 The qubit is the quantum analogue of the bit, the classical fundamental unit of information. It is a mathematical object with specific properties that can be realised in an actual physical system in many different ways. Just as the classical bit has a state (either 0 or 1), a qubit also has a state. Yet contrary to the classical bit, \(\lvert 0\rangle\) and \(\lvert 1\rangle\) are but two possible states of the qubit, and any linear combination (superposition) thereof is also physically possible. In general, thus, the physical state of a qubit is the superposition \(\lvert\psi \rangle = \alpha \lvert 0\rangle + \beta \lvert 1\rangle\) (where \(\alpha\) and \(\beta\) are complex numbers). The state of a qubit can be described as a vector in a two-dimensional Hilbert space, a complex vector space (see the entry on  quantum mechanics).  The special states \(\lvert 0\rangle\) and \(\lvert 1\rangle\) are known as the computational basis states, and form an orthonormal basis for this vector space. According to quantum theory, when we try to measure the qubit in this basis in order to determine its state, we get either \(\lvert 0\rangle\) with probability \(\lvert \alpha\rvert^2\) or \(\lvert 1\rangle\) with probability \(\lvert \beta\rvert^2\). Since \(\lvert \alpha\rvert^2 + \lvert\beta\rvert^2 = 1\) (i.e., the qubit is a unit vector in the aforementioned two-dimensional Hilbert space), we may (ignoring the overall phase factor) effectively write its state as \(\lvert \psi \rangle =\) cos\((\theta)\lvert 0\rangle + e^{i\phi}\)sin\((\theta)\lvert 1\rangle\), where the numbers \(\theta\) and \(\phi\) define a point on the unit three-dimensional sphere, as shown here. This sphere is often called the Bloch sphere, and it provides a useful means to visualise the state of a single qubit.