Quantum Computing Asked on May 13, 2021
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?
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.
Answered by Steve Wood on May 13, 2021
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
Answered by Cryoris on May 13, 2021
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