Quantum Computing – Decoherence – Extra History – #5
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Quantum Computing – Decoherence – Extra History – #5

Why does it take so long for us to increase the number of qubits we can effectively have a computer perform operations on? Why isn’t it just like slapping more circuits onto a board? The answer: decoherence. [Birth Of The People] by Demetori When running algorithms on a quantum computer, we get the results of those algorithms in the loosest sense, by seeing how they affected our entangled set of qubits. But in order to know what effects those algorithms are having, we have to know where we started from, we have to know what state our qubits were in before we ran the algorithm. And with something the size of quantum particles, that is actually ridiculously difficult. Because, and again, I’m explaining this in the loosest possible terms, while we can track all of the changes that happen on our set of entangled particles by the code we’re running on them, what happens when those particles are changed by something else that we’re not observing? What happens if something we don’t notice changes the state of our quantum particles and then we run code on them? Well, the data we get back is bad. We get back answers to our questions that are just plain wrong, because some variables we weren’t tracking influenced the outcome. But what sort of thing could change the properties of this incredibly complex entangled quantum system of ours? Well, annoyingly, just about anything. These systems are incredibly delicate; if our qubits get jostled by some stray atoms or bump into some air molecules or collide with other photons; any of those things could cause decoherence. They could affect our data in ways we can’t track. And as we talked about in the very first episode, even observation can cause our wave function to collapse and our carefully crafted set of entangled particles to decohere. This is why quantum computing projects are often done at incredibly low temperatures, in unbelievably sterile environments. As you might expect, one of the long-term goals for the advancement of quantum computing is not only to create computers with more qubits, but to figure out a way to maintain coherence in what we’d consider a much more… normal environment. But there’s another interesting limiting factor on quantum computing. It’s a limitation that everyone working on the problem has known was there from the get-go. But it might not be something that immediately springs to mind if you’re not already deeply immersed in the stuff. Okay, so think back to the very first episode of this series, when we talked about the double slit experiment. I know, back then, we were so young. The waves of possibility, the space where our photon could possibly be, pass through both slits and then interferes with itself to create a way of where the photon will probably be. With the peaks of that wave representing very likely spots and the valleys representing very unlikely spots. And then, when it hits the wall in the back, the wave form collapses and resolves itself into one actual point we can observe, right? Well, in the case of our quantum computer, we’re the wall. Remember, our array of entangled qubits is useful because it’s a quantum superposition. A probability waveform. All this interesting calculation is happening on that waveform. But as we know from the double slit experiment, any attempt at measuring or observing a quantum superposition, causes it to collapse and resolve down to an actuality, A single point. Something much more akin to our traditional classical computing form of data. So, if we can’t actually get at the data we’re processing, what use is it? What if we found a way to use the collapsed waveform? Suppose we were to ask our quantum computer questions that will cause the waveform to collapse in ways that provided us data which we can use classical computing methods to interpret. For example, just like last episode, let’s say we’re looking through a registry of guests at a hotel to determine if John Doe is staying there. in classical computing, we’d have to query each name: see if it matched “John Doe” and if it didn’t, then move on to the next one With a quantum computer we could, essentially, look at all the names at once, which would be way faster. But, since that act is being done at the level of the quantum superposition, we can’t actually extract the data. We can’t actually see each point on that calculation. Which means that we won’t be able to look at the data set and see the answer to, “Is this name, ‘John Doe’?” for each line of the registry. But if we can craft our algorithm correctly, we can engineer it so that the quantum waveform of our qubits collapses in a way that answers the overarching question, “Is John Doe at this hotel?”. With a simple yes-or-no answer; an answer which is completely understandable in classical computing terms. So, what does this all mean? Well, it means that quantum computing is not a replacement for classical computing. In fact, any given operation on a quantum computer will probably be slower than performing that same operation on a classical one. At least, for the foreseeable future. But it also means that for certain tasks for specific questions which we know how to tease out, we can perform certain computing tasks using exponentially fewer operations; meaning that even though the individual operations may run more slowly, the actual task will be completed much, much faster. So, while you might not be using quantum computing to browse the web, play a game, or write a word document anytime soon, it is possible that the classical computers of tomorrow will be querying quantum computers out there in the cloud to perform specific tasks a thousand times more quickly than we could process them on our own motherboards. For certain businesses, research, or governmental tasks which involve sorting massive data sets to pull out specific information or to find answers to specific questions, the benefits could be huge. What does that mean for us? What does that mean for Humanity at large? Join us next time as we wrap all this up by discussing the future of quantum computing, delving into the question of what quantum computing might eventually do for us, what pitfalls we might run into, and what the next few decades might bring. {Spoiler Alert} Skynet. Just kidding. Am I? Time travel or just tune in next time to find out. *Space Sound* Come with me if you want to see episode six. Okay, wait, how did you time travel with clothes? [Subatomic Fugue] by Tiffany Roman


