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I would like to keep the ordering of the bra ket-print but right now this doesn't happen. Any Idea? I tried playing with HoldForm but then i could evaluate the expression inside properly

Ns = 1;
 (Ket[(Replace[ Quotient[i - 1, Ns], {0 -> "\[UpArrow] ", 
     1 -> "\[DownArrow] "}] <> ToString@Mod[i, Ns])]
  Bra[(Replace[        Quotient[j - 1, Ns], {0 -> "\[UpArrow] ", 
     1 -> "\[DownArrow] "}] <> ToString@Mod[j, Ns])])

, {i, 1, Ns*2}, {j, 1, Ns*2}   ] // MatrixForm

Right now this gives:

bad braket

I would like it to give something more like:

good braket

Thank you

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up vote 10 down vote accepted

I would do it entirely without ToString. The main tool in combining mixed type output in a given order is Row:

Ns = 1;
        Quotient[i - 1, Ns], {0 -> "\[UpArrow] ", 
         1 -> "\[DownArrow] "}], Mod[i, Ns]}]
        Quotient[j - 1, Ns], {0 -> "\[UpArrow] ", 
         1 -> "\[DownArrow] "}], Mod[j, Ns]}]
  {i, 1, Ns*2}, {j, 1, Ns*2}] // MatrixForm

$\left( \begin{array}{cc} |\text{$\uparrow $ }0\rangle \langle \text{$\uparrow $ }0| & |\text{$\uparrow $ }0\rangle \langle \text{$\downarrow $ }0| \\ |\text{$\downarrow $ }0\rangle \langle \text{$\uparrow $ }0| & |\text{$\downarrow $ }0\rangle \langle \text{$\downarrow $ }0| \\ \end{array} \right)$

Edit to elaborate on comment

In the long run, it may be better to separate the formatting from the algebra. To that end, I would first observe that there is an algebraic reason for the order of the bra and ket to be maintained. You're really forming something that in a matrix representation corresponds to non-commutative multiplication. So it makes sense to enter these bras and kets with a specially defined multiplication symbol, say, \[CenterDot] -- the $\cdot$ symbol. This symbol has no pre-defined meaning and allows you to tailor any output format for it that you like. That is, you input a "multiplication" with $\cdot$ but we can set it up to output the product without explicit multiplication symbol. Moreover, if the labels of the kets and bras are always a spin arrow and a number, you can predefine that in a Format statement too:

arrows = {0 -> "\[UpArrow]", 1 -> "\[DownArrow]"};
Format[Ket[x_, y_]] := Ket@Row[{x /. arrows, "\[ThinSpace]", y}];
Format[Bra[x_, y_]] := Bra@Row[{x /. arrows, "\[ThinSpace]", y}];
Format[CenterDot[x__]] := Row[Riffle[{x}, "\[ThinSpace]"]];

Ns = 1;
    Quotient[i - 1, Ns], Mod[i, Ns]
    Quotient[j - 1, Ns], Mod[j, Ns]
  {i, 1, Ns*2}, {j, 1, Ns*2}] // MatrixForm

enter image description here

The point here is that where it says \[CenterDot] in the code, I actually only had to enter the shortcut Esc.Esc. So the ordered product is now very easy to enter, and the actual algebraic expression is much easier to read.

I again used Row for formatting, combined with \[ThinSpace] as a spacer between elements of the bras and kets, as well as in the product.

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That is quite useful. Are there any other good commands relevant to this I should look at too? Thank you Jens – AimForClarity Mar 11 '13 at 3:08
@AimForClarity Yes, the formatting could be separated further from the content, to improve readablilty. I'll add that to my answer. – Jens Mar 11 '13 at 3:44
Thank you, that is a good tip. I have been defining matrices to represent the algebra of a number space and a "qubit" space for a random walk by using the kronecker product. I wonder if instead I used the algebra and defined the non-commutative operations on the bra-ket row structure via the \cdot, if that would have been a better approach. Any thoughts? – AimForClarity Mar 11 '13 at 4:35
@AimForClarity Actually, I'd say your approach sounds just fine. For a finite-dimensional Hilbert space, bras and kets are completely analogous to row- and column vectors in a vector space defined by some working basis, and then you may as well leverage the built-in capabilities for matrix multiplication, instead of re-implementing the corresponding algebra symbolically. That's also how I've done it before. – Jens Mar 11 '13 at 4:45
Thanks Jens. I have also used the package but found it slow for any useful work. There they define all the associated operations on bra and kets – AimForClarity Mar 11 '13 at 5:19

Well, I just figured out a simple minded solution, but I hope that someone has a more elegant one perhaps.

Basically you can just use StringReplace & use string all the way

  HS2[[i, j]]*
   \"},\n\"Bra\"]\)", {"row" -> ( (Replace[
      Quotient[i - 1, Ns], {0 -> "\[UpArrow] ", 
       1 -> "\[DownArrow] "}] <> ToString@Mod[i, Ns])),
 "col" -> (Replace[
     Quotient[j - 1, Ns], {0 -> "\[UpArrow] ", 
      1 -> "\[DownArrow] "}] <> ToString@Mod[j, Ns])

 , {i, 1, Dimensions[HS2, 1][[1]]}, {j, 1, 
 Dimensions[HS2, 2][[1]]}   ] // MatrixForm

An Example output:


To turn into nice representation:


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