#### Wrong outputs in Qiskit Circuit even though the implementation seem correct

So, I am making a transformation circuit for a feistel structure.here is the code:

def apply_T1(qr, cr, qc, ancilla):

``````# Input: 0000 -> Output: 0110
qc.x([qr[0], qr[1], qr[2], qr[3]])
qc.mcx([qr[0], qr[1], qr[2], qr[3]], ancilla[1])
qc.mcx([qr[0], qr[1], qr[2], qr[3]], ancilla[2])
qc.x([qr[0], qr[1], qr[2], qr[3]])
qc.barrier()

# Input: 0001 -> Output: 1111
qc.x([qr[1], qr[2], qr[3]])
qc.mcx([qr[0], qr[1], qr[2], qr[3]], ancilla[0])
qc.mcx([qr[0], qr[1], qr[2], qr[3]], ancilla[1])
qc.mcx([qr[0], qr[1], qr[2], qr[3]], ancilla[2])
qc.mcx([qr[0], qr[1], qr[2], qr[3]], ancilla[3])
qc.x([qr[1], qr[2], qr[3]])
qc.barrier()

# Input: 0010 -> Output: 1011
qc.x([qr[0], qr[2], qr[3]])
qc.mcx([qr[0], qr[1], qr[2], qr[3]], ancilla[0])
qc.mcx([qr[0], qr[1], qr[2], qr[3]], ancilla[1])
qc.mcx([qr[0], qr[1], qr[2], qr[3]], ancilla[3])
qc.x([qr[0], qr[2], qr[3]])
qc.barrier()

# Input: 0011 -> Output: 0111
qc.x([qr[2], qr[3]])
qc.mcx([qr[0], qr[1], qr[2], qr[3]], ancilla[0])
qc.mcx([qr[0], qr[1], qr[2], qr[3]], ancilla[1])
qc.mcx([qr[0], qr[1], qr[2], qr[3]], ancilla[2])
qc.x([qr[2], qr[3]])
qc.barrier()

# Input: 0100 -> Output: 1110
qc.x([qr[0], qr[1], qr[3]])
qc.mcx([qr[0], qr[1], qr[2], qr[3]], ancilla[3])
qc.mcx([qr[0], qr[1], qr[2], qr[3]], ancilla[2])
qc.mcx([qr[0], qr[1], qr[2], qr[3]], ancilla[1])
qc.x([qr[0], qr[1], qr[3]])
qc.barrier()

# Input: 0101 -> Output: 1000
qc.x([qr[1], qr[3]])
qc.mcx([qr[0], qr[1], qr[2], qr[3]], ancilla[3])
qc.x([qr[1], qr[3]])
qc.barrier()

# Input: 0110 -> Output: 0000

# Input: 0111 -> Output: 0011
qc.x(qr[3])
qc.mcx([qr[0], qr[1], qr[2], qr[3]], ancilla[0])
qc.mcx([qr[0], qr[1], qr[2], qr[3]], ancilla[1])
qc.x(qr[3])
qc.barrier()

# Input: 1000 -> Output: 0101
qc.x([qr[1], qr[2], qr[3]])
qc.mcx([qr[0], qr[1], qr[2], qr[3]], ancilla[0])
qc.mcx([qr[0], qr[1], qr[2], qr[3]], ancilla[2])
qc.x([qr[1], qr[2], qr[3]])
qc.barrier()

# Input: 1001 -> Output: 1101
qc.x([qr[1], qr[2]])
qc.mcx([qr[0], qr[1], qr[2], qr[3]], ancilla[0])
qc.mcx([qr[0], qr[1], qr[2], qr[3]], ancilla[2])
qc.mcx([qr[0], qr[1], qr[2], qr[3]], ancilla[3])
qc.x([qr[1], qr[2]])
qc.barrier()

# Input: 1010 -> Output: 1100
qc.x([qr[0], qr[2]])
qc.mcx([qr[0], qr[1], qr[2], qr[3]], ancilla[3])
qc.mcx([qr[0], qr[1], qr[2], qr[3]], ancilla[2])
qc.x([qr[0], qr[2]])
qc.barrier()

# Input: 1011 -> Output: 0010
qc.x([qr[2]])
qc.mcx([qr[0], qr[1], qr[2], qr[3]], ancilla[1])
qc.x([qr[2]])
qc.barrier()

# Input: 1100 -> Output: 1010
qc.x([qr[1], qr[0]])
qc.mcx([qr[0], qr[1], qr[2], qr[3]], ancilla[1])
qc.mcx([qr[0], qr[1], qr[2], qr[3]], ancilla[3])
qc.x([qr[1], qr[0]])
qc.barrier()

# Input: 1101 -> Output: 1001
qc.x([qr[1]])
qc.mcx([qr[0], qr[1], qr[2], qr[3]], ancilla[0])
qc.mcx([qr[0], qr[1], qr[2], qr[3]], ancilla[3])
qc.x([qr[1]])
qc.barrier()

# Input: 1110 -> Output: 0100
qc.x([qr[0]])
qc.mcx([qr[0], qr[1], qr[2], qr[3]], ancilla[2])
qc.x([qr[0]])
qc.barrier()

# Input: 1111 -> Output: 0001
qc.mcx([qr[0], qr[1], qr[2], qr[3]], ancilla[0])

return qc

# following code is to test

sim = AerSimulator()

# Test Case: Input = 0000, Expected Output = 0110

qr = QuantumRegister(4)
cr = ClassicalRegister(4)
anc = QuantumRegister(4)
qc = QuantumCircuit(qr, anc, cr)
qc = apply_T1(qr, cr, qc, anc)
qc.measure(anc, cr)
result = sim.run(qc).result()
counts = result.get_counts(qc)
print(f"Input: 0000, Expected: 0110, Actual: {counts}")

# Test Case: Input = 0001, Expected Output = 1111

qr = QuantumRegister(4)
cr = ClassicalRegister(4)
anc = QuantumRegister(4)
qc = QuantumCircuit(qr, anc, cr)
qc.x([qr[3]])  # To create input 0001

qc = apply_T1(qr, cr, qc, anc)
qc.measure(anc, cr)

result = sim.run(qc).result()
counts = result.get_counts(qc)
print(f"Input: 0001, Expected: 1111, Actual: {counts}")
``````

and so on for each test case.

When I try running the code to see expected versus actual outputs, I see that I am getting incorrect results even though I hardcoded the values into the circuit.

Could someone pls look into this? I would really appreciate it!

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