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I am trying to work with Qiskit's PauliTrotterEvolution() module, but the resulting circuits contain weird gates that I know nothing about.

Here is a simple example: I want to implement the fermionic creation operator on one of two qubits

from qiskit_nature.mappers.second_quantization import BravyiKitaevMapper
from qiskit_nature.operators.second_quantization import FermionicOp 
from qiskit.opflow.evolutions import PauliTrotterEvolution

fermi_op = FermionicOp("+I", display_format="dense") 

The fermionic operator is mapped to a sum of Pauli gates using the Bravyi-Kitaev mapper:

mapper = BravyiKitaevMapper()
bosonic_op = mapper.map(fermi_op)

Now we apply the PauliTrotterEvolution to the exponential of the bosonic operator:

pauli_trotter = PauliTrotterEvolution("trotter", reps=1)

conv = pauli_trotter.convert(bosonic_op.exp_i())

circ = conv.to_circuit()

circ.draw("latex")

I have no clue what these "circuit" gates are. Has anyone seen something similar before?

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These "circuit" gates are empty, if you transpile the circuit to a basis gate set they will disappear:

from qiskit import transpile

circ = # your circuit
transpiled = transpile(circ, basis_gates=["h", "s", "sdg", "rz", "cx"])
print(transpiled.draw())

As the answer of @viwosh points out, that behavior is also fixed in the newer versions of Qiskit.

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When I do this in a recent version (0.19.0.dev0+6e0926f) I get enter image description here

For your version perhaps try "circ.decompose().draw('latex')" to see how "circuit-24" is defined.

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