Scott Aaronson: Quantum Computing | Lex Fridman Podcast #72
the following is a conversation with Scott Aaronson a professor UT Austin director of its quantum information center and previously a professor at MIT his research interest center around the capabilities and limits of quantum computers and computational complexity theory more generally he is an excellent writer and one of my favorite communicators of computer science in the world we only had about an hour and a half for this conversation so I decided to focus on quantum computing but I can see us talking again in the future on this podcast at some point about computational complexity theory and all the complexity classes that Scott catalogs and his amazing complexity Zoo wiki as a quick aside based on questions and comments I've received my goal with these conversations is to try to be in the background without ego and do three things one let the guest shine and try to discover together the most beautiful insights in their work and in their mind to try to play devil's advocate just enough to provide a creative tension in exploring ideas to conversation and three to ask very basic questions about terminology about concepts about ideas many of the topics we talk about in the podcast I've been studying for years as a grad student as a researcher and generally as a curious human who loves to read but frankly I see myself in these conversations as the main character for one of my favorite novels badesti husky called the idiot I enjoy playing dumb clearly it comes naturally but the basic questions don't come from my ignorance of the subject but from an instinct that the fundamentals are simple and if we linger on them from almost a naive perspective we can draw an insightful thread from computer science to neuroscience to physics the philosophy and the artificial intelligence this is the artificial intelligence podcast if you enjoy it subscribe on YouTube give it five stars an apple podcast supported on patreon or simply connect with me on Twitter at lex friedman 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Galaxy S 20 and of course right away Technium right home has a new episode that summarizes all that I needed to know about this new device they've also started to do weekend bonus episodes with interviews of people like a well founder Steve Case an investing and Gary Marcus on AI who I've also interviewed on this podcast you can find the Technium ride home podcast if you search your podcast app for ride home two words then subscribe enjoy and keep up to date with the latest tech news and now here's my conversation with Scott Aaronson sometimes get criticism from a listener here and there that while having a conversation with a world-class mathematician physicist neurobiologist aerospace engineer or the theoretical computer scientists like yourself I waste time by asking philosophical questions about freewill consciousness mortality love nature of truth super intelligence weather time travel as possible weather space-time as emergent fundamental even the crazy questions like whether aliens exist what their language might look like what their math might look like whether Malthus inventors discovered and of course whether we live in a simulation or not so I try with it out with it I try to dance back and forth from the deep technical to the philosophical so I've done that quite a bit so you're a world-class computer scientist and yet you've written about this very point the philosophy is important for experts in any technical discipline though they somehow seem to avoid this so I thought it'd be really interesting to talk to you about this point why should we computer scientists mathematicians physicists care about philosophy do you think well I would reframe the question a little bit I mean philosophy almost by definition is the subject that's concerned with the biggest questions that you could possibly ask all right so you know the ones you mentioned right are are we living in a simulation you know are we alone in the universe how should we even think about such questions you know is the future determined and what you know what do we even mean by being determined why are we alive at the time we are and not at some other time you know and and and you know when you when you sort of contemplate the enormity of those questions I think you know you could ask well then why why be concerned with anything else all right why why not spend your whole life on those questions you know I think I think in some sense that is the the right way to phrase the question and you know and and and actually you know what what we learned you know I mean throughout history but really starting with the Scientific Revolution we've got you know Galileo and so on is that there is a good reason to you know focus on narrower questions you know more technical you know mathematical or empirical questions and that is that you can actually make progress on them right and you can actually often answer them and sometimes they actually tell you something about the philosophical questions that sort of you know may be motivated your curiosity as a child right you know they don't necessarily resolve the philosophical questions but sometimes they reframe your whole understanding of them right and so for me philosophy is just the thing that you have in the background from the very beginning that you want to you know the you know these are these are sort of the reasons why you went into intellectual life in the first place at least the reasons why I did right but you know math and science are tools that we have for you know actually making progress and you know hopefully even you know changing our understanding of these philosophical questions sometimes even more than philosophy itself does what do you think computer scientists avoid these questions will run away from them a little bit at least in technical scientific discourse well I'm not I'm not sure if they do so more than any other scientists though I mean I mean I mean I mean I mean Alan Turing was famously you know interested and you know is his most famous one of his two most famous papers was in a philosophy journal mind you know it was the one where he proposed the Doering test he took a Vidkun Stein's course at Cambridge you know argued with him I just recently learned that a little bit and it's actually fascinating I I was I was trying to look for resources in trying to understand where the sources of disagreement and debates between Wittgenstein and touring war that's an interesting that these two minds have somehow met in the arc of history yeah well the transcript you know of their the course which was in 1939 right is one of the more fascinating documents that I've ever read because you know a vit concern is trying to say well all of these these four systems are just a complete irrelevance is right if a formal system is irrelevant who cares you know why does that matter in real life right and touring is saying well look you know if you use an inconsistent formal system to design a bridge you know the bridge may collapse right and you know soso touring in some sense is thinking decades ahead you know I think of where Vidkun Stein is the way with formal systems are actually going to be used you know in computers right to actually do things in the world you know and it's interesting that touring actually dropped the course halfway through why because he had to go to Bletchley Park and you know work on something of more immediate importance that's fascinating if you take a step from philosophy to actual like the biggest possible step to actual engineering the actual real impact yeah and I would say more generally right uh you know a lot of scientists are you know interested in philosophy but they're also busy right and they have you know a lot on their plate and there are a lot of sort of very concrete questions that are already you know not answered but you know look like they might be answerable right and so then you could say well then why you know break your brain over these you know metaphysically unanswerable questions when there were all these answerable ones instead so I think you know for me I I enjoy talking about philosophy I even go to philosophy conferences sometimes such as the you know fqx I conferences I enjoy interacting with philosophers I would not want to be a professional philosopher because I like being in a field where I feel like you know uh you know if I get too confused about the sort of eternal questions then I can actually make progress on something can you maybe a link on that for just a little longer yeah what do you think is the difference so like the corollary of the criticism that I mentioned previously that why ask the philosophical questions of the mathematician is if you want to ask for softball questions then invite a real philosopher on and ask that so what's the difference between the way a computer scientist and mathematician Ponder's a philosophical question and a philosopher Ponder's the falafels question well I mean I mean a lot of it just depends on the individual all right it's hard to make generalizations about entire fields but you know I think I think if we if we if we tried to we tried to stereotype you know we would say that uh you know as scientists very often will be less careful in their use of words you know I mean philosophers are really experts in sort of you know like when it when it when I talk to them and they will just pounce if you know use the wrong phrase for something versus a very nice word you could say cyclers yeah yeah where or you know they will they will sort of interrogate my word choices let's say to a much greater extent than scientists would write and and scientists you know will often if you ask them about a philosophical problem like the hard problem in in of consciousness or free will or whatever they will try to relate it back to you know recent research right you know research about about neurobiology or you know but you know the best of all was research that they personally are involved with right right and you know and and and and you know of course they will want to talk about that you know and it is what they will think of you know and then of course you could have an argument that maybe you know it's all interesting as it goes but maybe none of it touches the philosophical question right but you know but maybe you know as a science you know at