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Homework Help: Evaluating annihilation and creation operators

  1. Jan 17, 2014 #1
    1. Evaluate the following (i.e. get rid of the operators):


    2. Relevant equations


    3. The attempt at a solution

    The first one is [itex]\sqrt{6}\left|(6)\right\rangle[/itex] and second one [itex]\sqrt{5}\left|(4)\right\rangle[/itex]

    However I'm unsure how to evaluate for the others using the equations given. Could someone please point me in the right direction?

  2. jcsd
  3. Jan 17, 2014 #2


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    What does the cube of an operator mean ? And lose the round brackets inside the bra and ket | and <> symbols (the pun is not intended).
    Last edited: Jan 17, 2014
  4. Jan 17, 2014 #3
    I have no idea how powers affect operators. Can't find any examples in my lecturers notes nor when searching the internet. Really struggling to grasp quantum mechanics as the mathematics I know doesn't seem to apply.
  5. Jan 17, 2014 #4


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    It's just like ordinary algebra, e.g., ##x^2 := x \, x##.

    So can you evaluate ##(a^+)^2|n\rangle## and ##a^2 |n\rangle## now?

    Also, you should be able to evaluate ##a^+ a|n\rangle## with the "relevant equations" you already written down.
  6. Jan 18, 2014 #5
    Ahh I see, you just use the operators one by one. Thanks.
    There is one more thing.. what does a ket of a number actually mean?
    I understand that for example [itex]\hat{p}\left|\psi\right\rangle~=~ h/i*d/dx~\psi(x)[/itex] but I don't see how this relates to actual numbers?
  7. Jan 18, 2014 #6


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    You didn't specify the context in your original post, so I can only give a broad answer. For the quantized harmonic oscillator, one denotes the ground state (i.e., state of lowest energy) by ##|0\rangle##. The higher energy (eigenstates) are then successively numbered by 1,2,3... etc. The creation and annihilation operators are an example of so-called Ladded Operators which map from one such eigenstate to another. The Wiki page has more info about other contexts.
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