Quantum Country exercise 15

This is one in a series of posts working through the exercises in the Quantum Country online introduction to quantum computing and related topics. The exercises in the original document are not numbered; I have added my own numbers for convenience in referring to them.

Exercise 15. Find a way of implementing a NAND gate using just a single Toffoli gate and no other quantum gates.

Answer: As described in the previous exercise, a Toffoli gate with inputs \vert x \rangle, \vert y \rangle, and \vert 1 \rangle acts as a NAND gate producing the target output \vert 1 \oplus \left( x \wedge y \right) \rangle = \vert \neg \left( x \wedge y \right) \rangle.

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