least it it as I said it does tell us concrete things and you know even if like a deep dive into neurobiology will not answer the hard problem of consciousness you know maybe it can take us about as far as we can get toward you know expanding our minds about it you know toward thinking about it in a different way well I mean I think neurobiology can do that but you know with these profound philosophical questions I mean also art and literature do that right they're all different ways of trying to approach these questions that you know we don't for which we don't even know really but an answer would look like but and yet somehow we can't help but keep returning to the questions and you have a kind of mathematical beautiful mathematical way of discussing this with the idea of Q Prime oh you're right they usually the only way to make progress on the big questions like the full of the philosophical questions we're talking about now is to pick off smaller sub questions ideally sub questions you can attack using math empirical observation or both you define the idea of a Q Prime so given an unanswerable philosophical riddle q replace it with a mirror leak in quotes scientific or mathematical question q prime which captures part of what people have wanted to know when they first asked q yes then with luck once all q prime so you described some examples of such Q prime sub questions in your long essay titled white philosophers should care about computational complexity so you catalog the various Q Prime's on which you think of theoretical computer science has made progress can you mention a few favorites if any pop any pup to mind or boy yes so I mean I would say some of the most famous examples in history of that sort of replacement well you know I mean I mean to go back to Alan Turing right what he did in his Computing Machinery and intelligence paper was exactly you know he explicitly started with the question can machines think and then he said uh sorry I think that question is too meaningless but here's a different question you know could you program a computer so that you couldn't tell the difference between it and a human right and you know yeah in the very first few sentences he in fact just yeah I miss the Q prime precise he does precisely that or you know we could look at at girdle right where you know you had these philosophers arguing for centuries about the limits of mathematical reasoning right in the limits of formal systems and you know then by the early 20th century logicians you know starting with you know frag a rustle and then you know most spectacularly girdle you know manage to reframe those questions as look we have these formal systems they have these definite rules are there questions that we can phrase within the rules of these systems that are not provable within the rules of the systems and can we prove that fact right and so that would be another example you know III had this essay called the ghost in the quantum Turing machine it's you know one of the crazier things I've written but I I tried to do something or you know to to advocate doing something similar there for free will where you know instead of talking about is free will you know real where we get hung up on the meaning of you know what exactly do we mean by freedom and can you have can you be you know or do we mean compatibilist free will libertarian free will what are these things mean you know I suggested just asking the question how well in principle consistently with the laws of physics could a person's behavior be predicted you know without so let's say destroying the person's brain you know taking it apart in the process of trying to predict them and you know and that actually asking that question gets you into all sorts of meaty and interesting issues you know issues of what is the computational substrate of the brain you know or can you understand the brain you know just at the sort of level of the neurons you know it sort of the abstraction of a neural network or do you need to go deeper to the you know molecular level ultimately even to the quantum level right and of course that would put limits on predictability if you did so you need to reduce you need to reduce the mind to a computational device like formalize it so then you can make predictions about what you know whether you could predict a B if you were trying to predict a person yeah then presumably you would need some model of their brain all right and now the question becomes one of how accurate can such a model become can you make a model that will be accurate enough to really seriously threaten people's sense of free will you know not just metaphysically but like really I've written in this envelope what you were going to say next is you see the right term here so it's also a level of abstraction has to be right so if your yeah if you're accurate at the somehow at the quantum level hmm that may not be convincing to us at the human level well right but the question is what accuracy at the sort of level of the underlying mechanisms do you need in order to predict the behavior right at the end of the day the test is just can you you know foresee what the person is going to do right I am you know and and and and you know and and and and in discussions of freewill you know it seems like both sides want to you know very quickly dismiss that question is irrelevant well to me it's totally relevant okay because you know if someone says oh well you know I will applause demon that knew the complete state of the universe you know could predict everything you're going to do therefore you don't have free will you know that it doesn't trouble me that much because well you know I've never met such a demon hey you know uh you know and we you know we even have some reasons the thing you know maybe it you know it could not exist this part of our world you know it was only an abstraction a thought experiment on the other hand if someone said well you know I have this brain scanning machine you know you step into it and then you know every paper that you will ever write it will write you know every thought that you will have you know even right now about the machine itself it will force a you know a well if you can actually demonstrate that then I think you know that that you know that that sort of threatens my internal sense of having free will in a much more visceral way you know but now you notice that we're asking in a much more empirical question we're asking is such a machine possible or isn't it I mean if it's not possible then what in the woes of physics or what about the behavior of the brain you know prevents it from existing so if you could philosophize a little bit within this empirical question at where do you think would enter the the by which mechanism would enter the possibility that we can't predict the outcome so there would be something they'll be akin to a free will yeah well you could say the the sort of obvious possibility which was you know kick knives by Addington and many others about as soon as quantum mechanics was discovered in the 1920s was that if you know let's say a sodium ion channel you know in the in the in in in the brain right hey today you know it's its behavior is chaotic right it sort of it's governed by these hodja hodja ly hot skin equations in neuroscience right which are differential equations that have a stochastic component right now where does you know and this ultimately governs let's say whether a neuron will fire or not that's a basic chemical process or electrical process by which signals are sent in the brain exactly exactly and and you know and so you could ask well well where does the randomness in the process you know that that neuroscientists you're but what neuroscientists would would treat is randomness where does it come from you know ultimately it's thermal noise right where does thermal noise come from but ultimately you know there were some quantum mechanical events at the molecular level that are getting sort of chaotically amplified but you know a sort of butterfly effect and so you know even if you knew the complete quantum state of someone's brain you know at best you could predict the probabilities that they would do one thing or do another thing right I think that part is actually relatively uncontroversial right the the controversial question is whether any of it matters for the sort of philosophical questions that we care about because you could say if all it's doing is just injecting some randomness into an otherwise completely mechanistic process well then who cares right and more concretely if you could build a machine that you know could just calculate the even just up the probabilities of all of the possible things that you would do all right and you know um you know if all the things that said you had a 10% chance of doing you did exactly a tenth of them you know and and and and and and so on that somehow also takes away the feeling of freedom exactly I mean I mean to me it seems essentially just as bad as if the Machine deterministically predicted you it seems you know hardly different from that so that so then but a more a more subtle question is could you even learn enough about someone's brain to do that okay because you know another central fact about quantum mechanics is that making a measurement on a quantum state is an inherently destructive operation okay so you know if I want to measure the you know position of a particle right it was well before I measure it it had a superposition over many different positions as soon as I measure I localized it right so now I know the position but I've also fundamentally changed the state and so so you could say well maybe in trying to build a model of someone's brain that was accurate enough to actually you know make let's say even even well calibrated probabilistic predictions of their future behavior maybe you would have to make measurements that we're just so accurate that you were just fundamentally alter their brain okay or or or or maybe not maybe you only you know you it would suffice to just make some nano robots that just measured some sort of much larger scale you know macroscopic behavior like you know is that you know what is this neuron doing what is that neuron doing maybe that would be enough see but now you know III but what I claim is that we're now asking a question you know in which you know it is it is it is possible to envision what