How to visualize the resulting state vector using CVNeuralNetLayers and Lightning qubit device

Question

I'm trying to learn how to use PennyLane's CVNeuralNetLayers. I wrote this code below but I am not able to use the resulting state of the circuit generated by this layer. I looked for examples in the documentation but found nothing.

I already tried to change the device to "default.qubit" but a similar error occurs. Is it possible to visualize the resulting state of this layer? Should i change the pennylane device?

from pennylane.templates.layers import CVNeuralNetLayers
from pennylane import numpy as np
import pennylane as qml

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

n_layers = 1
n_wires = 4

shapes = CVNeuralNetLayers.shape(n_layers, n_wires)
weights = [np.random.random(shape) for shape in shapes]

@qml.qnode(dev)
def circuit(weights):
  CVNeuralNetLayers(*weights, wires=[0, 1, 2, 3])
  return qml.state()

print("\n\n", qml.draw(circuit, expansion_strategy="lightning")(weights))

ket = circuit(weights)
print("\nResulting State: ", ket)

The output is:

DeviceError: Gate Beamsplitter not supported on device lightning.qubit

Answer 1

The CVNeuralNetLayers use a very particular set of quantum gates that are unique to photonics. The only device that is available in PennyLane which can simulate these gates is the Strawberry Fields plugin device. It should be able to handle circuits with CVNeuralNetLayers.

As a secondary note, this plugin is not as feature-rich as some other ones, so I'm not 100% whether or not it will support the functionality you are looking to use (qml.draw and qml.state). You'll have to check whether it suits your particular needs.