Qiskit provides the qiskit.quantum_info.Operator class to get the unitary matrix operator from the corresponding quantum circuit, as in the following example:
from qiskit import QuantumCircuitfrom qiskit.quantum_info import Operatorfrom qiskit.visualization import array_to_latexqc = QuantumCircuit(2)qc.h(0)op = Operator(qc)array_to_latex(op)\begin{bmatrix}\frac{1}{\sqrt2} & \frac{1}{\sqrt2} & 0 & 0\\ \frac{1}{\sqrt2} & -\frac{1}{\sqrt2} & 0 & 0\\ 0 & 0 & \frac{1}{\sqrt2} & \frac{1}{\sqrt2} \\ 0 & 0 & \frac{1}{\sqrt2} & -\frac{1}{\sqrt2}\end{bmatrix}
However, Operator(QuantumCircuit) raises an error in the case of a parametric quantum circuit:
from qiskit.circuit import Parameterqc = QuantumCircuit(2)theta = Parameter(name='$\\theta$')qc.ry(theta, 0)op = Operator(qc) # ERROR!This brings me to the question: is there a way in Qiskit to get the matrix operator symbolic representation from a given arbitrary PQC? For instance, in this case I would like to get a sympy.matrices.dense.Matrix object (with just one parameter $\theta$) like this:
\begin{bmatrix}\cos\left(\frac{\theta}{2}\right) & -\sin\left(\frac{\theta}{2}\right) & 0 & 0\\ \sin\left(\frac{\theta}{2}\right) & \cos\left(\frac{\theta}{2}\right) & 0 & 0\\ 0 & 0 & \cos\left(\frac{\theta}{2}\right) & -\sin\left(\frac{\theta}{2}\right)\\ 0 & 0 & \sin\left(\frac{\theta}{2}\right) & \cos\left(\frac{\theta}{2}\right)\end{bmatrix}
EDIT: this is now possible by using the new qiskit-symb package