cost_function.py

import numpy as np 
from cloning_circuit import create_cloning_circuit
from fidelity_count import calculate_fidelity


def compute_cost(thetas, phis = [0, np.pi/2, np.pi, 3*np.pi/2]) : 
    """Computes the cost function, default values of phi are given as [0, np.pi/2, np.pi, 3*np.pi/2]"""

    cost_function = 0

    for phi in phis : 
        circuit = create_cloning_circuit(phi, thetas)
        F1, F2 = calculate_fidelity(circuit)
        cost_function += (1-F1)**2 +(1-F2)**2 + (F1-F2)**2

    return(cost_function)


## TO TEST AND SEE WHAT THE FUNCTION DOES 

# phis = [0, np.pi/2, np.pi, 3*np.pi/2]   # Values we need to train the variational cloning machine. These are the default values so in practice
# no need to put in in argument. 

# # A random seed for reproducibility
# np.random.seed(153)

# thetas = [np.random.uniform(0, 2 * np.pi) for _ in range(12)]  # random initial values

# COST = compute_cost(thetas)

# print("Cost function is :", COST)