there is that there appears to be a limit to the amount of information that quantum mechanics allows to be shared between entangled particles. Again, what makes quantum mechanics what it is? Deep down it seems to have nothing to do with mathematical notions... (incomprehensible) (mouse clicks) And this is precisely where some scientists think that we should start trying to reconstruct quantum mechanics from scratch, not from these ad conjectures. hoc, as they say.
We were really, to begin with, about waves and particles and Schrodinger's equations, but from some simple axioms, some simple assumptions about what is allowed with information, how it can be encoded, how it can be transferred, how it can be read. And I want to give you an idea of ​​what this type of reconstruction would look like. There are many of them that have been suggested. They are all somewhat similar. And I have randomly taken one that was suggested in 2009 by Borivoje Dakic and Caslav Brukner at the University of Vienna in Austria. They proposed what they said were three reasonable axioms about how the world could be.
And this is what the axioms were. That, first of all, the most basic components of the world, whatever they may be, cannot contain more than a bit of information inside them. Second, they assumed that the state of what they called locality, so the way they put it was the state of a composite system made up of many subsystems, is completely determined by the measurements made on its subsystems. What that really means is that there are no secret mechanisms, like hidden variables, that manipulate what is happening. Third, they assumed that this property of reversibility, which can convert any pure state from one to another and back again, like flipping a coin, is heads and tails.
So Dakic and Brukner showed that just these assumptions about information, in reality, will inevitably lead to only two possible sets of rules about how things can happen. And those are the rules that describe classical mechanics and quantum mechanics. What's more, if you modify this third axiom to assume that states can interconvert gradually, little by little rather than in one big leap, then you only get rules that conform to quantum mechanics. So for a classic coin toss, for example, a heads and tails abruptly interconvert in one big jump, it's either heads or tails. In a quantum system, you could have a situation where, you know, you have your hand on the coin and it can gradually, as you hold it down, interconvert into heads or tails without having to pick it up and flip it. on.
Okay, so the point is that these axioms about information can, on their own, lead to quantum behavior that we are familiar with, such as superpositions and entanglement. And some researchers think that these quantum reconstruction efforts could lead to a completely different perspective on quantum theory. One in which the physical meaning of all the seemingly strange behaviors finally becomes clear. Now it remains to be seen whether that will happen, but I think it is already enlightening that everyone is focusing on the information, on how the answers or the results of the measurements depend on the questions we ask and how we ask them.
And I think this is the most productive way to think about what quantum mechanics seems to tell us. And it's a perspective that I think is very well illustrated by a metaphor that the American physicist John Wheeler, who studied with Bohr and who, by the way, had Feynman as one of his students, suggests that perhaps we should think about mechanics quantum in this way. . Let's think of it as if it were a bit of the game of 20 questions. So I'm sure you know this game. So it's the one where, you know, one player leaves the room and everyone else in the room agrees on a certain person, a historical figure, a well-known person.
And then the person outside has to come back and start asking each person a question to try to figure out who they've chosen. And they have to be questions that can only be answered yes or no. You see, they are actually quantum questions. Now imagine you are the one asking here. So you go back, start asking questions and get answers. To start, is the person a man? And the first person says yes and the second person, then you go to the second person, is he alive? No, that's fine, and you continue, but you discover something strange: as you ask more and more people, it takes them longer and longer to answer your question.
It's like they have to think about it. Well, that's strange because, you know, if they agree on one person, the answer is yes or no. What is there to think? Anyway, you keep playing this game and eventually you think you're focusing on who he is and finally you think, ah, I got this. And you say it's Richard Feynman. And everyone laughs and tells you that you're right, it's Richard Feynman. And then you say, well, what was going on? Why did it take you so long to think of the answer? And they explain how they decided to play this.
So instead of settling on a particular person when you were out of the room, what they decided to do was say that the answer anyone gave to the question you asked had to be consistent with all the answers that had been given before. . They didn't decide on anyone, but the answer they gave had to correspond to all the answers that have been given so far, it had to correspond to a famous person of some kind. And then, of course, you know, as the answers went on, it became harder and harder to think, well, who does that apply to then?
