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There is an excellent answer to How do I add 1+1 using a quantum computer? that shows constructions of the half and full adders. In the answer, there is a source for the QRCA. I have also looked at this presentation.

I am still left with these questions:

  1. What does a truth table for a QRCA look like?

  2. What would a unitary matrix for a QRCA be?

  3. What would a circuit diagram for a QRCA look like? (There is an example of a 6-bit circuit in the arXiv paper)

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  • $\begingroup$ You acknowledge that there's a quantum circuit diagram for a particular size in the paper you're citing. What more are you after regarding quantum circuits? $\endgroup$ – DaftWullie Jun 14 '18 at 18:33
  • $\begingroup$ Here's the abstract: link.springer.com/chapter/10.1007/978-3-540-85194-3_19 $\endgroup$ – Sanchayan Dutta Jun 14 '18 at 18:34
  • $\begingroup$ also, the first two lines of figure 5 effectively define the truth table, from which you can directly derive the unitary matrix. $\endgroup$ – DaftWullie Jun 14 '18 at 18:36
  • $\begingroup$ Again, apologies. I do not actually know how to derive the matrix from the table (or how to dervive a truth table for a quantum gate). Nor am I able to create circuits (the 6-bit circuit is more complex than a 2 or 3 bit circuit). Obviously very new to logic cicuits & quantum computing! $\endgroup$ – meowzz Jun 14 '18 at 18:52
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What does a truth table for a QRCA look like?

You don't want to know. It will be a gigantic complicated table that provides no insight whatsoever. At the very least you need to use boolean algebra instead of a table, but even that will be cumbersome and will require many intermediate values that ultimately are just a less-visual way of describing an addition circuit.

If it helps, here is the set of equations for a simpler operation, an increment operation. The equations define how each output bit can be computed from the input bits:

$o_0 = i_0 \oplus 1$

$o_1 = i_1 \oplus i_0$

$o_2 = i_2 \oplus (i_0 \land i_1)$

$\vdots$

$o_n = i_n \oplus {\Large{\land}}_{k=0}^{n-1} i_k$

What would a unitary matrix for a QRCA be?

It's a permutation matrix.

As a starting point, here is the permutation matrix corresponding to a 2-bit increment:

$$\text{Inc}_2 = \begin{bmatrix} &&&1\\ 1&&&\\ &1&&\\ &&1&\\ \end{bmatrix}$$

and a 3-bit increment:

$$\text{Inc}_3 = \begin{bmatrix} &&&&&&&1\\ 1&&&&&&&\\ &1&&&&&&\\ &&1&&&&&\\ &&&1&&&&\\ &&&&1&&&\\ &&&&&1&&\\ &&&&&&1&\\ \end{bmatrix}$$

I suspect you see the pattern. Just start with an identity matrix and shift it down by 1 (with the bottom row wrapping around to the top). To add 2, instead of adding 1 (i.e. incrementing) you would just shift down by 2 instead of by 1.

In an addition circuit, the amount of shifting depends on the other input. So you end up with a series of sub-matrices with increasingly-shifted diagonals:

$$\text{Add}_2 = \begin{bmatrix} \begin{bmatrix} 1&&&\\ &1&&\\ &&1&\\ &&&1\\ \end{bmatrix} \\& \begin{bmatrix} &&&1\\ 1&&&\\ &1&&\\ &&1&\\ \end{bmatrix} \\&& \begin{bmatrix} &&1&\\ &&&1\\ 1&&&\\ &1&&\\ \end{bmatrix} \\&&& \begin{bmatrix} &1&&\\ &&1&\\ &&&1\\ 1&&&\\ \end{bmatrix} \end{bmatrix}$$

What would a circuit diagram for a QRCA look like?

There are many possible constructions. Here is one that works entirely inline:

addition circuit

You can play with this construction in Quirk.

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