Quantum Random Sweepstakes

Dockerfile

FROM mcr.microsoft.com/quantum/iqsharp-base:0.11.2004.2825
ENV IQSHARP_HOSTING_ENV=sckaiser-azure-community-live

# Make sure the contents of our repo are in ${HOME}.
# These steps are required for use on mybinder.org.
USER root
COPY . ${HOME}
RUN chown -R ${USER} ${HOME}
RUN pip install rise

# Finish by dropping back to the notebook user.
USER ${USER}

operations.qs

// Copyright (c) Sarah Kaiser. All rights reserved.
// Licensed under the MIT License.

namespace Sweepstakes {
    open Microsoft.Quantum.Intrinsic;
    open Microsoft.Quantum.Canon;
    open Microsoft.Quantum.Measurement;
    open Microsoft.Quantum.Arithmetic;
    open Microsoft.Quantum.Math;

    /// # Summary
    /// This operation uses a register of qubits in superposition to select
    /// a random integer in a range {0, numPlayers-1}.
    ///
    /// # Input
    /// ## numPlayers
    /// The number of participants in the random drawing.
    ///
    /// # Output
    /// An `Int` that represents the index of the player that has won the
    /// sweepstakes.
    ///
    /// # Remarks
    /// ## Example
    /// ```Q#
    /// // You need to pick one of 7 viewers of your stream to send them
    /// // a present! 🎁
    /// let winnerIndex = SelectWinner(7);
    /// ```   
    operation SelectWinner(numPlayers : Int) : Int {
        // Calculate number of qubits needed to store the number of players.
        let numQubits = BitSizeI(numPlayers);
        // Setup the variable that will track the result.
        mutable result = 0;

        // This repeat block is needed because unless the number of players is 
        // a power of two, you could get a result that does not index a player.
        // Since there is no fair way to reuse those spare values, we just try
        // again until we get a random index that does represent a player.
        repeat {
            // Prepare a register of qubits
            using (register = Qubit[numQubits]) {
                // Put all of the qubits in superposition with the `H` operation.
                ApplyToEach(H, register);
                // Measure and interpret as an integer.
                set result = MeasureInteger(LittleEndian(register));
            }
        // Make sure the result corresponds to the index of a player.
        } until(result < numPlayers);

        // Return the winning player!
        return result;
    }
}

START-HERE.ipynb