How Does a Quantum Computer Work?


A classical computer performs operations using
classical bits, which can be either zero or one. Now in contrast, a quantum computer users
quantum bits or qubits. And they can be both zero and one at the same time. And it is this
that gives a quantum computer its superior computing power. There are a number of physical objects that
can be used as a qubit. A single photon, a nucleus or an electron.
I met up with researchers who were using the outermost electron in phosphorous as a qubit.
But how does that work? Well, all electrons have magnetic fields, so they are basically
like tiny bar magnets. And this property is called spin. If you place them in a magnetic
field they will align with that field, just like a compass needle lines up with the magnetic
field of the earth. Now this is the lowest energy state, so you
could call it the zero state or we call it for the electron, spin down. Now you can put
it in a one state, or spin up, but that takes some energy.>>If you took out the glass from your compass
you could turn the needle the other way, but you would have to apply some force to it.
You have to push it to flip to the other side. And that is the highest energy state. In principle,
if you were so delicate to really put it exactly against the magnetic field, it would stay
there.>>Now so far this is basically just like
a classical bit. It has got two states, spin up and spin down, which are like the classical
one and zero. But the funny thing about quantum objects is that thy can be in both states
at once. Now when you measure the spin it will be either up or down. But before you
measure it, the electron can exist in what is called a quantum super position, where
these coefficients indicate the relative probability of finding the electron in one state or the
other. Now it is hard to imagine how this enables
this incredible computing power of quantum computers without considering two interacting
quantum bits.>>Hello.
>>Hi. Now there are four possible states of these
two electrons.>>You could think that, well, that is just
like two bits of a classical computer, right? If you have two bits you can write zero, zero;
zero, one; one, zero; one, one. Right? There is four numbers. But these are still just two bits of information.
Right? All I need to say to determine which one of the four numbers you have in your computer
code is the value of the first bit and the value of the second bit. Here, instead, quantum
mechanics allows me to make super position of each one of these four states. So I can
write a quantum mechanical state, which is perfectly legitimate, that is some coefficient
times this plus some coefficient times that plus some coefficient times that plus some
coefficient times that. So determine the state of this two spin system,
I need to give you four numbers, four coefficients, whereas in the classical example of the two
bits, I only need to give you two bits. So this is how you understand why two qubits
actually contain four bits of information. I need to give you four numbers to tell you
the state of this system, whereas here I only need two. Now if we make three spins, we would have
eight different states and it could give you eight different numbers to define the state
of those three spins, whereas classical it is just three bits.
If you keep going, what you find is that the amount of equivalent classical information
contained by N qubits is two to the power N classical bits. And, of course, the power of exponentials
tells you that once you have, let’s say, 300 of those qubits in what we call the folient
angle state, so you must be able to create these really crazy states where there is a
super position of all three angles being one way and another way and another way and so
on, then you have like two to the 300 classical bits, which is as many particles as there
are in the universe.>>But there is a catch, although the qubits
can exist in any combination of states, when they are measured they must fall into one
of the basis states. And all the other information about the state before the measurement is
lost.>>So you don’t want generally to have as
the final result of your quantum computation something that is a very complicated super
positional state, because our cannot measure a super position. You can only measure one
of these basis states.>>Like down, down, up, up.>>Yeah. So what you want is to design the
logical operations that you need to get to the final computational result in such a way
that the final result is something you are able to measure, just a unique state.>>That is not trivial.>>That is not trivial. And it is essentially
… I am kind of stretching things, but I guess it is to some degree the reason why
quantum computers are not a replacement of classical computers.>>They are not.>>No, they are not. They are not universally
faster. They are only faster for special types of calculations where you can use the fact
that you have all these quantum super positions available to you at the same time, to do some
kind of computational parallelism. If you just want to watch a video in high definition
or browse the internet or write some documenting work, they are not going to give you any particular
improvement if you need to use a classical algorithm to get the result. So you should
not think of a quantum computer as something where every operation is faster. In fact,
every operation is probably going to be slower than in the computer you have at your desk.
But it is a computer where the number of operations required to arrive at the result is exponentially
small. So the improvement is not in the speed of the individual operation. It is in the
total number of operations you need to arrive at the result.
But that is only the case in particular types of calculations, particular algorithms. It
is not universally, which is why it is not a replacement of a classical computer.

