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I ran the following code
which adds two 2-digit numbers on different back-ends and the simulator gives the expect result while all real QC's have a problem with the transpiling:
In fact, Melbourne was able to return some results but they were quite far from what expected and what the simulator returned. Has anybody encountered a similar problem? I paste below how the result looks when it fails:
I would add that you use 7 qubits, however, with exception of Melbourne the processors have less qubits (for example Armonk has only 1 qubit, Ourense 5 qubits etc.), so you cannot run the algorithm on them.
Melbourne has 15 qubits, so your algorithm is transpiled without error. Check number of qubits by click on particular processor on main page in IBM Q.
Note, simulator can be used for algorithms up to 32 qubits.
Unexpected results on Melbourne are given by too deep circuit.
So without the OpenQASM code to import this is hard (I do not want to input it by hand) to verify what went wrong. However, in the case of unexpected results, it is important to note that every SWAP gate gets decomposed into 3 CX gates, and every CCX (Toffoli) gate becomes six CX gates and a handful of single-qubit gates. So your circuit is already quite long when decomposed. Add to this the need for additional SWAP gates to match the device topology and one can then see that the actual circuit run will have its output greatly affected by noise.
Without the OpenQASM code to import this is hard to verify what went wrong. However, in the case of unexpected results, it is important to note that every SWAP gate gets decomposed into 3 CX gates, and every CCX gate becomes six CX gates and a handful of single-qubit gates. So your circuit is already quite long when decomposed. Add to this the need for additional SWAP gates to match the device topology and one can then see that the actual circuit run will have its output greatly affected by noise.
