How to use Initial States in Qiskits QAOA?

Question

The class QAOA from qiskit: https://qiskit.org/documentation/stubs/qiskit.aqua.algorithms.QAOA.html
has the parameter initial_state from the type InitialState. https://qiskit.org/documentation/apidoc/qiskit.aqua.components.initial_states.html#module-qiskit.aqua.components.initial_states

class QAOA(operator=None, optimizer=None, p=1, initial_state=None, mixer=None, initial_point=None, gradient=None, expectation=None, include_custom=False, max_evals_grouped=1, aux_operators=None, callback=None, quantum_instance=None) 

But this doesnt work.

qaoa_mes = QAOA(H, p=p, optimizer=optimizer, initial_state = Zero, quantum_instance=Aer.get_backend("qasm_simulator"))
results = qaoa_mes.run()  

Any Ideas?

Answer 1

You need to add the number of qubits for the initial state, this worked for me :

n_qubits = 2 #or whatever you want for your example 
qaoa_mes = QAOA(H, p=p, optimizer=optimizer, initial_state = Zero(n_qubits), quantum_instance=Aer.get_backend("qasm_simulator"))

You can also pass a list for the initial point, for example :

qaoa_mes = QAOA(H, p=p, optimizer=optimizer, initial_state = [0.,0.], quantum_instance=Aer.get_backend("qasm_simulator"))

You can even pass a circuit for the initial point, there :

n_qubits = 2
initial = QuantumCircuit(n_qubits)
#add any gate you want in the circuit, for example :
initial.h(0)
initial.cx(0,1)

qaoa_mes = QAOA(H, p=p, optimizer=optimizer, initial_state = initial, quantum_instance=Aer.get_backend("qasm_simulator"))

If you need something else feel free to ask ! :)