Nonlocality of the type first elucidated by Bell in 1964 is a difficult concept to explain to nonspecialists and undergraduates. We attempt to do so by showing how nonlocality can be used to solve a problem in which someone might find themselves as the result of a series of normal, even if somewhat unlikely, events. Our story is told in the style of a Sherlock Holmes mystery, and is based on Mermin’s formulation of the “paradoxical” illustration of quantum nonlocality discovered by Greenberger, Horne, and Zeilinger.

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P. K.
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Bell’s theorem without inequalities and only two distant observers
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See also
P. K.
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Spooky actions at a distance: Mysteries of the quantum theory
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19.

In the first case we must know the no-cloning theorem, and in the second one we must know that a qubit can contain only one bit of information, despite being preparable in infinitely many different ways.

20.
L.
Hardy
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Disentangling nonlocality and teleportation
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21.
R. W.
Spekkens
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H.
Engel
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Overlook
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D.
Pirie
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The Patient’s Eyes
(
St. Martin’s
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2002
);
The Night Calls
(
St. Martin’s
, New York,
2003
).
24.

For readers who are familiar with Mermin’s illustration of nonlocality (Ref. 11), upon which the situation here is based, it may be helpful to note which elements of our scenario correspond to those of Mermin’s. The three clients in our story take the place of Mermin’s three detectors, and the two settings on these detectors to the two sides of each of the robbers (setting 1 to the back and setting 2 to the front). The four different combinations of detector settings therefore correspond to the four different combinations of sides of the robbers seen by the four guards. Finally, the two colors that the detectors can flash correspond to the same two colors of the robbers’ suits. Thus, in Mermin’s case when he sets the detector settings to 111, for example, and states that an even number of the three detectors flash red, that corresponds in our case to saying that the fourth guard saw an even number of the robbers wearing red.

25.
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Quantum computation with quantum dots
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26.
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2001
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T.
Maudlin
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Quantum Non-locality and Relativity
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