A QUBIT CAN BE PHYSICALLY IMPLEMENTED IN SEVERAL DIFFERENT WAYS BUT AS LONG AS
If we are only interested in programming a quantum computer, then the actual physical implementation of a qubit on that computer is not of particular interest to us. As Alexandre Zagoskin says in his book Quantum Mechanics: A Complete Introduction: "In principle, any quantum system with a two-dimensional Hilbert space can be a qubit". A qubit can be physically implemented in several different ways, but as long as it has only two possible states, then it can be used in a quantum computer.
In fact, bearing this in mind, we have actually been using qubits throughout this book — without realising it. The example of the particle which can have only one of two colours — red or blue — forms a two-state system which represents a qubit.
One real-world example of a qubit arises from the spin of a particle. In the classical world, if we measure a spinning ball, we might expect to find its axis of rotation pointing in any random direction. However, if we measure the spin of an electron (by noting its deflection as it passes through a magnetic field), we find that only two distinct outcomes are possible: spin-up, or spin-down.