Nonsensical projection of initialized qubits in Qiskit

Question

I wanted to try Qiskit by setting up two qubits initialized to $ | 11 \rangle $ or $ | 01 \rangle $ and two classical bits to measure those two initialized qubits.

For this simple demonstration, I used the code below: (ignore the circuit name being "fivequbitcircuit." I started trying Qiskit because I wanted to try the five qubit error correcting code and realized that regardless of which single qubit error I was trying to simulate, the histogram was mostly giving me 0000 for the syndrome measurement.

import numpy as np
from qiskit import QuantumCircuit, QuantumRegister, ClassicalRegister

qr = QuantumRegister(2, name="q")    

cr = ClassicalRegister(2, 'c')

fivequbitcircuit = QuantumCircuit(qr, cr)
fivequbitcircuit.initialize( 1,[0])    #initializing the first qubit to 0
fivequbitcircuit.initialize( 1,[1])    #initializing the second qubit to 0

fivequbitcircuit.measure(qr, cr)        

# Drawing the Quantum Circudit
fivequbitcircuit.draw('mpl')

from qiskit import QuantumCircuit,Aer,transpile
from qiskit.visualization import plot_histogram

backend = Aer.get_backend("aer_simulator")
tqc = transpile(fivequbitcircuit, backend)
job = backend.run(tqc, shots=1000)
result = job.result()
counts = result.get_counts(tqc)

plot_histogram(counts)

For some reason, running this gives me something like the histogram below:

It made no sense to me, as both qubits are initialized to $ | 1 \rangle $ and no gate operation was done to either one before the projective measurement was made.

Answer 1

It seems the Aer backend is not compatible with initializing using integers in this version, here's a small change to make it work: You can use a string to tell the circuit to initialize in the $|1\rangle$ state.

fivequbitcircuit.initialize("1", [0])  
fivequbitcircuit.initialize("1", [1]) 

I've also opened an issue to keep track of this: https://github.com/Qiskit/qiskit-terra/issues/10143.

In future, it would be more efficient to directly open an issue on Qiskit (or the respective package), as there you can directly contact developers for package-specific questions like this one.