Endianness of Sparse Simulator DumpMachine is Stateful

[sam-jaques]

Describe the bug

When dumping states from the Sparsesimulator, it converts qubit labels as binary into integers. Which bit is considered the most or least significant changes throughout execution.

To Reproduce

@Test("SparseSimulator")
    operation dump() : Unit {
        for idLoop in 0..1 {
            use qubits = Qubit[4] {
                X(qubits[0]);
                X(qubits[1]);
                X(qubits[3]);
                for idx in 0..3 {
                    Message($"Qubit {idx}: {M(qubits[idx])}");
                }
                DumpMachine();
                ResetAll(qubits);
            }
        }
    }

Expected behavior

DumpMachine() should output |11> in both iterations.

Actual behavior

DumpMachine() outputs |11> in the first iteration but |13> in the second. In fact, repeating more than twice, every odd iteration outputs |11> and every even iteration outputs |13>.

System information

• OS: Ubuntu 20.04

• .NET Core Version: 6.0

• IQ# Version 0.28.302812

Additional context Using DumpRegister((), qubits) gives the expected behaviour. Thinking about this carefully, Q# doesn't seem to have a global notion of an ordering of the qubits, which would be necessary to expect/enforce an endianness to a machine state dump. So in some sense this is the expected behaviour, since we shouldn't expect an ordering; however, it feels very unexpected at first glance!

While DumpRegister will work for my use case, I can imagine other scenarios where a function does not have access to all qubits currently in the machine but still wants to run diagnostics, and still wants an ordering enforced on the qubits it can see. A clumsy solution might be to allow an array of qubits to be passed as on argument to DumpMachine, and it would put those qubits in that order as the first qubits of the output.