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It is claimed that

Informally, this means that any stabilizer circuit can be simulated using CNOT gates alone; the availability of Hadamard and phase gates provides at most a polynomial advantage.

How does one obtain, say a Hadamard gate or phase gate, using only CNOTs?

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Note that it's talking about simulation, not actually performing something like a Hadamard. In is not saying that you could just replace Hadamards in a circuit with controlled-nots.

For example, the simulation of a Clifford circuit on $n$ qubits usually involves writing down stabilizers, represented as binary strings of length $2n$. If you have $m$ such stabilizers, you'll put it all in an $m\times 2n$ matrix. A Hadamard operation just represents exchanging $X$ and $Z$ which, in this matrix-based representation, just means swapping two columns. But a swap operation can be broken down into three controlled-nots.

Similarly, think of the action of an $S$ gate on the Pauli matrices $X\rightarrow Y$, $Z\rightarrow Z$. As binary strings, this means $\{1,0\}\rightarrow \{1,1\}$ and $\{0,1\}\rightarrow\{0,1\}$. This is just the controlled-not acting on these strings (OK, there's a little bit of processing of phases as well, which I'm leaving out).

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    $\begingroup$ Ah so it's a classical CNOT acting on the representation of the quantum states. Gotcha, thanks! $\endgroup$ Jan 30 at 13:48

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