progress on it would look like yeah but just as you said that question may be slightly detached from the philosophical question in the sense if consciousness somehow has a role to the experience of free will because ultimately what we're talking about free will we're also talking about not just the predictability of our actions but somehow the experience of them predictability yeah well I mean a lot of philosophical questions ultimately like feed back to the hard problem of consciousness you know and as much as you can try to sort of talk around it or not right and you know and then and and there is a reason why people try to talk around it which is that you know Democritus talked about the hard problem of consciousness you know in 400 BC in terms that would be totally recognizable to us today right and it's really not clear if there's been progress since or what progress could possibly consist of is there a Q prime type of sub question that could help us get it consciousness it's something about cars oh well I mean well I mean there is the whole question of you know of AI right of you know can you build a human level or superhuman level AI and you know can it can it work in a completely different substrate from the brain I mean there's you know of course that was Alan Turing's point and you know and and and even if that was done it's you know maybe people would still argue about the hard problem of consciousness right and yet you know my claim is a little different my claim is that in a world where you know there were you know human-level AI is where we had been even overtaken by such a eyes the entire discussion of the hard problem of consciousness would have a different character right it would take place in different terms in such a world even if we hadn't answered the question and and my claim about free will would be similar right that if there if this prediction machine that I was talking about could actually be built well now the entire discussion of the you know a free will is sort of transformed by that you know even if in some sense the the metaphysical question hasn't been answered yeah exactly transforms it fundamentally because say that machine does tell you that it can predict perfectly and yet there is this deep experience of free will and then that changes the question completely yeah and it starts actually getting to the question of the a the a GI the touring questions if the demonstration of free will the demonstration of intelligence the demonstration of consciousness does that equal nauseousness intelligence and free will but see elects if every time I was contemplating a decision you know this machine had printed out an envelope you know where I could open it and see that it knew my decision I think that actually would change my subjective experience of making decisions you might mean doesn't knowledge change your subjective experience well well you know I mean I mean the knowledge that this machine had pretty did everything I would do I mean it might drive me completely insane right but at any rate it would change my experience to act you know to not just discuss such a machine as a thought experiment but to actually see it yeah I mean I mean you know you could say at that point you know you could say you know what why not simply call this machine a second instantiation of me and be done with it right what we know what why even privilege the original me over this perfect duplicate that that exists in the machine yeah or yeah there could be a religious experience with a Jew it's kind of what God throughout the generations is supposed to that God kind of represents that perfect machine is able to I guess actually well I I don't even know what a work what are the religious interpretations of freewill yeah does so if God knows perfectly everything in in religion in the various religions were does freewill fit into that do you know that has been one of the big things that theologians have argued about for thousands of years you know I am I am NOT a theologian maybe I shouldn't go there there's not a clear answer in a book like I mean I mean this is you know the Calvinists debated this the you know this has been you know I mean different religious movements have taken different positions on that question but that is how they think about it you know meanwhile you know a large part of sort of what what animates you know theoretical computer science you could say is you know we're asking sort of what are the ultimate limits of you know what you can know or you know calculate or figure out by you know entities that you can actually build in the physical world right and if I were trying to explain it to a theologian maybe I would say you know we are studying you know to what extent you know gods can be made manifest in the physical world I'm not sure my colleagues would like that so let's talk about quantum computers yeah sure sure as you said modern computing at least in the 1990s was a profound story at the intersection of computer science physics engineering math and philosophy so the there's this broad and deep aspect to quantum computing that represents more than just the quantum computer yes but can we start at the very basics what is quantum computing yeah so it's a proposal for a new type of computation I would say a new way to harness nature to do computation that is based on the principles of quantum mechanics okay now the principles of quantum mechanics have been in place since 1926 you know they haven't changed you know what's new is you know how we want to use them okay so what does quantum mechanics say about the world you know the the physicists I think over the generations you know convinced people that that is an unbelievably complicated question and you know just give up on trying to understand it I can let you in not not being a physicist I can let you in on a secret which is that it becomes a lot simpler if you do what we do in quantum information theory and sort of take the physics out of it so the way that we think about quantum mechanics is sort of as a generalization of the rules of probability themselves so you know you might say there's a you know there was a 30% chance that it was going to snow today or something you would never say that there was a negative 30% chance right that would be nonsense much less would you say that there was a you know an I percent chance you know square root of minus 1% chance now the central discovery that sort of quantum mechanics made is that fundamentally the world is described by you know these are let's say the possibilities for you know what a system could be doing are described using numbers called amplitudes okay which are like probabilities in some ways but they are not probabilities they can be positive for one thing they can be positive or negative in fact they can even be complex numbers okay and if you've heard of a quantum superposition this just means the sum state of affairs where you assign an amplitude one of these complex numbers to every possible configuration that you could see assist them in on measuring it so for example you might say that an electron has some amplitude for being here and some other amplitude for being there right now if you look to see where it is you will localize it right you will sort of force the amplitudes to could be converted into probabilities that happens by taking their squared absolute value okay and then and and then you know you can say either the electron will be here or it will be there and you know knowing the amplitudes you can predict the price the probabilities that it will that you'll see each possible outcome okay but while a system is isolated from the whole rest of the universe the rest of its environment the amplitudes can change in time by rules that are different from the the normal rules of probability and that are you know alien to our everyday experience so any time anyone ever tells you anything about the weirdness of the quantum world you know or assuming that they're not lying to you right they are telling you you know and yet another consequence of nature being described by these amplitudes so most famously what amplitudes can do is that they can interfere with each other okay so in the famous double slit experiment what happens is that you shoot a particle like an electron let's say at a screen with two slits in it and you find that there you know on a second screen now there are certain places where that electron will never end up you know after it passes through the first screen and yet if I close off one of the slits then the electron can appear in that place okay so by so by decreasing the number of paths that the electron could take to get somewhere you can increase the chance that it gets there okay now how is that possible well it's because we you know as we would say now the electron has a superposition state okay it has some amplitude for reaching this point by going through the first slit it has some other amplitude for reaching it by going through the second slit but now if one amplitude is positive and the other one is negative then note you know I have to add them all up right I have to add the amplitudes for every path that the electron could have taken to reach this point and those amplitudes if they're pointing in different directions they can cancel each other out that would mean the total amplitude is zero and the thing never happens at all I closed off one of the possibilities then the amplitude is positive or it's negative and now the thing can happen okay so that is sort of the one trick of quantum mechanics and now I can tell you what a quantum computer is okay a quantum computer is a computer that tries to exploit you know these exactly these phenomena superposition amplitudes and interference in order to solve certain problems much faster than we know how to solve them otherwise so is the basic building block of a quantum computer is what we call a quantum bit or a qubit that just means a bit that has some amplitude for being zero and some other amplitude for being what so it's a superposition of zero in one states right but now the key point is that if I've got let's say a thousand cubits the rules of quantum mechanics are completely unequivocal that I do not just need one amp but you know I don't just need amplitudes for each qubit separately okay in general I need an amplitude for every possible setting of all thousand of those bits okay