Man, they are dead, etc. There are physicists, etc. You have to think more and more. And what happens is that as the questions are asked, the possible answers are reduced and in the end, it can only be one person. It has to be Richard Feynman, but the nature of the questions forced everyone to converge on that same person. If you had asked different questions, you would have ended up with a different person, a different answer. So the answer you got depended on the context. There was never a predetermined answer. You brought that answer to life in a way that is entirely consistent with the questions you asked.
What's more, the very notion of an answer only makes sense when you're playing. You know, if you came back and said, "Oh, I can't be bothered to go through this." Just tell me who you chose." There would be no answer to that question. You have to do research before an answer comes. So quantum mechanics is like that. It seems to be a theory of what is and is not knowable and how those are related. knowledge. And I like to think of this in terms of a distinctionbetween a theory of existence and a theory of existence. It tells us what and with what calculable probability it could be along with a logic of the relationships between the different possible ones.
If this, then that, and what this means is that we truly describe the features of quantum mechanics to the best of our ability. We should replace all conventional isms with ifms. So, for example, we shouldn't say here that it is a particle. It's a wave there. We should say that if we measure things like this, the quantum object behaves in a way. We associate it with particles, but if we measure it like this, it behaves as if it were a wave. We should not say that the particle is in two states at once. We should say that if we measure it, we will detect this state with probability x and that state with probability y.
This certainty is disconcerting because it is not what we have come to associate with science. We are used to science telling us how things are. And if there is a problem, it is because we are partially ignorant about it. But in quantum mechanics, from what we know so far, conditions seem to be fundamental. Well, okay, you might say, but what does this statement refer to? Obviously, quantum mechanics doesn't tell us much about this. And all we have now are clues and guesses, and trying to focus them better is a complicated task. It seems to demand an almost poetic level of expression, which will send some physicists running for cover.
Here, for example, is an attempt to do so proposed by physicist Chris Fuchs. He said: "Perhaps quantum mechanics is telling us that the world is sensitive to our touch. It has a kind of spark that makes it fly in ways that were not classically imaginable. The whole structure of quantum mechanics may be nothing more than the optimal method of reasoning and information processing in light of such a fundamental and wonderful sensitivity." And what people mean here is not the mundane truism that the human observer disturbs the world. That's true in classical physics. Rather, quantum mechanics may be the machinery that humans need, we humans are located on a scale halfway between the subatomic and the galactic to try to compile and quantify information about a world that has this incredibly sensitive character.
It embodies what we have learned about how to navigate such a world. And I think that, in any case, it is vital that we understand that this identity does not imply that the world, our world, our home is hiding something from us. It's just that classical physics has led us to expect too much of it. We have become accustomed to asking questions and getting definitive answers. What color is it? How much does it weigh? How fast is it moving? Forgetting the almost ridiculous amount of things we don't know about everyday objects. We thought we could go on asking questions forever and receiving answers on ever finer scales.
When we discovered we couldn't, we felt let down by nature and pronounced it

weird

. I don't think that helps anymore. Nature does the best it can and we need to adjust our expectations, so we need to go beyond the strange. Thank you so much. (Zlatko laughing) - Thanks, Phillip. Thanks, Phil, for the wonderful talk. And I can pass on a few comments before moving on to audience questions. And people in the audience feel free to ask questions again in the chat. The chat is very active right now, so I'll try to get you guys off the stream here.
By the way, we have some people, I'd just like to tell you, since you can't see the chat yourself, you know, they said this is, you know, something nice and enlightening and fascinating, so I thought. that should convey that. We had quite a few discussions and questions, I think that came from the discussion of, let's see, I hope you can see me now. Here we go. Why is quantum strange in the sense of deterministic? You know, is it because we are used to deterministic theories? And I think you touched on this throughout the talk, and this is one of the essential points, but maybe if we come back to it, there was the locality, the deterministic versus maybe non-causal potential.
