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Physics behind quantum computer

  1. Jan 3, 2014 #1
    Ok so i'm reading up on quantum computers , and I want to ask some question to clear some things up.
    So in simple terms a quantum computer or the chip itself would have to be made of something , a enclosure somekind that can hold single particles seperated, that have spins (most common probably electrons or protons ) , I guess the more confined particles a computer could have the better , so far so good.I also understand that each of these seperated particles have to be kept in a confined area otherwise they might just crash into the container wall or whatever comes close , so their probably held up or would have to be held in place by the help of one of the fields those particles interact with like electric or magnetic.
    Now I hear and read about the cryogenic , freezing stuff also involved in the physical devices of experiments so I ahve my first question if a field could keep the particle confined and not to touch or crash into anything wouldn't that be enough , I guess there is a reason for that cooling , is it because when you cool them (the particles) down their " jiggling" or random movement also drops and that is necessary in order to determine the state in which they are in ?

    The last question leads to my next one , if those particles have to be confined and they cannot touch anything as that would cause decoherence at best and any interaction (physical) would probably loose the state in whic the particle is in then by what method can the computer tell or sense the particles state to make the computations based on that if you can't just probe the particle or do any other interaction with it?

    a ordinary cpu has millions of transistors which pass physical voltage (signal) through them , but how does one " attach a wire" to specially confined single particle to tell it's state, I guess I'm missing something here as the method must involve someting that I'm not aware of.

    anyways those are the questions as of now , thank you for answers:)
  2. jcsd
  3. Jan 3, 2014 #2


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    There are many different approaches to realize quantum bits (qbits - the "single particles" you refer to). Most of them focus on particles and states inside solid objects, as it is easier to keep those in place.

    Temperature is always "energy that can be used to change something in an uncontrolled way" - that can be collisions with container walls or changes in energy levels of your system. If you heat it up too much and wait too long, you destroy every system, qbits are not an exception.

    There are interactions that preserve coherence. Those are the "calculation steps" for quantum computers.

    Exchanging photons, physically moving particles, or whatever the specific design allows.
  4. Jan 4, 2014 #3
    ok excuse me for being a little lazy here , it's just that i;m not sure such info is so easy to find in google.
    Ok so we know that whether an electron or a proton or some other particle is used it has to be " sealed off' and confined to preserve it's state etc, now I wonder how the actual reading info exachaning process can happen , in a ordinary computer we take information convert it to represent 1 and 0 and then use those in a given order to represent that info because that's the way a transistor (mosfet in particular) operates.
    but in a quantum computer how does this information exchange happen? I mean the input and the output of the CPU so to speak.

    I have read about electrons or protons in a diamond material captured and then light of a specific wavelength used to operate with them , how would it happen since the qbit cannot be interacted with physically to not destroy it,
    Okay say we have an electron ,what's next , an incoming photon which then interacts with the electron and then after that there is a measurement device which upon receiving the outcoming photon can tell at what spin did the electron had upon measurement or what?

    the thing that is hard for me to put together is for a given output from the computer we need to put given things in like all the varyables of a given code so that the computer would understand between what numbers it needs to calculate and so on, and I'm having difficulty putting this together with the way , the physical way a beam of photons or other things could tell these tings to the qubits which would then form a given overall state represented by 1 and 0 which would the give a meaningful result as compared to the original 1 and 0 we put in.
  5. Jan 4, 2014 #4


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    Wikipedia has a list of proposed qubit realizations.
    To find out how they could be manipulated, I think you'll have to look for publications. Everything is still highly experimental.

    The polarization of photons is probably easy to understand for single-photon processes: a rotation of the polarization ("calculation step") is classical physics, and a measurement can be done with a polarizer. I don't know how interactions between polarized photons work, however.
    A very simple "calculation" process would be "start with horizontal polarization, rotate by 30°, measure polarization with many photons." The result is 1/4 vertical, 3/4 horizontal, corresponding to sin(30°)=sqrt(1/4) and cos(30°)=sqrt(3/4).

    That is not right. You have interactions to prepare some initial state, interactions to do calculations, and interactions to get a classical measurement afterwards.
    In terms of electron spin, I think all those operations would be done with light.
  6. Jan 5, 2014 #5
    As mfb said, a qubit can interact with the outside world without breaking superpositions and entanglement. Mfb gave an example and there are many more. All of the examples I can think of involve precession of spin, but the only important point is that it happens. You are probably already familiar with this. Consider the spin of an electron in a weak magnetic field. This comes up in a first course in QM and is an example of external, classical parameters changing a quantum state without decohering. The strength of the field changes the speed of the precession which can be used in a computation.

    In general, a computation takes an input [0,1] "magic" happens and there is an output with the same set. I use "magic" because most theoretical information science is not concerned with what happens at the physical level which get very technical.

    A quantum computation works the same as above, but instead of transistors that are always in a binary state, the "magic" part of a quantum computation involves oscillations of qubits that are in a "Bloch" state. I put the quotes because I don't know if this is a standard term for this, but the idea is definitely correct. So the information that goes in is classical and puts the qubit in a definite state. Then the qubit state is rotated around the Bloch sphere using some set of operations. When the computation is done, a classical measurement is performed and the information comes out as a 1 or 0 again. So I/O is always classical.

    A good source after wiki is the Perimeter Institute site. If you go to 4. Collections and select "Course" and "2012" there is a course taught by Prof. Gottesman (a leader in the field) called "Quantum Information (review)". This is not a simple course and the pace is awkward sometimes due to student questions, but I think that lecture two and three talk about physical realizations. The course requires at least a Griffiths level of knowledge.
  7. Jan 5, 2014 #6
    ok, thanks I will definitely watch the series , my take on this is rusted a little can you please remind me of how incoming photons could interfere with electron in such a way that the outcoming photon or whatever would be such that by it we could determine the spin state of the electron and so make a measurement , this kinda asks fora new thread but I don't want to spam full of threads here so maybe someone can explain right here as it is related to what I am asking.
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