The concept of $C_{ab} $ in the context of the Quantum Error-Correcting Code (QECC) conditions as described in Theorem 2.7 can indeed be confusing due to the mathematical notation and the terminology used. I will clarify this for you.
In the statement of Theorem 2.7, $C_{ab}$ appears to represent a scalar quantity based on the equation provided:
$\langle \psi | E_a^\dagger E_b | \phi \rangle = C_{ab} \langle \psi | \phi \rangle $
Here, for any two error operators $E_a $ and $E_b $ from the set $E$, and for any two states $\psi \rangle$ and $\phi \rangle$ from the code space $ Q $, the product $ E_a^\dagger E_b $ behaves in such a way that its matrix element between $ | \psi \rangle $ and $ | \phi \rangle $ is proportional to the inner product $ \langle \psi | \phi \rangle $ with a proportionality constant $ C_{ab} $.
However, the nature of $ C_{ab} $ as discussed in the proof might suggest it is not just a scalar but an element of a matrix $ C $ where $ C $ is composed of these elements $ C_{ab} $ for all combinations of $ a $ and $ b $. This matrix $ C $ is Hermitian, as indicated by the relation $ C_{ab}^\dagger = C_{ba}^* $, which makes it eligible for properties such as being diagonalizable and having eigenvalues.
Here’s how $ C_{ab} $ functions both as a scalar and a matrix element:
As a Scalar: In each specific instance of $ a $ and $ b $, $ C_{ab} $ is indeed a scalar, indicating the proportionality between $ \langle \psi | E_a^\dagger E_b | \phi \rangle $ and $ \langle \psi | \phi \rangle $.
As a Matrix Element: When considering all possible pairs of $ a $ and $ b $, $ C_{ab} $ forms the elements of the matrix $ C $. The entire matrix $ C $ is what is referred to when discussing properties like diagonalizability and eigenvalues in the proof.
The confusion arises due to the dual role of $ C_{ab} $ as both individual scalars and as components of the matrix $ C $. The matrix $ C $ is important because its properties (like being Hermitian and diagonalizable) are crucial for the QECC to effectively correct errors represented by the error set $ E $. The diagonalization of $ C $ means that you can choose a basis in which $ C $ is diagonal, simplifying the analysis and manipulation of the error correction properties of the code.
Thus, in summary, $ C_{ab} $ in each instance is a scalar, but collectively, these scalars make up the matrix $ C $, which possesses the described properties and is central to the theory and application of QECCs.