Perhaps if we could go over your thoughts again in a brief summary of the discussion taking place in the chat. - Yeah, okay, well, I mean, certainly, you know, it seems to me that as far as we can tell right now, quantum mechanics is not deterministic. There are ways of interpreting quantum theory that are deterministic that, in fact, return us to some kind of picture of the existence of real particles that have real properties and real places out there. There are interpretations that suggest that, and if you like, all the quantum, or what we have conventionally called the

weird

ness, is placed in another aspect of the theory, but we recover the particles.
That is possible, but it is not proven and is just an interpretation. And at the moment it seems safer to say that there seems to be this fundamental indeterminacy in quantum mechanics, so that if we make a measurement on a system, the only thing we can meaningfully say is what the probabilities are. One of the results will be that we may never be able to say more than that, so we will never be able to predict exactly what we are going to find. - That is very interesting. And I think, guys, I'm sorry, we're going to have to stop the videos, but I hope you can still hear us well.
We're having some bandwidth issues today, yeah, very interesting, and maybe a follow-up question from the audience here. In the classical world, are we perhaps too accustomed to trying to write deterministic laws? You know, could the classical world be modeled under some probable, or perhaps probabilistic theory, to rephrase the question here, and that should really be the comparison between the quantum and classical world? - Well, I'm certainly not an expert on this, but what I can say in response is that these quantum reconstruction efforts that I talked about, one way of looking at it is that, in reality, quantum mechanics is simply a type of theory of probability.
It's just a different kind of probability. You know, probabilities work a little differently than we're used to in the classical world. And that, you know, one way to look at it is that, you know, we should think probabilistically, you know, more generally and, you know, there's a whole set of possible probabilistic theories or probability theories and one of them correspond to quantum-type rules. So, you know, here again, we come back to quantum mechanics being a theory about knowability and how much the world is actually knowable. I mean, the way I think about it, which makes sense to me is to suggest that quantum mechanics seems to tell us that we are, (laughter), we may ask more questions than nature can have answers, can have. definitive answers for.
It seems to me that's really the point. And it doesn't seem like such a crazy thing to propose. Why should every question we can ask necessarily have a definitive answer? And quantum mechanics seems to say no. And then you have a choice. If you choose to ask these questions, you will get this set of answers. If you choose to ask this set of questions, you will get a different one and they may not be compatible with each other because you ask different questions, so it depends on the context of the questions you are asking. - Hmm, I think that's enlightening.
Yeah, and maybe that reminds me a little bit of this game that you mentioned earlier, which was very enjoyable and I think you concluded with something like, you generated that response, oh, it's Feynman, by asking that particular thing. set of questions. And I think it was, Korotkov, he's a physicist who also says, "In quantum mechanics, you don't see what you get, you get what you see." (Zlatko laughs) And that brings me to maybe your perspective, thoughts or opinion on knowability, right? So in these more recent, quantum trajectory type experiments, those experiments can't always, usually can't tell you the future.
They can't predict the future, but they can always tell you exactly what, in a sense, the state, the quantum description, the quantum state is doing, as you subject the system to a continuous interrogation, a measurement. And there seems to be a very close direct one-to-one correspondence. In a sense, randomness only happens if you now suddenly change your measurement to something else, but you have knowledge about the system that is conditional on what you are measuring and you can update it. And there is always, in the event that you can do that upgrade perfectly, that you can approach these days for some experiments.
There isn't always a complete set of questions that you can answer perfectly. As you said, there are always more quantum questions that maybe you can always get a deterministic answer to, but there is a subset of those questions that you can always know that if you ask the question, you know, there is the qubit along the x direction, let's say , you always get a perfect one. Even though you have been injecting randomness into the system throughout the entire measurement process. Maybe it was a bit of a long question, so apologize for that, Phil (laughs), but maybe we can get your opinion on QTT and a little bit of that, that would be great. - Well, of course, Zlatko, the one who knows this is you and not me.
