  Here I show that a classical or quantum bit state plus one simple operation,
an action, are sufficient ingredients to derive a quantum dynamical equation
that rules the sequential changes of the state. Then, by assuming that a freely
moving massive particle is the qubit carrier, it is found that both, the
particle position in physical space and the qubit state, change in time
according to the Pauli-Schroedinger equation. So, this approach suggests the
following conjecture: because it carries one qubit of information the particle
motion has its description enslaved by the very existence of the internal
degree of freedom. It is compelled to be no more described classically but by a
wavefunction. I also briefly discuss the Dirac equation in terms of qubits.
