Hybrid quantum-classical model

Hi, I have been tring to make hybrid quantum-classical model.

I have two questions about that.

  1. classical layer between quantum layers
    : First, I attempted hybrid layer codes as follows.

dev = qml.device(‘default.qubit’, wires = 8)
def QCNN (…):
embedding.data_embedding(X, embedding_type=embedding_type) ## embedding 256 classical data to amplitude of 8 qubits
QCNN_structure1(unitary.U_SU4, params, U_params)
torch.nn.Linear(16, 16) ## classical layer 1
QCNN_structure2(unitary.U_SU4, params, U_params)
torch.nn.Linear(4, 4) ## classical layer 2
QCNN_structure3(unitary.U_SU4, params, U_params)
result = qml.expval(qml.PauliZ(4))
return result

I checked my hybrid model including above codes runs well without any errors.
However, I don’t have confidence whether this code can run with my goal.
(i.e., the output of QCNN structure_1 pass to classical layer 1 and the output from classical layer 1 to QCNN_structure 2)
How can I check this? And is it possible to write code for my goal?

  1. quamtum layer between classical layers
    : Like the first question I mentioned above, I considered hybrid model with quantum layer between classical layers. In this case, how can I use quantum layer without measurement between classical layers?
    For example, (this is pseudo code)

classical_layer1 (torch.nn.Linear(256, 256))
quantum_layer1 (amplitude embedding with 8 qubits and without measurement)
classical_layer2 (torch.nn.Linear(64, 64)

Is it possible to use intermediate data from quantum circuit without measurement step as a input data for classical layer?

Thank you in advance! :slightly_smiling_face:

Hi @junggu.choi, thank you for your question. Unfortunately you always need to perform a measurement at the end of your quantum circuit. Moreover, your second classical layer will need classical inputs, which are the measurements from your quantum layer. This demo on turning quantum nodes into Torch Layers can be very helpful for you.

I also encourage you to go through the introduction module in the Xanadu Quantum Codebook, it can help you learn why measurement is important.

I hope this helps!