I was absolutely fascinated by those experiments for a number of reasons, but I think perhaps the main one was that it seemed to me that you were really starting to focus on what happens when we make a measurement and, in particular, to dispel this idea. that measurement is this mysterious black box that suddenly turns, you know, all the quantum vagueness into something concrete. What really excited me about them was that you can see how that process happens and that there are laws for it. You know, yeah, that it's not just, you know, magical and unpredictable and everything else, that, as you say, if you're able to follow it with enough temporal resolution, with enough detail, knowing, you know, almost everything about what's happening in the system, then you can see something that looks like a trajectory that takes the system from one state to another.
And that's why, I mean, it certainly convinced me that, you know, this notion of wave function collapse as something mysterious that suddenly turns, you know, a wave into a particle, which really, has never been really necessary in quantum mechanics. I understand why it got there, but it's time to put it out again, because, you know, we don't need it. And it seems to me that that is bringing us much closer to having a proper understanding of what happens when we make a measurement. Maybe the one thing, and I'd be interested to know if you have any ideas about this, because the one thing it doesn't seem to explain, and this is really the kind of fundamental mystery of quantum mechanics, is why do you ever only get one thing?
And of course there are some interpretations that say, well, it's not like that, you just get both in different universes. But, as far as we can see, quantum measurements are unique. So where does that uniqueness come from, given that we know that more than one outcome is possible in principle? - That is a very deep question. In fact, I think I was asked a similar question in my thesis defense, in the private part of my thesis, where, you know, they take you aside in a room and question you until you reach a point of... anyway, so (laughs) I don't know if I have a good answer, but maybe some ideas that you've probably written about quite a bit as well is this notion that you mentioned before.
And I think you said that Bell had perhaps introduced it, of two different disconnected worlds, the world of the quantum and the world of the classical. And there's this notion of the Heisenberg cut, the boundary between these two. And there is no recipe in any textbook or any document that I know of that I can find that actually identifies where this difference occurs between what you call a measuring device and where you get classical results from the system. And what is the quantum system? So, for example, going back to the Nature paper on quantum leaps that you mentioned earlier, you could take that Heisenberg slice at different stages of the experiment and pretend that, for example, only the qubit itself is quantum and is undergoing a particular process. .
Classic measuring device that has this particular type of question that interrogates the system, kind of like the game you mentioned earlier. But it turns out that what we had to do for the experiment was move the cut one step further. That device was a particular type of superconducting cavity and we also modeled all of its quantum behavior. Now, that particular cavity is followed by another quantum amplifier. And we could actually include the quantum amplifier itself and also create this now very large quantum system. You know, you have three different quantum systems that are all coupled and you have this sort of matryoshka doll of larger nested systems that are becoming more and more classical in a sense, because at each stage you are amplifying, in a sense, the signal, there is more quantitative excitement involved.
And maybe with that, once you have, like you said, the grain of dust floating in the air, when you have so many quanta, so many excitations, then you tend to couple more strongly to this, well, decoherence, right? Whatever that is precisely in itself. ANDThey tend to move already at the limit where much of quantum nature tends to seem erased or eliminated in some sense. Although at that stage it is still a purely quantum system. You could reverse and undo the measurement in the sense of taking that information, because you're keeping track of it and kind of putting it back into the system.
You still have all those degrees of freedom. And then after the next amplifier that comes after the cavity, then we don't know where to draw that Heisenberg cut. You know, in terms of the empirical experimental world, that's pretty much, as far as our models can take us, you know, after that, we don't really have an equation or a model that we can write down and that we're currently able to say, oh, You know, now it's quantum. So empirically we see that, okay, it happens somewhere in there, but is that a limitation on our ability to express it or not? - Well, it's like...
I mean, it seems to me like, again, it's really about information, but in, you know, this notion of cutting. It sounds like what that really means is where do you decide that there comes a point where you're not going to do it anymore or where you can't gather fairly complete information about everything in the system? And once, you know, once you abandon (laughter) that attempt, that's where the cutoff happens, because you have imperfect information. Is that the correct way to look at it? - I think that's definitely a practical way of looking at it that seems to work in practice. (laughs) Is this the definitive solution?
