Error in MultiQubitDumpMachineDemo Code

[bdg221]

Describe the bug

In the tutorials, The Qubit > Display the Quantum State of a Multi-Qubit Program includes the demo code DumpMachine for Multi-Qubit Systems. The comments about performing the X gate on the second qubit are incorrect:

First of all, the entire system is now in state |10> and also it is decimal notation that represents |2>. Finally, the Message() should also be changed to |10>.

This would be extremely confusing to someone who is new to quantum computing, especially if they run the code and see:

To Reproduce

Steps to reproduce the behavior:

1. Go to the Learn with Azure Quantum katas and navigate to The Qubit then Display the Quantum State of a Multi-Qubit Program.
2. Copy the demo code and run it.
3. See confusing/conflicting results.

Expected behavior

I would expect the comment in the code to read:
The entire system is now in state |10>, or, in decimal notation, |2>.

Then the Message("State |01>:"); would be changed to Message(State |10>:");

Screenshots
Current implementation:

Results from running the demo:

System information

N/A

Additional context

I would be happy to correct this issue if you would like to assign it to me.

[tcNickolas]

This is an artifact of migration from the "old" QDK used in https://github.com/microsoft/QuantumKatas/ to the new QDK we're using on the website (see https://devblogs.microsoft.com/qsharp/introducing-the-azure-quantum-development-kit-preview/ for the announcement). Old QDK had a different output format for DumpMachine, so the same code would output

(You can see that basis states are by default collapsed into integers, using little-endian notation, so the state 2 corresponded to a 01 bit string indeed - that's why the comment mentioned little endian. The new DumpMachine also uses a different order of bits, qubit 0 is printed last, so the bit string for this state would be indeed 10, but still 2 in big-endian.)

More broadly, the tutorials migrated to the website need to be double-checked for the discrepancies between the descriptions and the code output that arise from endianness - for example, the same issue is likely to show up in the MultiQubitSystems tutorial.

Thank you for reporting this!

[bdg221]

This same behavior can be seen in the MultiQubitSystemsDemo code from Multi-Qubit Systems > Multi-Qubit Systems in Q#.

[DmitryVasilevsky]

With changes #1079 (most important!), and #1162, #1169 this issue should be fixed in the repository. This bug should be fixed once these changes propagate to QCOM web site.

[minestarks]

@DmitryVasilevsky can this be closed now? I wasn't sure where to check on QCOM

[tcNickolas]

This shows up in Multi-Qubit Systems kata, MultiQubitSystemsDemo demo. I've checked that the output matches the statements in the message on QCOM, so we can close this.

(And thank you @bdg221 for the original report!)