qiskit/releasenotes/notes/0.18/parameterized-delay-3d55448e5cbc5269.yaml

---
features:
  - |
    The :class:`~qiskit.circuit.Delay` class now can accept a
    :class:`~qiskit.circuit.ParameterExpression` or
    :class:`~qiskit.circuit.Parameter` value for the ``duration`` kwarg on its
    constructor and for its :attr:`~qiskit.circuit.Delay.duration` attribute.

    For example::

        idle_dur = Parameter('t')
        qc = QuantumCircuit(1, 1)
        qc.x(0)
        qc.delay(idle_dur, 0, 'us')
        qc.measure(0, 0)
        print(qc)  # parameterized delay in us (micro seconds)

        # assign before transpilation
        assigned = qc.assign_parameters({idle_dur: 0.1})
        print(assigned)  # delay in us
        transpiled = transpile(assigned, some_backend_with_dt)
        print(transpiled)  # delay in dt

        # assign after transpilation
        transpiled = transpile(qc, some_backend_with_dt)
        print(transpiled)  # parameterized delay in dt
        assigned = transpiled.assign_parameters({idle_dur: 0.1})
        print(assigned)  # delay in dt