I couldn't say it. (Zlatko laughs) And, sorry, continue. - No, no, no, it's okay. That's just what occurred to me while you were describing the experiment. You know, it seemed to be a question of, well, to what extent can you keep advancing your apparatus and still retain the ability to monitor, you know, more or less everything that's happening at the quantum level. - Exactly, and now we've been able to push that further and further and see that as you go up the matryoshka doll or the nesting chain here, you can move that Heisenberg cut pretty far and still model things. but at the end of the day, we've just moved the cut away, but we still have the cut.
So is this a fundamental cut or not? I think, as far as I know, it's an open question. But another way that people think, and I think we mentioned this, is that you could say the opposite, I think, of what Bell had said, John Bell, I think you said that you had banned measurement as a dirty word. . I think for more experimentally minded people like me, measurement is probably the essence of the whole theory. It is what we do, it is what we model, it is what we describe. Actually, that should be the core, so maybe we should start, not with the Schrodinger equation, which describes what the state of the system would be, you know, if that existed in itself, but maybe we should start with a description of the measurements, so you can stay closer on the QTT line.
You know, you could take that interaction between what you call a system and the actual measurement, because the problem is that in the Schrodinger equation, a measurement is completely absent. So the Schrodinger equation is linear, deterministic. There is nothing really strange about the Schrodinger equation. In a sense, the randomness, if you will, the weirdness of quantum randomness that comes into play only happens at the actual measurement stage when you have the von Neumann postulate, and so on. And so it is that process that I think you so well pointed out, now the field of physics is able to open the hood and describe it.
But I think we may have said too much about this. (laughs) Maybe if you wanted to share this, an audience question from now on, and let me know if you wanted to answer this one, could you share your thoughts on Maxwell's demon and maybe how that? relates to all of quantum mechanics in the sense that you have, I'm reading the question here, you know, when it comes to knowability and trying to sift or analyze and overcome the laws of thermodynamics in a way, but now at the quantum level . So maybe if you wanted to offer some thoughts on that about Maxwell's...
Yeah, yeah. Well, very timely. I just wrote about Maxwell's daemon today and I've done it before and, you know, everything discussed within the context of Maxwell's daemon is classical, although there is also a quantum Maxwell's daemon. There is a quantum version. But, you know, classically, again, it illustrates how everything seems to come down to information, the search for information. And, in essence, what Maxwell's demon tells us is that information itself is a kind of resource that we can use to do things, which (laughter) I think is extraordinary. I mean, maybe not everyone knows what Maxwell's demon is.
So, you know, very, very, very briefly, it was a way that James Clerk Maxwell suggested in 1867 to subvert the second law of thermodynamics. The second law says that you can express yourself in many ways, I mean, the simplest is that heat always flows from hot to cold and never the other way around. But what it also means is that you can never get free energy. (laughs) There is no perpetual motion. In fact, in any functioning system, some energy will always be wasted and lost as sort of random motions in the system as entropy. So the entropy of the universal always increases.
And Maxwell didn't like this, partly for religious reasons. And then he came up with the idea of ​​a little demon that could actually reverse this by having a trap door between two gas compartments, both of which were gas. The gas starts out at the same temperature in both, but the hottest molecules are the ones that move the fastest in the gas. So the molecule, the demon looks at the molecules and sees, if a fast molecule approaches from one direction, he opens a trap door and lets it pass. If a cold molecule approaches from the other direction, it opens it and lets it pass through.
And then all the hot ones go in one direction or the cold ones go the other and you end up with hot gas on one side, cold gas on the other, which is basically then, there's a thermal gradient that you can harness as a kind of energy source. to do some useful work. So the energy is essentially obtained from nothing, from just, you know, an ordinary gas. For a long time people thought: you know what? There must be something wrong with this. How can you subvert the second law? And it turns out the answer is that it can't because of the information and in this case, it's the information that's recorded in the demon's mind that, you know, has to gather information about those gas particles in order to do what it wants. does.
And eventually that information, his mind is finite and he has to delete it, otherwise it's too full and he can't do more. And erasing that information costs energy, and you actually get back all the energy you thought you got from a demon's operation. So, you know, you still don't get anything for free, free energy. So it's about information and people have realized that actually what this means is that what appears to be a kind of randomness, statistical randomness, you know, on one level, if you can focus and get detailed information about what what's happening, then you can use that information to extract the kind of random fluctuations that are there and extract energy.
