Controlled NOT gate
Also known as the XOR or measurement gate, the controlled NOT (or CNOT) gate takes two qubits as input, |x> and |y>. The result is the |x> qubit and an XOR (Ä) of |x> and |y>. If |x>Õs value is 0, |y> remains the same; otherwise, |y>Õs value is flipped to itÕs opposite. This is diagrammed below.
One ramification of this include that, if the |y> bit is zero, the |x> bit is copied to a second qubit but only if |x> is 0 or 1 (similarly, NOT(|x>) is copied to the result if the |y> bit is one). The reason for this qualification on |x> is that a superpositioned |x> bit will result in a new entangled state for the second qubit (specifically, (a|0> + b|1>)Ä|0> -> a|00> + b|11> ).
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Created by Brian Patterson
Last Modified 11/22/00