so that what that means is two to the 1000 power amplitudes okay if I if I had to write those down let's or let's say in the memory of a conventional computer if I had to write down two to the 1000 complex numbers that would not fit within the entire observable universe okay and yet you know quantum mechanics is unequivocal that if these qubits can all interact with each other and in some sense I need to to the 1000 parameters you know amplitudes to describe what is going on now you know now I can do you know where all the popular articles you know about quantum computer and go off the rails is that they say you know they they sort of sort of say what I just said and then they say oh so the way a quantum computer works is just by trying every possible answer in parallel okay you know you know that that sounds too good to be true and unfortunately it kind of is too good to be true that the problem is I could make a superposition over every possible answer to my problem you know even if there were two to the one thousand of them right I can I can easily do that the trouble is for a computer to be useful you've got at some point you've got to look at it and see and see an output right and if I just measure a superposition over every possible answer then the rules of quantum mechanics tell me that all I'll see will be a random answer you know if I just wanted a random answer well I could have picked one myself with a lot less trouble right so the entire trick with quantum computing with every algorithm for a quantum computer is that you try to choreograph a pattern of interference of amplitudes and you try to do it so that for each wrong answer some of the paths leading to that wrong answer have positive amplitudes and others have negative amplitudes so on the whole they cancel each other out okay whereas all the paths leading to the right answer should reinforce each other you know should have amplitudes pointing the same direction so the design of algorithms in the space is the choreography of the interferences precisely that's precisely what it was take a brief step back and write you mentioned information yes so in which part of this beautiful picture that you've painted is information contained oh well information is that the core of everything that we've been talking about right I mean the bit is you know the basic unit of information since you know Claude Shannon's paper in 1948 you know and you know of course you know people had the concept even before that you know he popularized the name right but I mean but a bit at zero or one that's right basically that's right and what we would say is that the basic unit of quantum information is the qubit is you know the object any object that can be maintained tennis manipulated in a superposition of 0 and 1 States now you know sometimes people ask well but but but what is a qubit physically and there are all these different you know proposals that are being pursued in parallel for how you implement qubits there is you know superconducting quantum computing that was in the news recently because of Google's the quantum supremacy experiment right where you would have some little coils where a current can flow through them in two different energy states one representing a 0 another representing the 1 and if you cool these coils to just slightly above absolute zero like a hundredth of a degree then they super conduct and then the current can actually be in a superposition of the two different states so that's one kind of qubit another kind would be you know just in an individual atomic nucleus it has a spin it could be spinning clockwise it could be spinning counterclockwise or it could be in a superposition of the two spin States that is another qubit but she's just like in the classical world right you could be a virtuoso programmer without having any idea of what a transistor is right or how the bits are physically represented inside the machine even that the machine uses electricity right you just care about the logic it's sort of the same with quantum computing right qubits could be realized by many many different quantum systems yet all of those systems will lead to the same logic you know the logic of qubits and and how you know how you measure them how you change them over time and so you know that the subject of you know how qubits behave and what you can do with qubits that is quantum information so just a linger on that short so does the physical design implementation of a qubit mm-hmm does not does not interfere with the that next level of abstraction that you can program over it so the true is the idea of it is is the a is it okay well to be honest with you today they do interfere with each other that's because the all the quantum computers we can build today are very noisy right and so sort of the the the you know the qubits are very far from perfect and so the lower level sort of affect the higher levels and we sort of have to think about all of them at once okay but eventually where we hope to get is to what are called error corrected quantum computers where the qubits really do behave like perfect abstract qubits for as long as we want them to and in that future you know the you know which you know a future that we can already Street or sort of prove theorems about or think about today but in that future the the logic of it really does become decoupled from the hardware so if noise is currently like the biggest problem for quantum computing and then the dream is error correcting modern computers can you just maybe describe what does it mean for there to be noise in the system absolutely so yeah so the problem is even a little more specific than noise so that the fundamental problem if you're trying to actually build a quantum computer you know of any appreciable size is something called decoherence okay and this was recognized from the very beginning you know when people first started thinking about this in the 1990s now what decoherence means is sort of unwanted interaction between you know your qubits you know the state of your quantum computer and the external environment okay and why is that such a problem why I said talked before about how you know when you measure a quantum system so let's say if I measure a qubit that's in a superposition of 0 and 1 States to ask it you know are you zero or are you one well now I force it to make up its mind right and now probabilistically it chooses one or the other and now you know it's no longer a superposition there's no longer amplitudes there's just there's some probability that I get a zero and there's some that I get a one and now the the the the the trouble is that it doesn't have to be me who's looking guy or in fact it doesn't have to be any conscious entity any kind of interaction with the external world that leaks out the information about whether this qubit was a 0 or a 1 sort of that causes the zero Ness or the oneness of the qubit to be recorded you know the radiation in the room in the molecules of the air in the wires that are connected to my device any of that as soon as the information leaks out it is as if that qubit has been measured okay it is you know the the the state has now collapsed you know another way to say it is that it's become entangled with its environment okay but you know from the perspective of someone who's just looking at this qubit it is as though it has lost its quantum state and so what this means is that if I want to do a quantum computation I have to keep the qubits sort of fanatically well isolated from their environment but then at the same time they can't be perfectly isolated because I need to tell them what to do I need to make them interact with each other for one thing and not only that but in a precisely choreographed way okay and you know that is such a staggering problem right how do i isolate these qubits from the whole universe but then also tell them exactly what to do I mean you know there were distinguished physicists and computer scientists in the 90s who said this is fundamentally impossible you know the laws of physics will just never let you control qubits to the degree of accuracy that you're talking about now what changed the views of most of us was a profound discovery in the mid to late 90s which was called the theory of quantum error correction and quantum fault tolerance okay and the upshot of that theory is that if I want to build a reliable quantum computer and scale it up to you know an arbitrary number of as many qubits as I want you know and doing as much on them as I want I do not actually have to get the cube it's perfectly isolated from their environment it is enough to get them really really really well isolated okay and even if every qubit is sort of leaking you know it state into the environment at some rate as long as that rate is low enough okay I can sort of encode the information that I care about in very clever ways across the collective states of multiple qubits okay in such a way that even if you know a small percentage of my cube it's leaked well I'm constantly monitoring them to see if that week happened I can detect it and I can correct it I can recover the information I care about from the remaining qubits okay and so you know you can build a reliable quantum computer even out of unreliable parts right now the the in some sense you know that discovery is what set the engineering agenda for quantum computing research from the 1990s until the present okay the goal has been you know engineer qubits that are not perfectly reliable but reliable enough that you can then use these error correcting codes to have them simulate qubits that are even more reliable than they are regarded the error correction becomes a net win rather than a net loss right and then once you reach that sort of crossover point then you know your simulated qubits could in turn simulate qubits that are even more reliable and so on until you've just you know effectively you have arbitrarily reliable cubans so long story short we are not at that break-even point yet we're a hell of a lot closer than we were when people started doing this in the 90s like orders of magnitude closer but the key ingredient there is the more qubits the butter because well the more qubits the larger the computation you can do right I mean I mean a qubit Tsar what constitute the memory of your quantum computer it also for the sorry for the error correcting mechanism yes so so