This story is a supplement to the feature "Quantum Computing with Ions" which was printed in the August 2008 issue of Scientific American.

Entanglement: Spooky Action At a Distance
The “ambiguous cube” (a) is like an ion in a superposition state—a measurement of the ion will lock it into one of two definite states (0 or 1). When two ions are in an entangled superposition (b), a measurement will force both ions into the same state (either 0 or 1) even though there is no physical connection between them.

Truth Table
A trapped-ion computer would rely on logic gates such as the controlled not (CNOT) gate, which consists of two ions, A and B. This truth table shows that if A (the control bit) has a value of 0, the gate leaves B unchanged. But if A is 1, the gate flips B, changing its value from 0 to 1, and vice versa. And if A is in a superposition state (0 and 1 at the same time), the gate puts the two ions in an entangled superposition. (Their state is now identical to the one shown in the box above.)

Illustrations: Jen Christiansen

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