- #1
PhysiSmo
We are given that 2 systems can only be found in the states [tex]|00\rangle, |01\rangle, |10\rangle, |11\rangle[/tex]. We are also given that the density operator is
[tex]\rho=\frac{1}{2}\left(|00\rangle \langle 00|+|11\rangle \langle 00|+|00\rangle \langle 11|+|11\rangle \langle 11|\right)[/tex].
a)Write the matrix form of the density operator. Prove that it describes a pure state. Which one?
b)By taking the partial trace of one system, show that the yielding state is a mixed state. Which is the matrix for this new state?
Solution.
a)We express the matrix form by taking inner products, so that the [tex]\rho_{nm}[/tex] element of the matrix is [tex]\rho_{nm}=\langle n|\rho |m \rangle[/tex]. We find then
[tex]\rho=
\begin{bmatrix}
1/2 & 0 \\
0 & 1/2
\end{bmatrix}[/tex]
which clearly describes a pure state, since [tex]Tr(\rho^2)=1[/tex].
How can one find then the state vectors? Since [tex]\rho=|\psi \rangle \langle \psi |[/tex] in general, is it true to say that [tex]\rho |\psi \rangle = |\psi \rangle[/tex]?
b)Unfortunately, I don't have a clue for this one. What do we mean by taking the partial trace of one system? Any help please?
[tex]\rho=\frac{1}{2}\left(|00\rangle \langle 00|+|11\rangle \langle 00|+|00\rangle \langle 11|+|11\rangle \langle 11|\right)[/tex].
a)Write the matrix form of the density operator. Prove that it describes a pure state. Which one?
b)By taking the partial trace of one system, show that the yielding state is a mixed state. Which is the matrix for this new state?
Solution.
a)We express the matrix form by taking inner products, so that the [tex]\rho_{nm}[/tex] element of the matrix is [tex]\rho_{nm}=\langle n|\rho |m \rangle[/tex]. We find then
[tex]\rho=
\begin{bmatrix}
1/2 & 0 \\
0 & 1/2
\end{bmatrix}[/tex]
which clearly describes a pure state, since [tex]Tr(\rho^2)=1[/tex].
How can one find then the state vectors? Since [tex]\rho=|\psi \rangle \langle \psi |[/tex] in general, is it true to say that [tex]\rho |\psi \rangle = |\psi \rangle[/tex]?
b)Unfortunately, I don't have a clue for this one. What do we mean by taking the partial trace of one system? Any help please?