Question about code

[ComputationalBasis]

Hello everyone,

I am writing this to make sure my code works as intended. I already did some verifications and seems correct, but validation from an expert would be nice.

I want to build a circuit that, initializing the qubits in the |+-+-...> state, applies a global unitary exp(-i gamma Hc) followed by applying the unitary exp(-i beta X(i)) on each qubit. Finally, I just want to compute an expected value given the final state obtained by applying the unitaries. My code is the following:

prep = []

for i in range(n_qubits):
    prep.append(H(i))        # Apply Hadamard to all qubits to obtain plus state
    if i % 2 == 1:           # Also, if odd qubit, apply Z gate tu obtain minus state
        prep.append(Z(i))

initial_prep = chain(*prep)

beta  = Parameter("beta")
gamma = Parameter("gamma")

# Mixer
Hm = sum(X(i) for i in range(n_qubits))
Um = HamEvo(Hm, beta) # Time evolution operator exp(-i*Hm*beta)

# Cost Hamiltonian
Hc = H_Rydberg
Uc = HamEvo(Hc, gamma) # Time evolution operator exp(-i*Hc*gamma)

# QAOA Block
qaoa_block = chain(initial_prep, Uc, Um)

circuit = QuantumCircuit(reg, qaoa_block)

# Energy
def energy_exp(vals):
    E = expectation(circuit, observable=Hc, values=vals)
    return float(abs(torch.real(E).item()))

[awennersteen]

Hi!

This does look largely correct to me.

it's not clear to me that you necessarily want to the the absolute value of E based on what you've said.
But there are reasons to do so of course.

[ComputationalBasis]

Yes, thank you so much Aleksander, I just wanted to make sure about the circuit structure.