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I was recently trying to use Qiskit's inbuilt QDRIFT to construct a circuit for a simple Hamiltonian $H = XX + YY$. but I'm having an issue converting the result of QDrift.convert into a circuit. I get the constructor first by doing:

>>> from qiskit.opflow.evolutions.trotterizations.qdrift import QDrift
>>> sim = QDrift(1)

Then I construct my Hamiltonian by doing:

>>> from qiskit.opflow import X, Y
>>> pauli = (X ^ X) + (Y ^ Y)

I checked the type of pauli, it is <class 'qiskit.opflow.primitive_ops.pauli_sum_op.PauliSumOp'> which should not be an issue for my qdrift. I then get my new op by doing:

>>> com_op = sim.convert(pauli)

The type of com_op is <class 'qiskit.opflow.list_ops.composed_op.ComposedOp'>. This class's documentation exists here and it has a method to_circuit defined for it. But when I run it I get error:

qiskit.extensions.exceptions.ExtensionError: 'Input matrix is not unitary.'

Any way I can get my circuit? I do not see why it can't construct a circuit for this.

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1 Answer 1

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An easy way to use QDrift in Qiskit is to pass it as a synthesis method to PauliEvolutionGate

from qiskit.circuit.library import PauliEvolutionGate
from qiskit.opflow import X, Y
from qiskit.synthesis import QDrift

op = (X^X) + (Y^Y)
time = 1
reps = 1

evo_gate = PauliEvolutionGate(op, time, synthesis=QDrift(reps = reps))

circ = QuantumCircuit(2)
circ.append(evo_gate, [0, 1])
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