You might be wondering how a quantum computer simulator emulates the READ or measurement QPU instruction. It does it, relying on using pseudo-random numbers generated on the classical computer according to a certain distribution. For instance, in Python, you can use the random.choices function. The following Python code shows you a basic READ function that can be used in a quantum computing simulator.

# make a measurement of the Qubit System
def read(self):
     self.possible_outcome = np.arange(self.n_states)
     self.prob_qubit = np.square(np.absolute(self.qubit))
     # weighted random number generation
     x = random.choices(self.possible_outcome,weights=self.prob_qubit)
     print("Read Quantum State = ", x)

The Python code for multiple READs (measurements) and plotting capabilities is the following:

def read_multiple(self,n_shots=1000):
        self.possible_outcome = np.arange(self.n_states)
        self.prob_qubit = np.square(np.absolute(self.qubit))
        # initialize vector for measurements
        self.measurement_state = np.zeros((self.n_states),dtype=np.int)
        # make many shots with measurements
        for i in range(n_shots):
             x = random.choices(self.possible_outcome,weights=self.prob_qubit)
             self.measurement_state[x] = self.measurement_state[x] + 1
        #print(self.measurement_state)
        plt.grid(b=True)
        plt.bar(self.possible_outcome,self.measurement_state/n_shots)
        plt.xlabel('Quantum States')
        plt.ylabel('Probability')
        plt.xticks(self.possible_outcome, rotation = '65')
        plt.show()