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Multiple choice
A density matrix
ρ
=
[
0.75
0.25
0.25
0.25
]
\rho = \begin{bmatrix}0.75 & 0.25 \\ 0.25 & 0.25\end{bmatrix}
ρ
=
[
0.75
0.25
0.25
0.25
]
is evolved by the Z gate (
Z
=
d
i
a
g
(
1
,
−
1
)
Z = \mathrm{diag}(1,-1)
Z
=
diag
(
1
,
−
1
)
) as
ρ
′
=
Z
ρ
Z
†
\rho' = Z \rho Z^\dagger
ρ
′
=
Z
ρ
Z
†
. What is the off-diagonal element
ρ
′
[
0
]
[
1
]
\rho'[0][1]
ρ
′
[
0
]
[
1
]
?
🔬 try it before you answer
+0.25
0
-0.5
-0.25
Check answer