Relation between Mutual information and Expectation Values

AI Thread Summary
The discussion focuses on calculating mutual information and expectation values in quantum mechanics. The mutual information between Alice and Bob has been computed as I(ρ_A:ρ_B) = 2. A challenge arises in computing the expectation value ⟨ψ|O_A|ψ⟩ due to mismatched matrix sizes, with the user seeking guidance on resolving this issue. The user successfully calculated ⟨ψ|O_A ⊗ O_B|ψ⟩ as 7/12 by using the tensor product of the matrices. Suggestions were made to adjust the calculations by incorporating identity matrices to align the dimensions for the expectation values.
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Homework Statement
Alice and Bob share the Bell state
\begin{align*}
|\psi\rangle = \frac{1}{\sqrt{2}}(|00\rangle+|11\rangle).
\end{align*}
Consider the pair of observables
\begin{align*}
\mathcal{O}_A =
\begin{pmatrix}
1 & 0 \\ 0 & \frac{1}{2}
\end{pmatrix}
, \qquad \mathcal{O}_B =
\begin{pmatrix}
1 & 0 \\ 0 & \frac{1}{3}
\end{pmatrix}
.
\end{align*}
Show the mutual information between Alice and Bob is larger than $(\langle\psi | \mathcal{O}_A \otimes \mathcal{O}_B| \psi\rangle - \langle\psi |\mathcal{O}_A|\psi \rangle \langle\psi |\mathcal{O}_B|\psi \rangle)^2 $
Relevant Equations
\begin{align*}
|\psi\rangle = \frac{1}{\sqrt{2}}(|00\rangle+|11\rangle).
\end{align*}
Consider the pair of observables
\begin{align*}
\mathcal{O}_A =
\begin{pmatrix}
1 & 0 \\ 0 & \frac{1}{2}
\end{pmatrix}
, \qquad \mathcal{O}_B =
\begin{pmatrix}
1 & 0 \\ 0 & \frac{1}{3}
\end{pmatrix}
.
\end{align*}
I've make progress in obtaining the values for the mutual information using the following:
$I(\rho_A:\rho_B) = S(\rho_A) +S(\rho_B) - S(\rho_{AB}) = 1 + 1 - 0 = 2.$

I would like to compute the expectation but I'm facing a problem in the case of $\langle\psi |\mathcal{O}_A|\psi \rangle$ since the size of matrices in this multiplication do not match. namely, $\langle\psi$ is of size $1\times 4$ and $|\psi \rangle$ is of size $4\times 1$ and the matrix $\mathcal{O}_A$ is $2 \times 2$.
I'm very new to the subject and I would greatly appreciate if I could have some guidance on how the computations for this expectation would be carried out.

additionally I have computed the $\langle\psi | \mathcal{O}_A \otimes \mathcal{O}_B| \psi\rangle$ by first computing the tensor product of the two matrices $A,B$ and then taken the multiplication with the Bra and Ket of the state respectively deducing
$$\langle\psi | \mathcal{O}_A \otimes \mathcal{O}_B| \psi\rangle = \frac{7}{12}$$.

I would appreciate any insight on this.
 
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Here it is edited:
Show the mutual information between Alice and Bob is larger than ##(\langle\psi | \mathcal{O}_A \otimes \mathcal{O}_B| \psi\rangle - \langle\psi |\mathcal{O}_A|\psi \rangle \langle\psi |\mathcal{O}_B|\psi \rangle)^2 ##

I've make progress in obtaining the values for the mutual information using the following:
##I(\rho_A:\rho_B) = S(\rho_A) +S(\rho_B) - S(\rho_{AB}) = 1 + 1 - 0 = 2.##

I would like to compute the expectation but I'm facing a problem in the case of ##\langle\psi |\mathcal{O}_A|\psi \rangle## since the size of matrices in this multiplication do not match. namely, ##\langle\psi## is of size ##1\times 4## and ##|\psi \rangle## is of size ##4\times 1## and the matrix ##\mathcal{O}_A## is ##2 \times 2##.
I'm very new to the subject and I would greatly appreciate if I could have some guidance on how the computations for this expectation would be carried out.

additionally I have computed the ##\langle\psi | \mathcal{O}_A \otimes \mathcal{O}_B| \psi\rangle## by first computing the tensor product of the two matrices ##A,B## and then taken the multiplication with the Bra and Ket of the state respectively deducing...

I think that for ##\langle\psi |\mathcal{O}_A|\psi \rangle## you calculate ##\langle\psi | \mathcal{O}_A \otimes \mathcal I| \psi\rangle##, and for ##\langle\psi |\mathcal{O}_B|\psi \rangle##, it is ##\langle\psi |\mathcal I \otimes \mathcal{O}_B | \psi\rangle##.

(But I am new to it, too.)
 
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