Graph coloring - Q# -

Question

I am checking Q# - Graph coloring

                for (C in 0 .. (1 <<< N) - 1) {
                InitializeColor(C, register);

Can you help me explain this part of the code?

Thnx

        for (N in 1 .. 4) {
            using (register = Qubit[N]) {
                for (C in 0 .. (1 <<< N) - 1) {
                    InitializeColor(C, register);
                    let measurementResults = MultiM(register);
                    Fact(ResultArrayAsInt(measurementResults) == C, 
                        $"Unexpected initialization result for N = {N}, C = {C} : {measurementResults}");
                    ResetAll(register);
                }
            }
        }
    }

Answer 1

This is part of the Graph Coloring Kata. The InitializeColor is an operation that you must implement. The code above is part of the checker code to confirm the working of the implementation.

The code is used to check if the InitializeColor operation is implemented correctly. It is supposed to take an input C denoting the color as an non-negative integer. It is supposed to apply a unitary to convert the state $|000...0\rangle$ to state denoted by LittleEndian encoding of $|C\rangle$.

The InitializeColor operation is supposed to take a register which is in the state $|0..0\rangle$ and applies a Unitary which transforms it into a register in the state $|C\rangle$. The nextline measures the register using the MultiM operation. The Fact function essentially confirms if the measured result in the state $|C\rangle$ and if not it print the error statement. The last line resets the register and brings it back to the state $|000..0\rangle$.

An example can be with $C=2$ and $N=4$. Then InitializeColor should apply a Unitary Operation which transform $|0000\rangle$ into $|0100\rangle$ assuming LittleEndian encoding or $|0010\rangle$ assuming BigEndian encoding.