And recently there have been experiments that have shown this, that have actually shown that you can work on an object. In this case, it involved lifting a small particle floating in a liquid, if I remember correctly, by making observations on it, collecting information about its movements. And by using it in some clever way, you can actually do work, lift it against gravity. Then information becomes a resource. So, you know, again, there's this fundamental idea and then the quantum version of it, which I won't get into, you know, is just representing that same idea in terms of quantum information.
So it's really interesting how, again, so many ideas, fundamental ideas in physics, but we also see it happening, of course, in biology, come down to this question of what is information, how is it incorporated, how can it be use and manipulate, and how it can become a resource that we can exploit. - Thanks, Phil. That is wonderful. I see we have a few more questions. I won't be able to answer all of them, as I see it's been about 15 minutes, but maybe we'll take one more question from Pranav here, if of course, Phil, would you like to share with us about You know, how did you decide to go into physical?
And then if you could tell us a little bit about your transition to becoming a science writer and editor. I think there seems to be interest from the audience if they want to share it with us. - (laughs) Well, let's talk about random walks. I mean, this was certainly one of them because I started out, my first degree was in chemistry and it was at Oxford, where in the final year you have to do a year of research, which was fantastic. It was by far my favorite part of the course. And I did it in physical chemistry on, really, the quantum mechanical behavior of molecules absorbed on surfaces, which is right on the border, really, between physics and chemistry.
And when I started doing a PhD, it turned out that I ended up in a physics department looking at similar things, condensed matter and behavior of objects and fluids on surfaces. And that led me to physics, but in the end I thought this goes back a long way, I can tell you that unlike most people, I actually enjoyed writing my thesis and I enjoyed writing the papers. that arose from this, from the doctorate. And then I thought, well, you know, I'd never thought about this before. I had always liked to write, but I had never thought about combining my enjoyment of writing with science, but it is possible.
And I decided to see if I could do that in scientific editing. And I was very lucky that at just the right time a place came up at Nature, which in those days, in the late '80s, was just a magazine. It wasn't this big publishing empire. It was just a newspaper in a small office, a crowded office, you know, full of paper. And I got a job there as a physical sciences editor and I was there for a long time. It was a fantastic place to work. It meant I was exposed to all areas of science.
It was a very, very steep learning curve and also opened up other opportunities to write for newspapers etc. And I finally decided that maybe I would write a book, and I did, and it was a horrible experience in the sense that I was working full time on Nature, which was already, you know, work, it was already intense work. . So I thought I didn't want to do that again, but I wanted to write more, and gradually I started working part-time at Nature until I finally quit. I don't remember when it was, but it was probably about 20 years ago.
I stepped away from Nature and still wrote for them regularly as a freelancer, but I became completely independent and now split my writing time, I guess, more or less 50/50 between books, usually on scientific topics, often about how science interacts with broader culture and the writing of scientific journalism. - Thanks, Phil. That's an inspiring story. I think with that, maybe I'll take any final words that you would like to share with us before thanking the audience and yourself for joining us here today. - Well, just thank you again, Zlatko, for inviting me to give this talk. It was fun.
It's always been a lot of fun to have interactions with IBM in general and with IBM's quantum computing efforts in particular. So it's very nice to be able to maintain that relationship and, you know, I'm always fascinated to see where quantum computing at IBM and quantum technologies at IBM have advanced. So keep me posted on that. - Wonderful, and I think we all look forward to reading many more books and articles from you, Phil. This was an absolute pleasure. Thank you all for tuning in today. I still see that we have over 130 people tuned in. You know, we're 20 minutes late, so I appreciate your time.
Thank you so much. This talk will remain recorded and live on Qiskit's YouTube channel, so feel free to subscribe for future updates. Thank you, Phil, for accepting our invitation. It has been a true privilege and pleasure to have you here today. And thank you all. See you next Friday at noon Eastern Time. - Thank you.

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