so the way I would say it is that error correction imposes an overhead in the number of qubits and that it is actually one of the biggest practical problems with building a scalable quantum computer if you look at the error correcting codes at least the ones that we know about today and you look at you know what would it take to actually use a quantum computer to you know a I'm hack your credit card number because you know you know maybe you know the most famous application people talk about right let's say to factor huge numbers and thereby break the RSA cryptosystem well what what that would take would be thousands of several thousand logical cube but now with the known error correcting codes each of those logical qubits would need to be encoded itself using thousands of physical qubits so at that point you're talking about millions of physical qubits and in some sense that is the reason why quantum computers are not breaking cryptography already it's because of this these immense overheads involved so that overhead is additive or multiplicative I mean it's like you take the number of logical qubits that you need in your abstract quantum circuit you multiply it by a thousand or so so you know there's a lot of work on you know inventing better trying to invent better error correcting codes okay that is the situation right now in the meantime we are now in what physicist John Prescott called the noisy intermediate scale quantum or NIST era and this is the era you can think of it as sort of like the vacuum you know we're now entering the very early vacuum tube era of quantum computers the quantum computer analog of the transistor has not been invented yet right that would be like true error correction right where you know we are not or or something else that would achieve the same effect right we are not there yet and but but but where we are now let's say as of a few months ago you know as of Google's announcement of quantum supremacy you know we are now finally at the point where even with a non error corrected quantum computer with you know these noisy devices we can do something that is hard for classical computers to simulate okay so we can eke out some advantage now will we in this noisy era be able to do something beyond what a classical computer can do that is also useful to someone that we still don't know people are going to be racing over the next decade to try to do that by people I mean Google IBM you know a bunch of startup companies or you know a player's apps yeah and in research labs and governments and yeah you just mentioned a million things well backtrack for a sec yeah sure sure so we're in these vacuum tube days yeah just entering and I'm just entering Wow okay so yeah how do we escape the vacuum so we get to how to get to where we are now with the cpu is this a fundamental engineering challenge is there is there breakthroughs in on the physics side they're needed on the computer science side what Oh is there an is it a financial issue we're a much larger just sheer investment and excitement is new so you know those are excellent questions oh my god well no no my my my guess would be all of the above yeah I mean my my guess you know I mean I mean you know you could say fundamentally it is an engineering issue right the theory has been in place since the 90s you know at least you know you know this is what you know error correction what you know would look like you know we we do not have the hardware that is at that level but at the same time you know so you could just you know try to power through you know maybe even like you know if someone spent a trillion dollars on some quantum computing Manhattan Project right then conceivably they could just you know build a an error corrected quantum computer as it was envisioned back in the 90s right I think the more plausible thing to happen is that there will be further theoretical breakthroughs and there will be further insights that will cut down the cost of doing this so let's take good briefs yeah to the faux soft goal I just recently talked to Jim Keller who's a sort of like the famed architect and then in the microprocessor world okay and he's been told for decades every year that the Moore's law is going going to die this year and he tried tries to argue that the the Moore's law is still alive and well and it'll be alive for quite a long time to come how long how long he's is the the main point is it still alive but he thinks there's still a thousand X improvement just on shrinking a transition as possible whatever the point is that the exponential growth you see it is actually a huge number of these s curves just constant breakthroughs at the philosophical level mm-hmm why do you think we as a descendants of apes were able to to just keep coming up with these new breakthroughs on the CPU side is this something unique to this particular endeavor or will it be possible to replicate in the quantum computer space okay all right the other there was a lot there too but didn't it to to break off something I mean I think we are in an extremely special period of human history right I mean it's it is you could say obviously special you know in many ways right there you know you know way more people alive than there than there than there have been and you know the you know the whole you know future of the planet is in is in is in question in a way that it it hasn't been you know through for the rest of human history but but you know in particular you know we are in the era where you know we we finally figured out how to build you know Universal machines it's that you know the things that we call computers you know machines that you program to simulate the behavior of whatever machine you want and you know and and and and and and and and and once you've sort of crossed this threshold of universality you know you've built you could say you know touring you've instantiated touring machines in the physical world well then the main questions are ones of numbers there you know ones of how many of how much memory can you access how fast does it run how many parallel processors you know at least until quantum computing quantum computing is the one thing that changes what I just said right you know in fear as well as well as long as it's classical computing then it's all questions of numbers and you know the you could say at a theoretical have all the computers that we have today are the same as the ones in the 50s they're just millions of times you know faster and with millions of times more memory and you know I mean I think there's been an immense economic pressure to you know get more and more transistors you know get them smaller and smaller get you add more and more cores and you know and and and and in in in some sense like a huge fraction of sort of all of the technological progress that there is in all of civilization has gotten concentrated just more narrowly into just those problems right and so you know it has been one of the biggest success stories in the history of technology right there's you know I mean it is I am as amazed by it as anyone elses right but at the same time you know we also know that it you know and I I really do mean we know that it cannot continue indefinitely okay because you will reach you know fundamental limits on you know how small you can possibly make a processor and you know if you want a real proof you know that would justify my use of the word you know we know that you know Moore's law has to end I mean ultimately you will reach the limits imposed by quantum gravity you know you know if you were doing if you tried to build a computer that operated at 10 to the 43 Hertz so did 10 to the 43 operations per second that computer would use so much energy that it would simply collapse to a black hole okay so you know that you know we you know in reality we're going to reach the limits long before that but you know that is a sufficient proof that there's a limit yes yes but it would be interesting to try to understand the mechanism the economic pressure these said just like the Cold War was a pressure on getting us getting us cuz I'm both my us is both the Soviet Union and the United States yeah getting us the two countries to get to hurry up to get the space to the moon there seems to be that same kind of economic pressure that somehow created a chain of engineering breakthroughs there resulted in the Moore's law yeah what'd be nice to replicate yeah well I mean I mean some people are sort of get depressed about the fact that technological progress you know may seem to have slowed down in in many many realms outside of computing right there was this whole thing of you know we wanted flying cars and we only got Twitter instead right and yeah go Peter - yeah yeah yeah right right so when then jumping to another really interesting topic the invention so Google mmm announced with their work in the paper in nature with quantum supremacy yes can you describe again back to the basic what is perhaps not so basic what is quantum supremacy absolutely so quantum supremacy is a term that was coined by again by John Prescott in 2012 not not everyone likes the name you know but uh you know it's sort of stuck you know we don't and we sort of haven't found a better alternative wantem computational compromise dude that's right that's right and but but the basic idea is actually one that goes all the way back to the beginnings of quantum computing when richard fineman and david deutsch people like that we're talking about it in the early 80s and-and-and and quantum supremacy just refers to sort of the point in history when you can first use a quantum computer to do some well-defined task much faster than any known algorithm running on any of the classical computers that are available okay so you know notice that I did not say a useful task okay you know it could be something completely artificial but it's important that the task be well-defined so in other words you know there is it is something that has right and wrong answers you know and that are knowable independently of this device right and we can then you know run the device see if it gets the right answer or not can you clarify a small point you said much faster than a classical implementation what about sort of what about the space with where the class there's no there's not it doesn't even exist a classical algorithm so so so so maybe I should clarify everything that a quantum