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Quantum Mechanics Operator/eigenstates problem

  1. Jan 27, 2012 #1
    1. The problem statement, all variables and given/known data

    A quantic system can be found in the states |a0> and |a1>, and those are eigenstates of the operator A with the respective eigenvalues: 0 and 1. Consider the operator B defined by: B|a0> = 7|a0> - 24i|a1> and B|a1> = 24i|a0> - 7|a1>.

    After measured by the operator B the system is in the state |a0> , right after that measure the observable described by A is measured. What is the probability of this measure yealding zero?

    2. Relevant equations

    I already found the eigenvalues of B they are -25 and +25.

    And its eigenstates:

    |bλ=-25> = (4/5)|a0> - i(3/5)|a1>

    |bλ=25> = (3/5)|a0> + i(4/5)|a1>

    Those are correct.

    3. The attempt at a solution

    Now, I am not sure on how to do this. Probability of |bλ=±25> colapsing on |a0> and then probability of |a0> colapsing on zero? How would you do it? I actually have a correction but I don't really understand it =\

    The correction is as follows:

    Pa0(bλ=25) = |<bλ=25|a0>|^2 = (4/5)^2

    Pbλ25(a0=0) = |<a0|bλ=25>|^2 = (4/5)^2

    Pa0(bλ=25,a0=0) = (4/5)^4

    Pa0(bλ=-25) = |<bλ=-25|a0>|^2 = (3/5)^2

    Pbλ25(a0=0) = |<a0|bλ=-25>|^2 = (3/5)^2

    Pa0(bλ=-25,a0=0) = (3/5)^4

    Total Probability = (4/5)^4 + (3/5)^4

    It does make some sense, but some things I just don't get.

    First, probability of |bλ=25> colapsing into |a0>, ok, but.. what is the square for? Doesn't the <bλ=25|a0> already do it? doesn't it mean "the probability of bλ colapsing in a0" ?

    Second, probability of a0 colapsing into zero,ok , but then I don't understand why is it equal to the one above it, only it is the other way around.. isn't it the same thing?

    Then you multiply the probabilities, do the same for the other eigenvalue and add, that makes sense.

    Another thing I don't really understand very well is how the actual values, the results in front, are actually obtained.

    I hope someone is able to help me. Thanks in advance for any attention.
  2. jcsd
  3. Jan 30, 2012 #2


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    This is confusing. I can't work out what the question is meant to be. There is some system to begin with, right? And then a measurement of the variable represented by B is made on the system. And then after that, a measurement of the variable represented by A is made on the system? And we are meant to work out the probability that the second measurement will give zero. Did I get all this right? So what is the state meant to be at the beginning, before any measurements were made?
  4. Jan 30, 2012 #3
    It seams we are meant to work out the probability of after measuring the system with B and it becoming in state a0, getting the result 0 with the operator A. It's like a composed probability, something like that, one probability times the other.

    The state at the beginning isn't told, I suppose it's not important, since you are told you are in state a0 after the measuring of B.
  5. Feb 3, 2012 #4


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    (sorry I've taken a while to reply). So you mean that the system is measured with B, and then measured with A, we want to know the probability that this measurement with A will yield zero?

    This will surely depend on the state at the beginning. For example, the state might have been an eigenstate of B, in which case the measurement of B will leave the state unchanged, and we know that the eigenstates of B each have a different overlap with the a0 state, and therefore will have a different probability of getting the zero result.

    P.S. also, I had a go at calculating the eigenstates of B, and I think you have got them the wrong way around (i.e. -25 and 25 mixed up)?
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