qiskit: IQFT acting on subsystem of reversed-ordered qubits state

Question

I have a state psi as an ndarray of shape (2 ** 3,) s.t.

psi[0]= amplitude of 000

psi[1] = amplitude of 001.

So my qubit ordering is reversed w.r.t. qiskit's. To initialize the circuit correctly and apply the IQFT on the first 2 qubits from the left I tried the following code:

import qiskit as qt
from qiskit.aqua.circuits import FourierTransformCircuits as QFT

    circuit = qt.QuantumCircuit(3)
    circuit.initialize( psi, [i for i in reversed(circuit.qubits)])

    QFT.construct_circuit(circuit=circuit, qubits=circuit.qubits[:2], inverse=True)

    backend = qt.Aer.get_backend('statevector_simulator')
    final_state = qt.execute(circuit, backend, shots=1).result().get_statevector()

From the tests I've run, final_state is not what I expected: defining

exact = np.kron(IQFT_matrix(2),np.eye(2)).dot(state) with IQFT_matrix(2)= IQFT_matrix for 2 qubits.

np.testing.assert_array_almost_equal(final_state, exact)

fails. Can you please help me find the problem?

Answer 1

There is the parameter do_swaps when you construct the fourier transform circuit.

do_swaps (bool): Boolean flag to specify if swaps should be included to align
                 the qubit order of input and output. The output qubits would
                 be in reversed order without the swaps.

Answer 2

To change the endianness of a circuit in Qiskit you can use the reverse_bits method of the QuantumCircuit. To do so for the QFT you can pick the QFT from the circuit library, reverse it, and add it to your circuit:

from qiskit import QuantumCircuit
from qiskit.circuit.library import QFT

iqft = QFT(3, inverse=True)  # get the IQFT
reversed_bits_QFT = iqft.reverse_bits()  # reverse bit order

circuit = QuantumCircuit(3)
circuit.compose(reversed_bits_QFT, inplace=True)  # append your QFT