computer can do a classical computer can also eventually do okay and the reason why we know that is that a a classical computer could always you know if it had no limits of time and memory it could always just store the entire quantum state you know of your you know of the quantum store in a list of all the amplitudes you know in the state of the quantum computer and then just you know do some linear algebra to just update that state right and so so anything that quantum computers can do can also be done by classical computers albeit exponentially slower okay so on and computers don't go into some magical place outside of Alan Turing's a definition of computation precisely they do not solve the halting problem they cannot solve anything that is uncomputable in Alan Turing sense what they what we think they do change is what is efficiently computable okay and you know since the 1960s you know the word efficiently you know as well as been a central word in computer science but it's sort of a code word for something technical which is basically with polynomial scaling you know that as you get to larger and larger inputs you would like an algorithm that uses an amount of time that scales only like the size of the input raised to some power and not exponentially with the size of the input right yeah so I I do hope we get to talk again because one of the many topics that there's probably several hours with a competent conversation on is complexity which probably won't even get a chance to touch today but you briefly mentioned it but let's let's maybe try to continue so you said the definition of quano supremacy is basically design is achieving a place where much faster on a formal that quantum computer is much faster on a formal well-defined problem yes it's not that is or isn't useful yeah yeah yeah right right and and I would say that we really want three things right we want first of all the quantum computer to be much faster just in the literal sense so like number of seconds you know it's a solving this you know well-defined you know problem secondly we want it to be sort of a you know for a problem where we really believe a quantum computer has better scaling behavior right so it's not just an incidental you know matter of hardware but it's that you know as you went to larger and larger inputs you know the classical scaling would be exponential and the scaling for the quantum algorithm would only be polynomial and then thirdly we want the first thing the actual observed speed-up to only be explainable in terms of the scaling behavior right so you know I want I want you know a real world you know a real problem to get solved let's say by a quantum computer with 50 cubits or so and for no one to be able to explain that in any way other than well you know the to the this this computer involved a quantum state with 2 to the 50th power amplitudes and you have a classical simulation in at least any that we know today would require keeping track of 2 to the 50th numbers and this is the reason why it was faster so the intuition is that then if you demonstrate on 50 cubits then once you get to 100 cubits then it'll be even much more faster precisely precisely yeah and and you know and and and quantum supremacy does not require error correction right we don't you know we don't have you could say true scalability yet or true you know uh error correction yet but you could say quantum supremacy is already enough by itself to refute the skeptics who said a quantum computer will never outperform a classical computer for anything but one  demonstrate quantum yeah supremacy and two what's up with these new news articles I'm reading that Google did so yeah what they actually do great great questions because now you get into actually you know a lot of the work that I've you know I and my students have been doing for the last decade which was precisely about how do you demonstrate quantum supremacy using technologies that you know we thought would be available in the near future and so one of the main things that we realized around 2011 and this was me and my Alyx arkhipov at MIT at the time and independently of some others including a Bremner joseon shepard okay and the realization that we came to was that if you just want to prove that a quantum computer is faster you know and not do something useful with it then there are huge advantages to sort of switching your attention from problems like factoring numbers that have a single right answer to what we call sampling problems so these are problems where the goal is just to output a sample from some probability distribution let's say over strings of 50 bits right so there are you know many many many possible valid outputs you know your computer will probably never even produce the same output twice you know if it's running as even you know assuming it's running perfectly okay but but the key is that some outputs are supposed to be like clearer than other ones so sorry to clarify is there a set of outputs that are valid and said they're not or is it more that the distribution of a particular kind of output is more is the specific distribution yeah particular there's there's there's a specific distribution that you're trying to hit right or you know that you're trying to sample from now there are a lot of questions about this you know how do you do that right now now how you how you do it you know it turns out that with a quantum computer even with the noisy quantum computers that we have now that we have today what you can do is basically just apply a randomly chosen sequence of operations all right so we you know we in sometimes you know we you know that part is almost trivial right we just sort of get the qubits to interact in some random way although a sort of precisely specified random way so we can repeat the exact same random sequence of interactions again and get another sample from that same distribution and what this does is it basically well it creates a lot of garbage but you know very specific garbage right so you know of all of the so if we're gonna talk about Google's device there were 53 qubits there okay and so there are two to the 53 power possible outputs now for some of those outputs you know there are there was a little bit more destructive interference in their amplitude okay so their amplitudes were a little bit smaller and for others there was a little more constructive interference you know the amplitudes were a little bit more aligned with each other you know that and so those those that were a little bit likelier okay all of the outputs are exponentially unlikely but some are let's say two times or three times you know unlikely er than others okay and so so you can define you know the sequence of operations that gives rise to this probability distribution okay now the next question would be well how do you you know even if you're sampling from and how do you verify that right how do you exam how do you know and so my students and I and also the people at Google we're doing the experiment came up with statistical tests that you can apply to the outputs in order to try to verify you know what is you know that that at least that some hard problem is being solved the the test that Google ended up using was something that they called the linear cross entropy benchmark okay and it's basically you know so the the drawback of this test is that it requires like it requires you to do a two to the 53 time calculation with your classical computer okay so it's very expensive to do the test on a classical computer the good news I think of a numbers - it's about 9 quadrillion ok doesn't help well well you know you want to be like scientific notation oh no what I mean is yeah it is it is it is impossible to run on us yes so we will come back to that it is just barely possible to run we think on the largest supercomputer that currently exists on earth which is called summit at Oak Ridge National Lab ok so I ironically for this type of experiment we don't want a hunch Kubitz okay because with a hundred cubits even if it works we don't know how to verify the results okay so we want you know a number of qubits that is enough that you know click the biggest classical computers on earth we'll have to sweat you know and we'll just barely you know be able to keep up with with the quantum computer you know using much more time but they will still be able to do it in order that we can verify that was just where the 53 comes from for the qubit well I mean I mean I mean I mean I mean that's also that sort of you know the mote I mean that's that's that's sort of where they are now in terms of scaling you know and then you know soon you know that point will be passed and and then when you get to larger numbers of qubits then you know these these types of sampling experiments will no longer be so interesting because we won't even be able to verify the results and we'll have to switch to other types of computations so with it with the sampling thing you know so so the test that Google applied with this linear cross entropy benchmark would basically just take the samples that were generated which are you know a very small subset of all the possible samples that there are but for those you calculate with your classical computer the probabilities that they should have been output and you see are those probabilities like larger than the mean you know so is the quantum computer bias toward outputting the strings that it's you know that you want it to be biased toward okay and then finally we come to a very crucial question which is supposing that it does that well how do we know that a classical computer could not have quickly done the same thing right how do we know that you know this couldn't have been spoofed by a classical computer right and so well the first answer is we don't know for sure because you know this takes us into questions of complexity theory you know you know the I mean questions on the of the magnitude of the P versus NP question and think that right we you know we don't know how to rule out definitively that there could be fast classical algorithms for you know even simulating quantum mechanics and for you know simulating experiments like these but we can give some evidence against that possibility and was sort of the you know the main thrust of a lot of the work that my colleagues and I did you know over the last decade which is then sort of in around 2015 or so