  • Extra Credits

    Why does it take so long for us to increase the number of qubits we can effectively have a computer perform operations on? Why isn’t it just like slapping more circuits on a board? The answer: Decoherence.
    Join the Extra History community: https://patreon.com/extracredits

  • Matt Kuhn

    I'm glad you touched on the true utility (at least initially) of quantum computing, because it ties in quite nicely with my work. I'm a computational linguist, which means I use computer systems to process and reproduce human language in various ways. Basically, I program AI to learn how to speak and understand language. One of the big problems that CL has is that natural human languages follow Zipf's law, and thus the vast majority of words occur very sparsely in practice. The words that are common are "function" or "stop" words that basically carry no inherent meaning of their own, and so aren't really that important as far as "understanding" a text or recording. Therefore, in order to learn, say, how to translate a document, your system has to process an incredibly large amount of data. We're talking tens of millions of words at the minimum – billions if you can get them. Not only are such large datasets (or as we call them, corpora) rare and hard to get, processing them takes an incredible amount of processing time, and so a lot of my field involves tricks to process them more quickly. Although the training takes a long time (sometimes weeks or even months of processing), the actual translation step is actually really fast – that's why you can put in a sentence into Google translate and get an answer basically instantly. Quantum computing holds vast possibilities for AI, because it would allow us to process these datasets orders of magnitude faster. Excited to see the next episode, and great job so far guys!

  • The Nothing Nobody

    "Time travel, or just wait for episode 6"

    Well TECHNICALLY, aren't we all always traveling forward in time at a rate of 1 second per second?

  • Tiara Ashdoll

    The code at 0:56 has a problem, assuming <3 is a valid operator, the first for loop, it should be x < qubit.length, not i < qubit.length

  • Stephanie Vite

    Can you guys do a series for vaccinations and the history behind them? I am genuinely curious on the history, and it would hopefully help people realize that it doesn't cause autism which is always a plus.

  • Konstantin Khitrin

    I actually don't have any serious complaints for once. Photons marked with 'p' threw me off for a moment. That's usually how we label protons. Photons are marked with Greek gamma, as in gamma radiation. But that's not even a real complaint. Just something that momentarily confused me.

  • Thibaut Hanson

    Sure, an operation on an array done faster "in the cloud" than in local.
    I can imagine a quantum coprocessor, eventually, but with communication protocols, latency and others, don't even hope to be faster if you ask "the cloud" for an answer. First of all, all the data would have to be shared between local and distant (since some operations are faster locally and others are faster "out there") thus being a nightmare to synchronize.
    On top of that, a network operation is thousands, if not tens of thousands slower than a local operation. It would require databases with tens of billions entry before the "quantum cloud" could be faster.

  • Madame Muffin

    Soooo, quantum computers are not really that good as computers, however, as a sort of server for mass calculations, they are very useful.

  • BoilingJD

    Doesn't the uncertainty principle prevent this from happening ? In order to get accurate results accurate measurement has to be made, but the more accurately you measure A the less accurate is B. So if accurate measurement is impossible, how would you find a way to generate an initial setup if you are not operating with absolute values?

  • Suheil Gonzalez

    John Doe must be the one hell of a spy cause it takes quantum computing broken down into classical computing just to find out where he is.

  • Sahuagin

    it seems likely that rather than avoiding computational complexity, quantum computation just trades exponentially scaling computational complexity for a different kind of exponential scaling, such as energy. it's really really hard to maintain a perfect environment in which to do the computations because you've traded time-complexity for the energy that it takes to maintain that environment. there are ways for it to take space instead of energy or time, too, but it will still scale exponentially.

  • Halloucinogenic Clown Genitals

    This video came out in Oct, 2018. In March, 2018, Google made a 72 qubit computer. But, Matt says the most amount of qubits in a computer was 20. ???

