Removing barriers from a QuantumCircuit object in Qiskit

Question

I find barriers useful for visualization purposes, naturally it helps observing a circuit that is separated to some logical segments than just watching a mixture of gates and qubits.

However, before transpiling I would love to remove the barriers to avoid possible inefficiencies that can be caused by the presence of the barriers.

I have tried to use qiskit.transpiler.RemoveBarriers as in the example inside the link, and the following behavior occurs:

1. The barriers seem to be deleted from the QuantumCircuit object - verified by drawing using QuantumCircuit.draw, and by examining the content of the QuantumCircuit.__dict__ attribute.
2. After transpiling the barriers are back again inside the transpiled QuantumCircuit object returned by the function transpile, along with the inefficiencies caused by the barriers which I tried to avoid in the first place.

Is there any simple way to get rid of barriers? Am I misunderstanding the functionality of RemoveBarriers?

Thanks.

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Edit:

I am adding a debugging piece of code:

print(f"operator.count_ops() = {operator.count_ops()}")

no_baerriers_operator = RemoveBarriers()(operator)
print(f"no_baerriers_operator.count_ops() = {no_baerriers_operator.count_ops()}")

transpiled_operator = transpile(no_baerriers_operator, **transpile_kwargs)
print(f"transpiled_operator.count_ops() = {transpiled_operator.count_ops()}")

print(f"transpile_kwargs = {transpile_kwargs}")

And its output:

operator.count_ops() = OrderedDict([('barrier', 3), ('([2] == [3])', 1), ('([0] == [1])', 1), ('ccx', 1), ('Uncomputation', 1)]) 

no_baerriers_operator.count_ops() = OrderedDict([('([2] == [3])', 1), ('([0] == [1])', 1), ('ccx', 1), ('Uncomputation', 1)]) 

transpiled_operator.count_ops() = OrderedDict([('cx', 14), ('u', 12), ('barrier', 2)])

transpile_kwargs = {'basis_gates': ['u', 'cx'], 'optimization_level': 3}

It can be seen, somewhat strangely, that in the original QuantumCiruit object (i.e operator) there are 3 barriers, then they vanish after using RemoveBarriers (no barriers in no_barriers_operator), and finally 2 barriers are back to transpiled_operator arfter transpilation.

The custom gates in operator don't contain any barriers.

Answer 1

Here, how to use RemoveBarriers:

from qiskit.transpiler.passes import RemoveBarriers
rb = RemoveBarriers()
circuit_out = rb(circuit_in)

This eliminates the barriers that are visible in the circuit. That is, if an instruction contains barriers as part of its decomposition, it wont be removed.