How do I check what is wrong in my full-adder code?

Question

I am trying to solve the first question on the qiskit test which is writing a code for a full adder.

So based on my research if I have $A$ q[0], $B$ q[1] and $C$ in q[2] as input and Sum and Cout as output, I should be able to produce the correct outputs by the following gates:

q[0] XOR1 q[1] ---> q[4]
q[0] AND1 q[1] ---> q[3]
q[2] XOR2 q[4] ---> q[5] (SUM)
q[2] AND2 q[4] ---> q[6]
q[3] OR q[6] ---> q[7] (COUT)

Writing the following program I get that my answer is producing wrong results :

from qiskit import QuantumRegister, ClassicalRegister, QuantumCircuit
from qiskit import IBMQ, Aer, execute
build your quantum circuit here
#Define registers and a quantum circuit
q = QuantumRegister(8) 
c = ClassicalRegister(2)
qc = QuantumCircuit(q,c)
Preparing inputs
qc.x(q[0])  # Comment this line to make Qbit0 = |0>
qc.x(q[1])  # Comment this line to make Qbit1 = |0>
qc.x(q[2])  # Comment this line to make Qbit2 = |0> ( carry-in bit )
qc.barrier()
AND gate1 implementation
qc.ccx(q[0],q[1],q[3])
qc.barrier()
XOR gate1 implementation
qc.cx(q[0],q[4])
qc.cx(q[1],q[4])
qc.barrier()
XOR gate2 implementation
qc.cx(q[2],q[5])
qc.cx(q[4],q[5])
qc.barrier()
AND gate2 implementation
qc.ccx(q[2],q[4],q[6])
qc.barrier()
#OR gate implementation
qc.cx(q[3],q[7])
qc.cx(q[6],q[7])
qc.ccx(q[3],q[6],q[7])
qc.barrier()
Measuring and put result to classical bit
( sum )
qc.measure(q[5],c[0])
( carry-out )
qc.measure(q[7],c[1])
execute the circuit by qasm_simulator
backend = Aer.get_backend('qasm_simulator')
job = execute(qc, backend, shots=1000)
result = job.result()
count = result.get_counts()
print(count)
qc.draw(output='mpl')

Grading tells me that my results are not matching, but I cannot figure out what is wrong with my code. Thank you so much for help.

score 2 · Accepted Answer · edited Nov 19 '20 at 09:50

If I am correct, I suppose you are talking about the Qiskit Challenge 2020. A possible reason why your circuit is being graded wrong is because the question asks you to construct the circuit for full adder and give it the input $A=1$, $B=0$ and $X=1$. However, I think as per your code, you are preparing the qubits to be $|ABX\rangle = |111\rangle$ instead of $|101\rangle$. Baring that, your circuit works perfectly fine from what I could analyze.

How do I check what is wrong in my full-adder code?

build your quantum circuit here

Preparing inputs

AND gate1 implementation

XOR gate1 implementation

XOR gate2 implementation

AND gate2 implementation

Measuring and put result to classical bit

( sum )

( carry-out )

execute the circuit by qasm_simulator

1 Answers1