  • klklkl427586

    Stopped watching after this video. If you made this amount of mistakes on entanglement i can't trust you with things i don't know.

  • Amber Armstrong

    If the photons ‘possibility wave’ can be ‘observed’ by anything else on the subatomic scale through their interactions, (decoherence) then I have two question. First, could the atoms we know the least about, dark matter (most of the universe) or antimatter (or other as of yet unknown particles) be having some interactions with photons we can’t detect? 2nd (this is grasping at straws for my mind to not admit to that spooky action for no reason) I’ve seen the compassions made between a creature living in two dimensions to three (us) and ideas of what reality could look like for a 4th dimensional being. Now if spacetime are the same thing, then a being in that dimension may see a field of grass, that same field as a forest later and then barren much later after as the exact same thing. A being that adds another axis to our three dimensions of spacetime maybe by using the these quantum properties, maybe it can ‘see’ the probability wave, or much like that spooky action across a galaxy, it has spooky action across spacetime. Being instantly connected to its whole life, from start to finish at all times, or (more insane) all the possibilities it could be as well. It could live in its own possibility wave, or maybe produce, or maybe it could be a living being that IS a possibility wave.Basically, could these almost magic actions of the quantum realm be applied to make (or explain) a living being in a 4th dimension? Ahh I’m just a country boy from Mississippi, can’t really make my out my question well.

  • Jason J

    what if we are not looking at it collapse but we are actually looking at something in the 4th dimension with our 3rd dimension brain interpretation of what it should look like.

  • Rekuzan Rikudo

    @Extra Credits The biggest 'killer app' that springs to mind when thinking about possible practical applications for Quantum computing (in my mind anywho) is calculating Pi (much, much faster!).

    That and downloading the latest viral cat video at speeds never dreamed possible back in the days of dial-up…

  • youtoober2013

    4:42 Is this the record for the longest animation on EC? 14 frames, well done.
    The 5 lb weight wrecked that last wooden board though, damn.

  • Damien King-Acevedo

    Your classical computing explanation of your "John Doe" search bugs the hell out of me; I understand that you're simplifying things to make it easier for people to understand, but your simplification is flat wrong.

  • Syko _G

    so thinking about this, wouldn't making and keeping entangled quantum bits prevent the random untrackable thing from interfering with the entanglement therefore solving decoherence

  • Robby Frith

    how are you suppose to find two matching particals if they could be anywhere or change almost anytime are you suppose to just somehow make particals

  • Austin M

    Wouldn't it make better sense to have a Quantum CPU on the same motherboard with a regular CPU in the same computer? Then the regular CPU would use the QCPU the same way we use a GPU now – offload certain operations it's better for to the other chip, then send back the result when it's done. I mean, even if you put the CPU+QCPU system in the cloud, it'll still have to be in the same physical box to be a viable thing, right?

    Granted this is less of a question about quantum mechanics and more about motherboard design, but it seems to me it's still important. After all, if you've got to have a traditional system to make a quantum system actually useful, then why sink so much time and resources into a quantum system if there is no production means to utilize it? That is, the guy who invented the first internal combustion engine surely knew he could use it to turn a wheel and thus move people/stuff around before he sank a whole bunch of time into it. Are we relatively certain we can do the same with a quantum CPU, or does this wind up being nothing more than a very powerful toy at CERN?

    And no, I am not saying the particle accelerator is a useless toy. CERN is a lot more than the LHC. And a lot of CERN's projects seem to be utterly useless on a practical level, even if we define "practical" as "learning physics theories that we literally can never prove or even test because human beings literally cannot observe/survive/measure any results they might ever produce" which is itself somewhat useless. I just…I feel like sometimes, in science, asking "why?" is every bit as valid a question as "why not?" is. There's limited money to fund all this stuff and we should've had a colony on Mars by my 30th birthday.

    Well I'm 32 and it's still in the planning phases, even though we already know EVERYTHING we need to know to make it happen. Priorities, people.

  • Polar Vortex

    Me: PC, do a thing!
    PC: This is tricky, lemme ask a quantum buddy
    PC: Hey, quantum buddy, help me out here
    QC: OK
    QC: frdkcjnenroslnaklrqkrnfilwmsjjdlrndbxjnen (waveform collapsing noises)
    QC: Here’s your data
    PC: Thanks. Lemme work on this
    PC: Here’s your thing
    Me: Thanks

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