Exercise - Create a quantum random bit generator

  • 8 minutes

You build out your quantum random number generator in two phases. In this unit, you build out the first phase, which is to generate a single random bit.

Note

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Define a random bit generator

You start by allocating qubit. When you allocate a qubit, it's in the $\ket{0}$ state, which isn't very useful for generating random numbers. You need to put the qubit into superposition. You can do this by applying the Hadamard gate, H, to the qubit. The Hadamard gate puts the qubit into an equal superposition of $\ket{0}$ and $\ket{1}$.

When you measure the qubit, you'll get a random bit, either 0 or 1, with equal 50% probability. The value of this bit is truly random, there's no way of knowing what you get after the measurement.

Create the Q# program

  1. Open Visual Studio Code and select File > New Text File to create a new file.

  2. Save the file as RandomNumberGenerator.qs. This file will contain the Q# code for your program.

  3. Copy the following code into the RandomNumberGenerator.qs file.

    namespace QuantumRandomNumberGenerator {

    @EntryPoint()
    operation GenerateRandomBit() : Result {
        // Allocate a qubit.
        use q = Qubit();

        // Set the qubit into superposition of 0 and 1 using the Hadamard 
        H(q);

        // At this point the qubit `q` has 50% chance of being measured in the
        // |0〉 state and 50% chance of being measured in the |1〉 state.
        // Measure the qubit value using the `M` operation, and store the
        // measurement value in the `result` variable.
        let result = M(q);

        // Reset qubit to the |0〉 state.
        // Qubits must be in the |0〉 state by the time they are released.
        Reset(q);

        // Return the result of the measurement.
        return result;
    }
}

Let's take a moment to review the new code.

  • First, you allocate one qubit with the Qubit() operation and the use keyword.
  • Then, the Hadamard operation puts the qubit into superposition, H(q).
  • Then, you measure the qubit to get a random bit, M(q).
  • Finally, Reset(q) resets the qubit to the zero state. Qubits must be in the $\ket{0}$ state by the time they are released.

Run the program locally

To run your program locally on the built-in simulator, Run from the list of commands below @EntryPoint(), or press Ctrl+F5. Your output appears in the debug console in the terminal.

The result is truly random. You can run the program again to see a different result.

In the next unit, you'll implement the second phase of your quantum random number generator: combining multiple random bits to form a larger number.