quantum_layer.py

from qiskit.aqua.operators import Z, Y, X
from qiskit.aqua.operators import StateFn

QUBITS = 4
operatorZ = Z ^ Z ^ Z ^ Z
operatorX = X ^ X ^ X ^ X
operatorY = Y ^ Y ^ Y ^ Y

def quantum_layer(initial_parameters):
    # expecting parameters to be a numpy array
    quantumRegister = QuantumRegister(QUBITS)
    quantumCircuit = QuantumCircuit(quantumRegister)
    
    quantumCircuit.h(range(4))

    for i in range(len(initial_parameters)):
      quantumCircuit.ry(initial_parameters[i] * np.pi, i)
   
    psi = StateFn(quantumCircuit)
    
    # two ways of doing the same thing
    expectationX = (~psi @ operatorX @ psi).eval()
    expectationZ = psi.adjoint().compose(operatorZ).compose(psi).eval().real
    expectationY = (~psi @ operatorY @ psi).eval()
    
    expectationZ = np.abs(np.real(expectationZ))
    expectations = [expectationX, expectationY, expectationZ, 
                    expectationX + expectationY + expectationZ] 

    return np.array(expectations)