Set the initial state of the QuantumCircuit using the Statevector object

Question

Suppose we have a 4-qubit quantum state state which is a Statevector object:

>>> type(state)
qiskit.quantum_info.states.statevector.Statevector

I would like to initialize a 6-qubit QuantumCircuit, such that registers 0, 1, 3, 4 are initialized in state, something like this:

qc = QuantumCircuit(6)
qc.initialize(state, [0, 1, 3, 4])

Now, the initialize method takes a list of complex amplitudes as an input. However, is there a way to feed it state directly, without converting it into a list?

Answer 1

You can do this with QuantumCircuit.prepare_state or QuantumCircuit.initialize.

I'll show a simple example.

from qiskit import QuantumCircuit
from qiskit.quantum_info import Statevector

state = Statevector.from_label("+-rl")
qc = QuantumCircuit(4)
qc.prepare_state(state, [0, 1, 2, 3])

Here I've constructed my Statevector object using Statevector.from_label but you should be able to construct it however you like and plug it into QuantumCircuit.prepare_state.

QuantumCircuit.initialize resets all qubits to the $|0\rangle$ state before state preparation.

QuantumCircuit.prepare_state does not add resets.