One of the most unusual and fascinating aspects of quantum mechanics is the fact that particles or systems can become entangled. For the simplest two quantum systems case we denote the systems A and B. If these systems are entangled, this means that the values of certain properties of system A are correlated with the values that those properties will assume for system B. The properties can become correlated even when the two systems are spatially separated—leading to the phrase spooky action at a distance.

The roots of this idea go back a long way—all the way back to the year 1935 when Einstein and two colleages, Podolsky and Rosen (now commonly known as EPR), published a paper titled “Can the quantum-mechanical description of reality be considered complete?” This paper—written by the quantum skeptic Einstein—was actually designed to show that quantum theory is incomplete and to make absurd predictions.

A core value held by EPR and other “realists” was that the properties of physical systems have definite values (an objective reality) whether you observe the system or not. Another way to say this is that a given property of a system has a sharply defined value before a measurement is made.

Quantum mechanics, however, tells a different story. Suppose that we have a qubit in the state 〉 = |0〉. Quantum mechanics tells us that prior to measurement a property of the system does not have a definite or sharply defined value. For our qubit, say that we want to measure ...

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