codeforces#P1357B2. "Is the number divisible by 3?" oracle

"Is the number divisible by 3?" oracle

Description

Implement a quantum oracle on $N$ qubits which checks whether the input bit string is a little-endian notation of a number that is divisible by 3.

Your operation should take the following inputs:

  • an array of $N \le 8$ qubits "inputs" in an arbitrary state.
  • a qubit "output" in an arbitrary state.

Your operation should perform a unitary transformation on those qubits that can be described by its effect on the basis states: if "inputs" is in the basis state $|x\rangle$ and "output" is in the basis state $|y\rangle$, the result of applying the operation should be $|x\rangle|y \oplus f(x)\rangle$, where $f(x) = 1$ if the integer represented by the bit string $x$ is divisible by $3$, and $0$ otherwise.

For example, if the qubits passed to your operation are in the state $\frac{1}{\sqrt{2}}(|110\rangle + |001\rangle)_x \otimes |0\rangle_y = \frac{1}{\sqrt{2}}(|3\rangle + |4\rangle)_x \otimes |0\rangle_y$, the state of the system after applying the operation should be $\frac{1}{\sqrt{2}}(|3\rangle_x\otimes |1\rangle_y + |4\rangle_x |0\rangle_y) = \frac{1}{\sqrt{2}}(|110\rangle_x\otimes |1\rangle_y + |001\rangle_x |0\rangle_y)$.

Your code should have the following signature (note that your operation should have Adjoint and Controlled variants defined for it; is Adj+Ctl in the operation signature will generate them automatically based on your code):

namespace Solution {
    open Microsoft.Quantum.Intrinsic;
operation Solve (inputs : Qubit[], output : Qubit) : Unit is Adj+Ctl {
    // your code here
}

}

Your code is not allowed to use measurements or arbitrary rotation gates. This operation can be implemented using just the X gate and its controlled variants (possibly with multiple qubits as controls).

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