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Multiple choice
Security from a Bell violation relies on the monogamy of entanglement, expressed (for CHSH) as
S
A
B
2
+
S
A
E
2
≤
8
S_{AB}^2 + S_{AE}^2 \le 8
S
A
B
2
+
S
A
E
2
≤
8
. If Alice and Bob observe the maximal violation
S
A
B
=
2
2
S_{AB} = 2\sqrt{2}
S
A
B
=
2
2
, what does this bound force for Eve's CHSH correlation
S
A
E
S_{AE}
S
A
E
with Alice?
S
A
E
=
2
2
S_{AE} = 2\sqrt{2}
S
A
E
=
2
2
, because Eve can share the maximal correlation symmetrically
S
A
E
=
4
S_{AE} = 4
S
A
E
=
4
, the algebraic maximum
S
A
E
=
0
S_{AE} = 0
S
A
E
=
0
, because
S
A
B
2
=
8
S_{AB}^2 = 8
S
A
B
2
=
8
saturates the bound, driving Eve's correlation to zero (below even the classical value of 2)
S
A
E
=
2
S_{AE} = 2
S
A
E
=
2
, the classical bound, the most Eve is allowed
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