what led to Google deciding to do this experiment so is the kind of evidence you first of all the hard P equals NP problem that you mentioned and the kind of evidence the year were looking at is that something you come to on a sheet of paper or is this something are these empirical experiments it's it's math for the most part I mean it you know it's also trot you know you know we have a bunch of methods that are known for simulating quantum circuits or you know quantum computations with classical computers and so we have to try them all out and make sure that you know they don't work you know make sure that they have exponential scaling on on you know these problems and and not just theoretically but with the actual range of parameters that are actually you know arising in Google's experiment okay so so there is an empirical component to it right but now on on the theoretical side you know what basically what we know how to do in theoretical computer science and computational complexity is you know we don't know how to prove that most of the problems we care about are hard but we know how to pass the blame to someone else yeah we know how to say well look you know I can't prove that this problem is hard but if it is easy then all these other things that you know if you know first you you probably we're much more confident or we're hard that then those would be easily as well okay so so we can give what are called reductions this has been the basic strategy in you know an NP completeness right in all of theoretical computer science and cryptography since the 1970s really and so we were able to give some reduction evidence for the hardness of simulating these sampling experiments these sampling based quantum supremacy experiments the reduction evidence is not as satisfactory as it should be one of the biggest open problems in this area is to make it better but you know we can do something you know certainly we can you say that you know if there is a fast classical algorithm to spoof these experiments then it has to be very very unlike any of the algorithms that we know which is kind of in the same kind of space of reasoning that people say P equal not equals NP yeah it's in the same spirit yeah in the same spirit okay so Andrew yang a very intelligent and presidential candidate with a lot of interesting ideas in all kinds of technological fields tweeted that because of quantum computing no code is uncrackable is he wrong or right he was premature let's say so well okay wrong look i you know i i'm actually i'm you know i'm a fan of andrew yang I like his cat you know I like his ideas I like his candidacy I think that uh you know he you know he may be ahead of his time with you know the universal basic income and you know and so forth and he may also be ahead of his time in that tweet that you referenced so guarding regarding using quantum computers to break cryptography so the situation is this okay so the famous discovery of Peter shor you know 26 years ago that really started quantum computing you know as an autonomous field was that if you build a full scalable quantum computer then you could use it to efficiently find the prime factors of huge numbers and calculate discrete logarithms and solve a few other problems that are very very special and character right they're not np-complete problems we're pretty sure they're not okay but it so happens that most of the public key cryptography that we currently use to protect the internet is based on the belief that these problems are hard okay what sure showed is that once you get scalable quantum computers then that's no longer true okay but now you know uh you know before people panic there are two important points to understand here okay the first is that quantum supremacy the milestone that Google just achieved is very very far from the kind of scalable quantum computer that would be needed to actually threaten public key cryptography okay so you know we touched on this earlier bright but Google's device has 53 physical qubits right you threaten cryptography you're talking you know with any of the known error correction method you're talking millions of physical qubits because error correction would be required yes yes yes yeah yes yeah it's a it certainly would right and uh you know how much you know how great will the overhead be from the error correction that we don't know yet but with the known codes you're talking millions of physical qubits and of a much higher quality than any that we have now okay so you know I I don't I don't think that that is you know coming soon although people who have secrets that you know need to stay secret for 20 years you know are already worried about this you know for the good reason that you know we presume that intelligence agencies are already scooping up data you know in the hope that eventually they'll be able to decode it once quantum computers become available okay so so there is so so so so this brings me to the second point I wanted to make which is that there are other public key cryptosystems that are known that we don't know how to break even with quantum computers okay and there so there's a whole field devoted to this now which is called post quantum cryptography okay and so there is already so so we have some good candidates now the best-known being what are called lattice based crypto systems and there is already some push to try to migrate to these crypto systems so NIST in the u.s. is holding a competition to create standards for post quantum cryptography which will be the first step in trying to get every web browser and every router to upgrade you know and use a you know some like SSL that is would be based on on you know what we think is quantum secure cryptography but you know this will this will be a long process but you know it is it is something that people are already starting to do and so so you know I'm sure as algorithm is was sort of a dramatic discovery you know it could be a big deal for whatever Intelligence Agency first gets a scalable quantum computer if no at least certainly if no one else knows that they have it right but eventually we think that we could migrate the internet to the post quantum cryptography and we'd be more or less back where we started okay so this is sort of not the application of quantum computing I think that's really going to change the world in a sustainable way right the C big by the way the biggest practical application of quantum computing that we know about by far I think is simply the simulation of quantum mechanics itself in order to you know learn about chemical reactions you know design maybe new chemical processes new materials new drugs new solar cells new superconductors all kinds of things like that what's the size of a quantum computer that would be able to simulate the you know quantum mechanical systems themselves that would be impactful for the real world for the kind of chemical reactions and that kind of work what what scalar were talking about now you're asking a very very current question a very big question people are going to be racing over the next decade to try to do useful quantum simulations even with you know 100 or 200 cubic quantum computers of the sort that we expect to be able to build over the next decade ok so that might be you know the first application of quantum computing that we're able to realize you know or or maybe it will prove to be too difficult and maybe even that will require fault tolerance or you know will require error correction that's an aggressive race to come off with the one case study kind of like the computer sure the with a with the idea that would just capture the world's imagination of yeah look we can actually do something very yeah right but I think you know within the next decade the best shot we have is certainly not you know Shor's algorithm to break cryptography you know it's just just because it requires you know too much in the way of error correction the best shot we have is to do some quantum simulation that tells the material scientists or chemists or nuclear physicists you know something that is useful to them and that they didn't already know you know and you might only need one or two successes in order to change some you know billion-dollar industries right like you know the way that people make fertilizer right now is still based on the hopper Boetsch process from a century ago and it is some many-body quantum mechanics problem that no one really understands right if you could design a better way to make fertilizer right that's you know billions of dollars right there so so so those are sort of the applications that people are going to be aggressively racing toward over the next decade now I don't know if they're gonna realize it or not but you know it is you know there's there sir they certainly at least have a shot so it's gonna be a very very interesting next decade justify what's your intuition is if a breakthrough like that comes would is it possible for that breakthrough to beyond 50 to 100 cubits or is scale a fundamental thing like 500 1000 of us qubits yeah so I I can tell you what the current studies are saying you know I I think probably better to rely on that that my intuition but you know there there was a group at Microsoft had a study a few years ago that said even with only about 100 cubits you know you could already learn something new about this the chemical reaction that makes fertilizer for example the trouble is they're talking about a hundred qubits and about a million layers of quantum gates okay so there so basically they're talking about a hundred nearly perfect qubits so the logical qubits is you measure exactly a hundred logical qubits and and now you know the hard part for the next decade is going to be well what can we do with a hundred to two hundred noisy qubits yeah yeah is that an error correction breakthroughs that might come without and need to do thousands or millions of yeah yeah so people are gonna be pushing simultaneously on a bunch of different directions one direction of course is just making the cube it's better right and you know there's there there is tremendous progress there I mean you know the fidelity is like the the the accuracy of the qubits isn't improved by several orders of magnitude you know in the you know and the last decade or two okay the second thing is designing better our you know let's say lower overhead error correcting codes