microsoft/qdk

/// # Sample
/// Cat State
///
/// # Description
/// A cat state is a highly entangled state where the qubits are in a
/// superposition of all |0...0〉 or all |1...1〉.
///
/// This Q# program implements a cat state of 5 qubits.
import Std.Diagnostics.*;

operation Main() : Result[] {
    use register = Qubit[5];

    // Prepare a cat state using the allocated register.
    PrepareCatState(register);

    // Show the cat state.
    DumpMachine();

    // Measure and reset qubits before releasing them.
    MResetEachZ(register)
}

/// # Summary
/// Prepares state (|0...0〉 + |1...1〉) / √2 (a generalized GHZ state
/// or a cat state) across the `register` of qubits.
/// All qubits are assumed to be in |0〉 state on input.
operation PrepareCatState(register : Qubit[]) : Unit {
    Fact(Length(register) > 0, "Qubit register must not be empty.");

    // Set the first qubit in the register into a (|0〉 + |1〉) / √2 superposition.
    // Then apply a CNOT to the remaining qubits using the first qubit as control.
    H(register[0]);
    ApplyToEach(CNOT(register[0], _), register[1...]);
}