In the Classical Theory of Information, the basic unit of information is the bit (from “binary digit”). It represents the state vector of a physical system and takes exclusively one of two values, commonly 0 and 1. In computers’ hard drives (HD), the referred physical system is a sub-micrometer ferromagnetic region whose dipole may point up (bit 0) or down (bit 1).
As computer components get smaller in size, quantum effects may play an important role on the way one codes and decodes information. This is due to the intrinsic quantum nature of physical systems. Thus, in the Quantum Theory of Information, the quantum bits, or qubits, arise as the description of state vector in a two-dimensional Hilbert space: $$ |\psi\rangle = \alpha |0\rangle + \beta |1\rangle \ . $$ The equation above could represent, for instance, the superposition quantum state of a single magnetic dipole on a computer that would operate on the molecular level (sub-nanometer dimensions).
In Optics, the polarization state of a photon can also be interpreted as a qubit: it suffices, for instance, to associate the states “0” and “1” to vertical and horizontal polarizations. A qubit coded on a photon is also called a photonic qubit. Such a system may be utilized in the realization of quantum logical gates and implementation of communication protocols, amongst other important tasks, and has the advantage of being easily produced and controlled on an optics laboratory.