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A little notation help, on quantum coding

  1. May 18, 2009 #1
    Hi, I wanted to know how to solve this question , its not a homework question i am really asking for , more the general way to solve these types of questions.......... I dont understand how it forms into another matrix. I have the answer attached aswell , but could some one explain ? (I dont think the matrix A is needed , i just copied it aswell )

    Cheers for any help guys :)

    Attached Files:

  2. jcsd
  3. May 18, 2009 #2


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    Do you know what [itex]\langle v_2|[/itex] represents? It is the conjugate transpose of [itex]|v_2 \rangle[/itex]. So [itex]\langle v_2|=\frac{1}{\sqrt{2}}(-i,-1)[/itex].

    |v_1 \rangle \langle v_2|=\frac{1}{2} \binom{i}{1}(-i,-1)
    Last edited: May 18, 2009
  4. May 18, 2009 #3
    aint <v2 is the transpose of v2> ???
  5. May 18, 2009 #4


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    Almost, we are working with complex vectors here so it is the conjugate transpose. Do you know how to form a matrix from the the expression in my previous post?
  6. May 18, 2009 #5


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    To represent an operator as a matrix, you must choose a basis. In this case, the question presumably wants you to write it in the same basis that [tex] |v_1\rangle [/tex] and [tex] |v_2\rangle [/tex] are given in. You are given that

    [tex]|v_1\rangle = \frac{1}{\sqrt{2}} ( i|1\rangle + |2\rangle) [/tex]

    [tex]|v_2\rangle = \frac{1}{\sqrt{2}} (i|1\rangle -|2\rangle) [/tex]

    The operator is

    [tex] |v_1\rangle\langle v_2| [/tex].

    The matrix elements of this operator in the basis [tex] |1\rangle, |2\rangle [/tex] are

    [tex] A_{ij} = \langle i|v_1\rangle\langle v_2|j\rangle [/tex].

    For example, the (1,1) element of the matrix will be

    [tex] A_{11} = \langle 1|v_1\rangle\langle v_2|1\rangle = \langle 1|v_1\rangle \langle 1 | v_2 \rangle ^{*} = \frac{i}{\sqrt{2}}\frac{-i}{\sqrt{2}} = \frac{1}{2} [/tex].
    Last edited: May 18, 2009
  7. May 18, 2009 #6
    Cheers Cyosis I understand that better now , but i still dont understand how what you wrote is i matrix and not a scalar ? , can u show me for a general case ? , or just show me how the answer is acheived ?
    Last edited: May 18, 2009
  8. May 18, 2009 #7


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    To calculate [itex]|v_1 \rangle \langle v_2|=\frac{1}{2} \binom{i}{1}(-i,-1)[/itex] just think of it as two matrices. You multiply the first row of v1 (i) with the first column of v2 (-i).

    What you will get is this:
    [tex]\frac{1}{2}\left( \begin{matrix} i*-i & i*-1 \\ 1*-i & 1*-1 \end{matrix}\right)= \frac{1}{2}\left( \begin{matrix} 1 & -i \\ -i & -1 \end{matrix}\right)[/tex]
  9. May 18, 2009 #8
    Hi I just wanted to ask quickly , does this only apply to only complex vectors ? , what if they was all real values ? , is it still the same then ????
    Last edited: May 18, 2009
  10. May 18, 2009 #9
    Any one ?
  11. May 18, 2009 #10


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    Yes it's the same. The only difference is that you usually take the normal transpose for real valued vectors since there is nothing to conjugate.
  12. May 18, 2009 #11


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    Or the way I like to think about it, the conjugate transpose for real vectors (or matrices) is the normal transpose, since the conjugate of a real number is just the same number.
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