Is there a function in qiskit that allows me to insert quantum gates at certain indices of a circuit?

Question

Here's my quantum circuit with mid-circuit measurements:

qr = QuantumRegister(1, 'q')
cr = ClassicalRegister(1, 'c')
base_circ = QuantumCircuit(qr, cr)
for i in range(N):
    base_circ.measure(0,0)
    base_circ.barrier()
base_circ.draw()

I wonder is there a method in qiskit that allows me to insert quantum gates at certain indices? For example, in this case, can I add another instruction to add two x gates before the first two measurements (one for each)? I found this post might be helpful, but I'm not able to implement instructions like qc.h(anc[0],i=6) or circuit.gate(qubit).at(index) in my case as it returns errors. Thanks so much for the help!

Answer 1

From my experience, most of the time you can restructure your code to avoid the need for inserting gates in the middle of a circuit.

That said, if you already know the insertion points at the time of circuit creation but you don't know the gates to be inserted, you can add placeholders at these places and replace them later with whatever gates you want.

from qiskit import QuantumCircuit
from qiskit.circuit import Instruction
from typing import Union

class Placeholder(Instruction):
    def __init__(self, num_qubits, label):
        self.name = "placeholder"
        super().__init__(self.name, num_qubits, 0, [], label = label)

    def inverse(self):
        return Placeholder(self.name, self.num_qubits)

# This function accepts a QuantumCircuit in addition to single instructions
# Note: you can modify it to accept a dictionary of label-instruction pairs.
def replace(self, placeholder_label, instruction: Union[Instruction, QuantumCircuit]):
    if isinstance(instruction, QuantumCircuit):
        instruction = instruction.to_gate()

    self._data = [(instruction, _inst[1], _inst[2]) if _inst[0].name == 'placeholder' and _inst[0].label == placeholder_label else _inst for _inst in self._data]

QuantumCircuit.replace = replace

As an example, let's create a new circuit with some placeholders

circ = QuantumCircuit(2)
circ.h(0)
circ.x(1)
circ.cx(0, 1)
circ.append(Placeholder(2, 'First'), [0, 1])
circ.cx(1, 0)
circ.append(Placeholder(1, 'Second'), [0])
circ.measure_all()
circ.draw('mpl')

Now, let's replace these placeholders with actual gates:

from qiskit.circuit.library import HGate, SwapGate

circ.replace('First', SwapGate())
circ.replace('Second', HGate())
circ.draw('mpl')

The final circuit should look like

Answer 2

The short answer is no, there is no way to insert gates in a middle of a circuit.

As explained, the issue https://github.com/Qiskit/qiskit-terra/issues/4736 has a longer explanation on why not.

The mid size explanation is the following: generally speaking, a circuit is not a sequence of instructions and, therefore, there is no indices to insert things in. A circuit is more like a directed acyclic graph where the gates are nodes and the edges are dependency relation on qubits. It is very hard to define an API that is simple but expressive enough to refer to an unambiguous position in it.