Making custom gate in Qiskit?

Question

I have been trying to make a gate in qiskit in terms of the basic gates, but I keep on getting an error when I apply it to a circuit.
This is my gate:
class LGate(Gate):
 
    def __init__(self, label=None):
    
        super().__init__('l', 1, [], label=label)

def _define(self):

from qiskit.circuit.quantumcircuit import QuantumCircuit
        from qiskit.circuit.library import U3Gate
        q = QuantumRegister(1, 'q')
        qc = QuantumCircuit(q, name=self.name)
        rules = [
            (U3Gate(pi, 0, pi), [q[0]], [])
        ]
        qc._data = rules
        self.definition = qc

Of course this is just an X gate, but I was just trying a basic example.
Running the program:
circ = QuantumCircuit(2)
circ.l(0)
print(circ)

Error:
AttributeError: 'QuantumCircuit' object has no attribute 'l'

Cryoris · Answer

You only defined the gate, but not you haven't added the according method to the circuit. Add the following to the QuantumCircuit class:
    def l(self, qubit): 
        # adjust this import to the location of your gate
        from .library.standard_gates.l import LGate
        return self.append(LGate(), [qubit], [])

See for instance here how it works for the SwapGate.
Just as a note: It's best practice to not use the letter l because that can be difficult to distinguish from an I in some fonts.

Yixiong_chen · Answer

I think you can also use the method of composite gates which might be easier to implement. The idea is that you create a circuit with gates and then turn it into an instruction by using the to_instruction() method. Once you've done this, you can consider this instruction as a predefined gate and add it to your new circuit by using the append() method. Let me show you an example code. I'll follow your example for X gate, but this code can be generalized to arbitrary gates.
from qiskit import QuantumCircuit

# customize a gate instruction (e.g., X gate)
qc = QuantumCircuit(1, name='X')
qc.x(0)
custom_gate = qc.to_instruction()

# append custom gate to a new circuit
new_circ = QuantumCircuit(2)
new_circ.append(custom_gate, [0])
print(new_circ)

┌───┐
q_0: ┤ X ├
     └───┘
q_1: ─────

As you can see, we first define single-quibit circuit qc with a X gate and turn it into an instruction. Then it can be appended to any new circuit. Here we attach it to a two-qubit circuit with name new_circ.
I hope my answer would help.

Making custom gate in Qiskit?

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