I read Chapter 2.1.3 (page 11) of this master's thesis, the content of which I'll summarize in what follows:
Let $\lbrace \vert c_1\rangle, \vert c_2\rangle, \ldots \vert c_k\rangle\rbrace$ be the basis of codewords in the codespace. It is necessary that $$ \langle c_i \vert E^{\dagger}_{a}E_b \vert c_j\rangle = 0, \tag{1}$$ for all $i, j \in \lbrace 1, 2, \ldots, k \rbrace \;\land i \neq j$. This is equivalent to $$ \langle c_i \vert E^{\dagger}_{a}E_b \vert c_i\rangle = \langle c_j \vert E^{\dagger}_{a}E_b \vert c_j\rangle, \tag{2}$$ for all $i, j \in \lbrace 1, 2, \ldots, k \rbrace$. Equation $(1)$ and $(2)$ can be combined as (Knill and Laflamme) $$ \langle c_i \vert E^{\dagger}_{a}E_b \vert c_j\rangle = C_{ab} \delta_{ij},$$ where $C_{ab} \in \mathbb{C}$ and $\delta_{ij} = \begin{cases} 1 \text{ if }\, i = j, \\0 \text{ if }\, i \neq j. \end{cases}$
Furthermore, since $$\langle c_i \vert E^{\dagger}_{a}E_b \vert c_i\rangle = (\langle c_i \vert E^{\dagger}_{b}E_a \vert c_i\rangle)^*,$$ for all the codewords in the codespace, we can write $C_{ab}$ as a Hermitian matrix.
Questions
- I cannot figure out how equations $(1)$ and $(2)$ are equivalent. My humble explanation suggests that for $(1)$, they are equivalent since $E^{\dagger}_a E_b$ would cancel out to $I$ and we are left with $\langle c_i\vert c_j \rangle = 0$, since they are orthogonal. For $(2)$ we have $E^{\dagger}_a E_b$ would cancel out to $I$ and we are left with $\langle c_i\vert c_i \rangle = \langle c_j\vert c_j \rangle = 1$, since we are projecting a state on itself.
- How can we convert $C_{ab}$ to matrix, what is the matrix dimension and what is the nature of the elements in the cells of this matrix (a binary matrix or elements in $\mathbb{C}$ or something else)?
- Links to questions two, how would $\delta_{ij}$ affect the matrix?
I am certainly not looking for complete answers, I am trying to understand this myself. I am open to any suggestions and scholarly articles that helps. Any hints and tips are appreciated :).