and even short of doing the full recursive error correction you know there were these error mitigation strategies that you can use you know that may you know allow you to eke out a useful speed-up in in the near term and then the third thing is just taking the quantum algorithms for simulating quantum chemistry or materials and making them more efficient you know and those algorithms are already dramatically more efficient than they were let's say five years ago and so when you know I quoted these estimates like you know circuit depth of 1 million and so you know I hope that because people will care enough that these numbers are going to come down so you're one of the the world-class researchers in this space there's a few groups that we mentioned Google and IBM working at this there's there's other research labs but you put also you have an amazing blog you just you put a lot on your put a you paid me to say it you put a lot a lot of effort sort of to communicating the science of this and communicating exposing some of the BS and sort of the natural just like in the AI space the natural charlatan ism that's the word in this in quantum mechanics in general but quantum computers and so on can you give some notes about people or ideas that people like me or listeners in general from outside the field should be cautious of when they're taking in news headings that Google achieved quantum supremacy what should we look out for where's the Charlatans in the space where's the BS yeah so a good question unfortunately quantum computing is a little bit like cryptocurrency or deep learning and like there is a core of something that is genuinely revolutionary and exciting and because of that core it attracts this sort of vast penumbra of you know people making you know just utterly ridiculous claims and so with with quantum computing I mean I would say that the main way that people go astray is by you know not focusing on sort of the question of you know are you getting a speed-up over a classical computer or not right and so so you know people have like a dismissed quantum supremacy because it's not useful right or you know it's not itself let's say obviously useful for anything okay but you know ironically these are some of the same people who will go and say well we care about useful applications we care about solving traffic routing and Optima you know and and financial optimization and all these things and that sounds really good you know but they're you know they're their entire spiel is sort of counting on nobody asking the question yes but how well could a classical computer do the same thing yes right you know it I really mean the entire thing is is is you know it you know they they say well a quantum computer can do this a quantum computer can do that right and they just avoid the question are you getting a speed-up over a classical computer or not and you know if so how you know how how do you know have you really thought carefully about classical algorithms to do you know to solve the same problem right and a lot of the application areas that you the you know companies and investors are most excited about that the popular press is most excited about you know for quantum computers have been things like machine learning AI optimization okay and the problem with that is that since the very beginning you know even if you have a perfect you know fault tolerant you know quantum c'mon um computer you know we have known of only modest speed ups that you can get for these problems okay so so there is a famous quantum algorithm called Grover's algorithm okay and what it can do is it can solve many many of the problems that arise in AI machine learning optimization including np-complete problems okay but it can solve them in about the square root of the number of steps that a classical computer would need for the same problems okay now a square root speed-up is you know important it's impressive it is not an exponential speed-up okay so it is not the kind of game-changer that lets say Shor's algorithm for factoring is or for that matter that simulation of quantum mechanics is okay it is a more modest speed up let's say you know roughly you know in theory it could roughly double the size of the optimization problems that you could handle right and and so what you know because people found that I guess to to boring or you know to unimpressive you know they've gone on to to like invent all of these heuristic algorithms where you know because no one really understands them you can just project your hopes on to them right that well maybe it gets an exponential speed-up you can't prove that it doesn't you know and the burden is on you to prove that it doesn't get a speed-up right and you know so they've done an immense amount of that kind of thing and a really worrying amount of the case for building a quantum computer has come to rest on this stuff that those of us in this field know perfectly well is on extremely shaky foundations so the fundamental question is yeah show that there's a speed-up yes Icicle absolutely in this space that you're referring to which is actually interesting the area that a lot of people excited about is machine learning yeah so your senses do you think it will so I know that there's a lot of smoke currently yeah but do you think they're actually eventually might be breakthroughs where you do get exponential speed ups in the machine learning space absolutely there might be I mean I think we know of modest speed ups that you can get for these problems I think you know whether you can get bigger speed ups is one of the the biggest questions for quantum computing theory you know for people like me to be thinking about now you know we had actually recently a really you know a super exciting candidate for an exponential quantum speed-up for a machine learning problem that people really care about this is basically the Netflix problem the problem of recommending products to users given some sparse data about their preferences Karen etus and Prakash in 2016 had an algorithm for sampling recommendations that was exponentially faster than any known classical algorithm right and so I you know a lot of people were excited I was excited about it I had an eighteen-year-old undergrad by the name of Alain Tang and she was you know she was obviously brilliant she was looking for a project I gave her as a project can you prove that this speed-up is real can you prove that you know any classical algorithm would need to access exponentially more data right and you know this this was a case where if that was true this was not like a P versus NP type of question right this this might well have been provable but she worked on it for a year she couldn't do it eventually she figured out why she couldn't do it and the reason was that that was false there is a classical algorithm with a similar performance to the quantum algorithm so even succeeded in D quantizing that machine learning algorithm and then in the last couple of years building on a wins breakthrough a bunch of the other quantum machine learning algorithms that were proposed have now also been D quantized yeah okay and so I would say again backwards step yes a like a yes or a forward step for science but well yeah step for a machine-learning yeah that that precedes the big next forward step right right right now if it's bright now some people will say well you know there's a silver lining in this cloud they say well look thinking about quantum computing has led to the discovery of potentially useful new classical algorithm it's true right and so you know so you get these spin-off applications but if you want a quantum speed-up you really have to think carefully about that you know e winds work was a perfect illustration of why right and I think that you know the the challenge you know that you know that the the field is now open right find a better example find you know where quantum computers are going to deliver big gains for machine learning you know I and I am you know not only do i ardently support you know people thinking about that I'm trying to think about it myself and have my students and postdocs think about it but we should not pretend that those speedups are already established and and the problem comes when so many of the companies and you know and and journalists in this space are pretending that like all good things like life itself this conversation must soon come to an end let me ask the most absurdly philosophical last question okay what is the meaning of life what gives your life fulfillment purpose happiness and yeah meaning I would say you know number one trying to discover new things about the world and and share them and you know communicate and and learn what other people have discovered you know number two you know my friends my family my kids my students you know they're just the people around me number three you know trying you know when I can to you know make the world better and some small ways and you know it's the pressing that I can't do more and that you know the world is you know in you know facing crises over you know the climate and over you know resurgent authoritarianism and all these other things but you know trying to stand against the things that I find horrible when I can let me ask ya one more absurd question yeah what makes you smile well yeah I guess your question just did I don't know I thought I tried that absurd one on you well is a huge honor to talk to you we'll probably talk to you for many more hours Scott thank you so much well thank you thank you it was great thank you for listening to this conversation with Scott Aaronson and thank you to our presenting sponsor cash app download it used coal XPath cast you'll get $10 $10 will go to first an organization that inspires and educates young minds to become science and technology innovators of tomorrow enjoy this podcast subscribe on youtube give it five stars an apple podcast supported on patreon or simply connect with me on Twitter Alex Friedman now let me leave you awards from a funny and insightful blog post Scott wrote over 10 years ago on the ever-present Malthusian isms in our daily lives quote again and again I've undergone the humbling experience of first lamenting how badly something sucks then only much later having a crucial insight that it's not sucking wouldn't have been a Nash equilibrium thank you for listening I hope to see you next time you