# Preparing Bell state $(1/\sqrt{2}) (|01\rangle + |10\rangle)$ in Qiskit

I'm working through the Qiskit textbook right now, and wanted to complete part 1 of exercise 3.4, which asks me to use qiskit to produce the Bell state:

$$\frac{|01\rangle + |10\rangle}{\sqrt{2}}$$

which is equivalent to

$$\begin{bmatrix} 0 \\ \frac{1}{\sqrt2} \\ \frac{1}{\sqrt2}\\ 0 \end{bmatrix}.$$

When you apply a CNOT to this, we get:

$$\begin{bmatrix} 0 \\ 0 \\ \frac{1}{\sqrt2}\\ \frac{1}{\sqrt2} \end{bmatrix}$$

Which is just $$|1\rangle\otimes |+\rangle$$. However, I can't figure out how to set my qubits up in the right way.

Any tips on how to use qiskit to get that last matrix?

The circuit to prepare the state $$|\psi \rangle = \dfrac{|01\rangle + |10\rangle}{\sqrt{2}}$$ is as follows:

     ┌───┐
q_0: ┤ H ├──■───────
└───┘┌─┴─┐┌───┐
q_1: ─────┤ X ├┤ X ├
└───┘└───┘


This can be written in matrix notation as:

\begin{align} U &= (I \otimes X)\cdot CNOT \cdot (H\otimes I) \\ &= \begin{pmatrix} 1 & 0 \\ 0 & 1 \end{pmatrix} \otimes \begin{pmatrix} 0 & 1 \\ 1 & 0 \end{pmatrix} \cdot \begin{pmatrix} 1 & 0 & 0 & 0 \\ 0 & 1 & 0 & 0 \\ 0 & 0 & 0 & 1\\ 0 & 0 & 1 & 0\end{pmatrix} \cdot \dfrac{1}{\sqrt{2}}\begin{pmatrix} 1 & 1 \\ 1 & -1 \end{pmatrix} \otimes \begin{pmatrix} 1 & 0 \\ 0 & 1 \end{pmatrix} \\ &= \begin{pmatrix} 0 & 1 & 0 & 0 \\ 1 & 0 & 0 & 0 \\ 0 & 0 & 0 & 1\\ 0 & 0 & 1 & 0\end{pmatrix} \cdot \begin{pmatrix} 1 & 0 & 0 & 0 \\ 0 & 1 & 0 & 0 \\ 0 & 0 & 0 & 1\\ 0 & 0 & 1 & 0\end{pmatrix} \cdot \dfrac{1}{\sqrt{2}} \begin{pmatrix} 1 & 0 & 1 & 0 \\ 0 & 1 & 0 & 1 \\ 1 & 0 & 1 & 0\\ 0 & 1 & 0 & 1\end{pmatrix} \\ &= \dfrac{1}{\sqrt{2}}\begin{pmatrix} 0 & 1 & 0 & 1 \\ 1 & 0 & 1 & 0 \\ 1 & 0 & -1 & 0\\ 0 & 1 & 0 & -1 \end{pmatrix} \end{align}

And $$U |00\rangle =\dfrac{1}{\sqrt{2}}\begin{pmatrix} 0 & 1 & 0 & 1 \\ 1 & 0 & 1 & 0 \\ 1 & 0 & -1 & 0\\ 0 & 1 & 0 & -1 \end{pmatrix} \begin{pmatrix} 1 \\ 0 \\ 0 \\ 0 \end{pmatrix} = \dfrac{1}{\sqrt{2}} \begin{pmatrix} 0 \\ 1 \\ 1 \\ 0 \end{pmatrix} = \dfrac{|01\rangle + |10\rangle}{\sqrt{2}}$$

If you want to generate this circuit with qiskit you can do it as follows:

from qiskit import QuantumRegister, ClassicalRegister, QuantumCircuit

qreg_q = QuantumRegister(2, 'q')
creg_c = ClassicalRegister(2, 'c')
circuit = QuantumCircuit(qreg_q, creg_c)

circuit.h(qreg_q[0])
circuit.cx(qreg_q[0], qreg_q[1])
circuit.x(qreg_q[1])

• Cheers for that @KAJ226! I wanted to clear something up about the convention that qiskit uses. Do operations translate from right to left, top to bottom? For example, you placed $𝐼⊗𝑋$ first, which is the rightmost set of qubit values. Additionally, you wrote the identity matrix first, which is $q_1$'s value. Is that the right way of thinking about this? I've been going from the left to the right. Feb 4 '21 at 3:10
• It also looks like the convention is inconsistent with qiskit. Feb 4 '21 at 3:22
• The reason it is in the order $(I \otimes X)\cdot CNOT \cdot (H\otimes I)$ is because you multiply this to the state $|\psi \rangle = |00\rangle$ from the left. That is: $U |\psi \rangle = (I \otimes X)\cdot CNOT \cdot (H\otimes I) |00 \rangle$ and hence the operation $(H\otimes I) |00 \rangle$ get done first. Follow by $CNOT$ then follow by $(I \otimes X)$. Now, it is good to note that qiskit uses little-endian for both classical bit ordering and qubit ordering. So the state $|0001\rangle$ would be $|1000\rangle$ in qiskit. Feb 4 '21 at 4:06
• That almost answers everything. The last thing I'm confused about is the order that tensor product takes. In this case, qiskit would say ($q_0 ⊗ q_1$), right? Feb 4 '21 at 4:11
• it reads backward. Feb 4 '21 at 6:14

To provide general answer, Bell states are prepared with circuit described by matrix $$CNOT(H \otimes I)$$. Which Bell state is returned depends on input to the circuit:

• input $$|00\rangle$$ returns $$\frac{1}{\sqrt{2}}(|00\rangle + |11\rangle)$$
• input $$|01\rangle$$ returns $$\frac{1}{\sqrt{2}}(|01\rangle + |10\rangle)$$
• input $$|10\rangle$$ returns $$\frac{1}{\sqrt{2}}(|00\rangle - |11\rangle)$$
• input $$|11\rangle$$ returns $$\frac{1}{\sqrt{2}}(|01\rangle - |10\rangle)$$

Clearly, value of the second input qubit specifies whether both output qubits will be same (the second input qubit is $$|0\rangle$$) or opposite (the second input qubit is $$|1\rangle$$). The first qubit specifies a phase - when the qubit is $$|0\rangle$$, the phase is 0, when it is $$|1\rangle$$, the phase is $$\pi$$ ($$\mathrm{e^{i\pi}}=-1$$).

Concerning the input, to obtain for example $$|01\rangle$$, put $$X$$ gate on second qubit and then apply the circuit for Bell state. Similarly for other inputs.