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Consider the Bell state
∣
Φ
+
⟩
=
(
1
/
2
)
(
∣
00
⟩
+
∣
11
⟩
)
|\Phi^+\rangle = (1/\sqrt{2})(|00\rangle + |11\rangle)
∣
Φ
+
⟩
=
(
1/
2
)
(
∣00
⟩
+
∣11
⟩)
of system S and environment E. Trace out E to get the reduced density matrix
ρ
S
\rho_S
ρ
S
, then compute the purity
P
=
T
r
(
ρ
S
2
)
P = \mathrm{Tr}(\rho_S^2)
P
=
Tr
(
ρ
S
2
)
. What is
P
P
P
?
🔬 try it before you answer
0.707
0.5
1
0.25
Check answer