How can I code for an anticontrol U1 gate in qiskit? There are references for anticontrol X but no U gate

Question

Some of them use https://github.com/Qiskit/qiskit-terra/blob/3b3536bcdb83124d49723dd205573f169c82ea9c/qiskit/circuit/add_control.py#L24 this code to implement the anticontrol X gate

Answer 1

U1Gate is being replaced by PhaseGate (aka p). If you still want to use u1, replace the import PhaseGate in this example by import U1Gate as PhaseGate:

from qiskit import QuantumCircuit
from qiskit.circuit.library import PhaseGate
circuit = QuantumCircuit(2)
circuit.append(PhaseGate(3.14159).control(1, ctrl_state="0"), [0, 1])
print(circuit)

q_0: ─o─────
      │P(π) 
q_1: ─■─────
            

The method control takes the amount of qubits to control on (in this example, 1) and the ctrl_state. In this case, an anticontrol (aka open control) applies phase rotation on q_1 if the q_0 == 0.