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OPENQASM2.0 has only one two-qubit gate: controlled not. For a teleportation experiment, I need to perform a measurement in the Bell basis. That is, I need a two-qubit gate with matrix representation

$$\begin{bmatrix} 0&1&1&0\\ 0&-1&1&0\\ 1&0&0&1\\ 1&0&0&-1 \end{bmatrix}/\sqrt{2}.$$

To use this library, I need to decompose this gate into a combination of CNOTs and elementary single-qubit gates such as X,Y,Z, etc.

I don't expect the person answering this question to give the decomposition, but hope that they can point me to a helpful resource. I am familiar with linear algebra and have tools such as Mathematica.

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  • $\begingroup$ If you know how to create a Bell pair from any of the four states |00>, |01>, |10>, |11> using H and CNOT gates, then running the circuit backward (H and CNOT are self-inverse) will map any on of the 4 Bell states to the 4 computational basis states. Measure them and this will have implemented the Bell state measurement. $\endgroup$
    – holl
    Dec 13, 2021 at 15:52

1 Answer 1

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The key observation is that swapping the first and third column yields

$$ \frac{1}{\sqrt2}\begin{bmatrix} 0 & 1 & 1 & 0 \\ 0 & -1 & 1 & 0 \\ 1 & 0 & 0 & 1 \\ 1 & 0 & 0 & -1 \end{bmatrix} \to \frac{1}{\sqrt2}\begin{bmatrix} 1 & 1 & 0 & 0 \\ 1 & -1 & 0 & 0 \\ 0 & 0 & 1 & 1 \\ 0 & 0 & 1 & -1 \end{bmatrix} = I\otimes H\tag1 $$

where $H=\frac{1}{\sqrt2}\begin{bmatrix} 1 & 1 \\ 1 & -1 \end{bmatrix}$ is the Hadamard gate. The coefficients $\frac{1}{\sqrt2}$ are needed to ensure unitarity. The column swap corresponds to the permutation matrix

$$ P = \begin{bmatrix} 0 & 0 & 1 & 0 \\ 0 & 1 & 0 & 0 \\ 1 & 0 & 0 & 0 \\ 0 & 0 & 0 & 1 \end{bmatrix}\tag2 $$

which maps $|00\rangle$ to $|10\rangle$ and $|10\rangle$ to $|00\rangle$. In other words, it flips the first qubit when the second qubit is in the $|0\rangle$ state. This is the usual controlled-NOT gate with two modifications: the first qubit is the target and the second is the control and the gate acts non-trivially if the control qubit is in the $|0\rangle$ state rather than the usual $|1\rangle$. Thus,

$$ P = (I\otimes X)\circ\text{CNOT}_{2,1}\circ(I\otimes X).\tag3 $$

Putting it all together, we get

$$ \frac{1}{\sqrt2}\begin{bmatrix} 0 & 1 & 1 & 0 \\ 0 & -1 & 1 & 0 \\ 1 & 0 & 0 & 1 \\ 1 & 0 & 0 & -1 \end{bmatrix} = (I\otimes H) \circ (I\otimes X)\circ\text{CNOT}_{2,1}\circ(I\otimes X)\tag4 $$

where the last expression is written in the linear algebraic convention, i.e. the corresponding gates are executed from right to left.

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  • $\begingroup$ This CNOT gate seems to be different from what IBM does; the matrix for their gate has identity for the upper left quadrant. The one needed to use this answer appears to be {{1,0,0,0},{0,0,0,1},{0,0,1,0},{0,1,0,0}} $\endgroup$
    – Anna Naden
    Dec 7, 2021 at 23:47
  • $\begingroup$ It's the same gate, but with control and target qubits swapped. $\endgroup$ Dec 7, 2021 at 23:51

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