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  • 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.

A classical computer performs operations using classical bits, which can be either zero or

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B1 中級

量子コンピュータはどのように機能するのか? (How Does a Quantum Computer Work?)

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    林宜悉 に公開 2021 年 01 月 14 日
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