# Implementing deutsch_problem(seed=None) and deutsch() from qiskit text

I'm attempting to work through the exercise at the end of https://learn.qiskit.org/course/ch-gates/phase-kickback#phase-8-0 where I am to build and implement Deutsch's algorithm. I understand that deutsch_problem(seed=None) will randomly determine which function is in the black box, but I am not sure how to implement it using Python coding in the def deutsch(function).

My attempt is below and though I have not yet run this on qasm_simulator to see what result I get, I feel that since qc.draw() only gives me the circuit leading up to the black-box, something must be incorrect.

# My attempt


# My code within def deutsch(function):
deutsch_problem(seed=None)    #I THINK this randomly decides which of the 4 functions are in the black box

#The next three lines are my attempt to turn the random function into a unitary for the circuit
usim = Aer.get_backend('unitary_simulator')
qobj = assemble(problem)
unitary = usim.run(qobj).result().get_unitary()

#The next section builds Deutsch's algorithm to measure the outcome
qc=QuantumCircuit(2,1)
qc.x(1)
qc.h([0,1])
qc.compose(unitary)
qc.h(0)
qc.measure(0,0)


This is how I do it, I tried to append gate directly with to_gate, remember to transpile or decompose the label gate before run simulator. if you want to get unitary of the gate, you can use Operator directly


def deutsch(function):
"""Implements Deutsch's algorithm.

Args:
function (QuantumCircuit): Deutsch function to solve.
Must be a 2-qubit circuit, and either balanced,
or constant.
Returns:
bool: True if the circuit is balanced, otherwise False.
"""

qc = QuantumCircuit(2,1)
qc.x(1)
qc.h([0,1])
qc.barrier()
################## Three way to append the deutsch_problem()
qc.append(problem.to_gate(label='unitary'),[0,1])
#qc.append(Operator(problem),[0,1])
#qc.unitary(Operator(problem),[0,1])
###################
qc.barrier()
qc.h(0)
qc.measure(0,0)
qc = qc.decompose('unitary')
display(qc.draw())
svsim = Aer.get_backend('aer_simulator')
#qc = transpile(qc,svsim)
final_state = svsim.run(qc).result().get_counts()#.get_statevector()
final_state = list(final_state.keys())
if final_state == '0':
return 'constant'
if final_state == '1':
return 'balance'
from qiskit import Aer, assemble
from qiskit.quantum_info.operators import Operator
problem =deutsch_problem()
#display(problem.draw())
deutsch(problem)

$$$$
`
• Thank you so much! Much of python is still new and I had not yet seen append, to_gate, Operator(), ... so this has taught me more than I had hoped! Jun 20, 2022 at 13:08
• also don't forget the decompose and transpile step for the label gate(oracle), otherwise simulation will pop up error. @PGibbon
– poig
Jun 20, 2022 at 16:32