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99 thoughts on “How Does a Quantum Computer Work?

  1. This is the best short clip to explain how it works. I'm happy they actually went through the basics of how the hardware works.

  2. Wow, sure is a lot of energy going toward a concept that can only be useful in specific or special situations and is definitely not a replacement for classical computers! Well, at least this guy was honest and straight forward about it! It seems like Quantum Computers were never intended to replace classical computers, unfortunately that was the impression I had gotten when these concepts went more mainstream. Hey, maybe one day Quantum Computing will be built into classical computers as a functional option for special operations and applications!

  3. Actually, this isn't hard to understand, but it is hard to get your head around. Quantum states exist in probabilities. However, to be observed, they must be in a fixed state. When not being observed, they are "probably" in one state or the other. The act of observation fixes the state of the particle, according to its probability of being in that state. Look up, "quantum gates" to see an explanation based on unitary matrices for an idea of how the math works.

  4. I had problem in sleep i.e. I didn't get sleepy. Then I started this video and slept halfway,though it costed me my daily Internet Limit 🤣😂🤣😂🤣😂🤣😂

  5. Sure but how can you construct such logic gates that generate a measurable result without measuring an electron's initial state? An electron's spin is in superposition as long as its not observed, afterwards it collapses to either 1 or 0. Those logical gates must somehow observe the electron to measure its state then pass that data to you. Isnt that counterintuitive?

  6. Praggo Thanks for trying. Why o why must every video about quantum anything be beset by superpositions of wacky sound effect life cheese music-oid sound my liege?

  7. Let me get this right. So you input the calculations, the quantum computer does the calculation in a few steps, then does the computer identify the qubit by choosing whether the qubit is closer to 1 or 0, or does it choose randomly but the closer the qubit is to 0 or 1, the more likely the qubit will be identified as a 1 or a 0?

  8. A team of Russian and American scientists recently used a quantum computer to essentially reverse time by decreasing entropy on the atomic level.

  9. Veritasium's other video about quantum entanglement is a good setup for this video, and his explanations of insane concepts is quite easy to grasp.

  10. I have watched 3 videos about quantum computing in order to get idea what it is.
    And they have convinced me, that it is one huge hoax.

  11. so it’s just the reason that it’s better is that you don’t know if it’s up or down which literally means nothing

  12. if you havent measured it how do you know its even doing what youre describing?

    saying its "both probabilities at once" and then saying you have never actually observed that property is like saying you know something is true just because you think its true

  13. if nobody has ever observed the phenomena of superposition due to its physical property of collapsing into a binary state upon observance, then how can you say it exists?

  14. Quantum computing is a .90 probability hoax and .10 probability real deal. Yes, at the same time. Because, you know, a bit can only be either 1 or 0 and a qubit can be either both at once, or 1 with one probability and 0 with another (is that a particular probability random number generator then?), or an infinity of values between 0 and 1 at the same time or just freaking 42, but only once!

  15. SERIOUS QUESTION:
    SInce the coefficient Mr. Morello showed were complex numbers, the actual number of corresponding classical bits shouldn't be double?

  16. Should have written something about qubit in Quran. So that "they" can "wrongly" interpret something about Quantum world, at least.

  17. Guys, this is really a great explanation of Qubits and the best one I've seen so far.

    I suggest that you guys look at some videos on the basic properties/phenomenons of Quantum particles/physics before diving into Quantum Computers.

  18. I replayed the video 2^300 times but only watched it once. It was a super entanglement to me, and the cat is always dead when I look in the box.

  19. As an ignorant I ask: wouldn't be easier to assign different voltages to read, let's say, hexadecimal instead of binary?

