Qiskit QuantumCircuit.save_unitary() is showing an error

Question

OS: lubuntu 20.04 Mozilla Firefox browser jupyter-lab environment

Error message: AttributeError: 'QuantumCircuit' object has no attribute 'save_unitary'

Below is the code:

from qiskit import QuantumCircuit, Aer, assemble
from math import pi
import numpy as np
from qiskit.visualization import plot_bloch_multivector, plot_histogram, array_to_latex
import matplotlib.pyplot as plt


def show_figure(fig):

    # create a dummy figure and use its
    # manager to display "fig"

    
    dummy = plt.figure()
    #print("plt.figure is set")
    new_manager = dummy.canvas.manager
    #print("canvas.manager assigned")
    new_manager.canvas.figure = fig
    fig.set_canvas(new_manager.canvas)

qc = QuantumCircuit(2)
qc.x(0)
qc.x(1)
qc.h(0)
qc.h(1)
qc.cx(0,1)
qc.h(0)
qc.h(1)
display(qc.draw(initial_state=True, output='mpl'))
plt.show()
print(qc)


qc.save_unitary()

usim = Aer.get_backend("aer_simulator")
qobj = assemble(qc)
unitary = usim.run(qobj).result().get_unitary()
#array_to_latex(unitary, prefix="\\text{Circuit}\n")
```

Answer 1

You need to instantiate the simulator at first. That is, to move this line before calling save_unitary:

usim = Aer.get_backend("aer_simulator")

Another workaround is to import the method:

from qiskit.providers.aer.library import save_unitary

Although QuantumCircuit is defined in Qiskit Terra, save_unitary method is added to it in Qiskit Aer.

You can find more details here.