Using qml.AmplitudeEmbedding two times in a circuit raises an error

Question

I'm trying to use qml.AmplitudeEmbedding() two times in a circuit but it raises an error saying that the device used does not support this feature.

Answer 1

This is a current restriction with PennyLane, in that only a single, initial state preparation routine can be used.

However, you can use the qml.transforms.merge_amplitude_embedding transform to merge multiple amplitude embeddings in your quantum function, as long as they act on different wires:

dev = qml.device('default.qubit', wires=4)

@qml.qnode(dev)
@qml.transforms.merge_amplitude_embedding
def qfunc():
    qml.CNOT(wires=[0, 1])
    qml.AmplitudeEmbedding([0, 1], wires=2)
    qml.AmplitudeEmbedding([0, 1], wires=3)
    return qml.probs(wires=[0, 1, 2, 3])

Using the transformation we can join the different amplitude embeddings into a single one:

>>> print(qml.draw(qfunc)())
  0: ──╭C───────────────────────╭┤ Probs
  1: ──╰X───────────────────────├┤ Probs
  2: ──╭AmplitudeEmbedding(M0)──├┤ Probs
  3: ──╰AmplitudeEmbedding(M0)──╰┤ Probs
  M0 = [0.+0.j 0.+0.j 0.+0.j 1.+0.j]