  20. so in simplified terms.. its a complex software that is easy to use when solving really really complex problems, and it saves time as its faster than a normal computer..If its a simple problem a normal computer will solve things much faster

  21. when i saw those 0.xx numbers with the electron pointing both up and down i understood the whole concept immediately, because it reminded me of cells in neural networks.

  22. Mum: HOW DO YOU EXPLAIN GETTING A ZERO IN YOUR EXAM!!?

    Me: Relax Mum. My test paper is quantum state. It's both a 0 and 100 at the same time. Just measure it again and you'll see I actually got a 100 😉

    Internet: YOU FOOL! THE POSSIBLE STATES ARE NOT BINARY ….

    Me: Shut the …

  23. So basically, quantum computing allows you to have a ton of information with less bits, the main problem is however, you can't really get that information until the algorithm is done… got it!

  24. you need 2 ^ 300 numbers as coefficients, to "define" one of possible states of 300 qubits. Now, where do you store those 2 ^ 300 numbers in?

  25. The first time I’ve seen it explained without the hype. I wish he had an actual use case as an example. Perhaps how it will compute 1+1 =2.

  26. Quantum Computing Class Final: Professor: "Ok, did everyone hear our silly explanation of how quantum computing works? I mean in theory at least?"
    Everyone in the class: "Yes professor."
    Professor: "Ok, you all Pass!"

  27. ok so you have to design the quantum computer in such a way that you predict the qbits value at given time of use?

  28. the summary:

    the quantum computation – for now- can be used only for huge amount of data processing but can not be translated into our classical computational systems
    that means we need an inter-translation system to connect between both worlds
    I believe that what are we going to see in the near future

  29. The perpetual parroting of "…cubits can be both 0 and 1 at the same time…" might sound intriguing but sadly makes no sense and isn't the case at all. A clear explanation still has to appear on the net. The error in the illustration of the 4 possible states at 2:01 adds to the confusion (gamma & beta).

  30. If you set your computer code in a quantum state on a quantum computer, then it would be next to impossible to hack into it. Do i have that right?

  31. Not very well explained…
    electrons and polarity, ok I can understand that
    then you add alpha, beta, gama, delta… what are this ?!?

  32. Look at this: It means that the set of classical algorithms admitting quantum speeding up has probability measure zero. __ https://arxiv.org/abs/quant-ph/9803064 ___So, SOME problems quantum comp will do much faster, like password cracking… In fact quantum comp is based on probability. It means that it can calculate something true with probability…99.2%. That is all. So, how will we protect our passwords?:)

  33. If I understand correctly (sure), you start with a vague question and if the qubits are superpositioned in every possible state, every possible answer your computer could give is already there. So it's kind of like trying to have the largest pool of answers possible to draw from, so the answer can be as accurate as possible. Imagine asking someone to guess a number between 1 and 10 and they respond with 3.58, where you saw ten possible answers, they saw a thousand. After that, your supposed to go with the most probable answer, but I'm not sure how that measuring is accomplished, since just measuring forces the qubits out of superposition.

  34. I feel like I understand some of it but it leads me to think that an entangled pair don't communicate at all they simple have a fixed relationship. So by knowing the state of one in the pair you know the state of the other. But this only works on a given moment in time. After that moment the second one continues to move and change thus without knowing it's period of change you have a 50/50 chance of which state it is in if measured later.So the spooky action at a distance just seems like it is another piece of data being time. But how could it be this simple and not have been noticed by someone other than me?

  35. Its like explaining Calculus to a stone age human. How on earth do these people even communicate with the sub atomic particles or whatever and get them to work?. mind bogging

  36. a quantum bit ie a unit of light cannot be a one and zero at the same time.
    light is polarized, meaning it has its own magnetic field, a computer uses
    magnetic fields to name bits ones or zeros, a bit or unit cannot be both a
    one and a zero at the same time. what this is speaking of are fallen angels
    who are androgynous, ie both sexes, female body, male genitals.

  37. Could you please explain how we can detect these spins? It seems we need very sensitive sensors to determine the spin of the electron

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