Visualizing the state vectors of multiple qubits
So far, we’ve learned the various ways to represent a qubit, both as a vector 
and visually on a Bloch sphere. We did something similar with the qsphere. In this section, we will learn how to represent multiple qubits and how to represent them in their general state. We will start by making a slight update to the notation. A single qubit is presented as the following vector:
We can therefore represent two qubits similarly, in the following form:
From the preceding equation, you can see that the state is used to represent multiple qubits, versus for single qubits. The difference is case sensitive: lowercase for single qubits and uppercase for multiple qubits. The probability amplitudes, along with the constraint by the normalization of 1, can therefore be represented as follows:
Let’s look at an example that comprises two qubits, the first one in the state , as follows:
The other qubit...
