A molecular protein receptor or a complex of receptors with very specific 3D-shapes can bind to molecules floating in the air, as  in an enzyme lock-and-key fashion and result in neural signals being sent to the brain, which are noted and recorded as a distinct smell (say rotten eggs).

   Recent studies by researchers at Rockerfeller U. suggest that humans can distinguish more the 1 trillion scents. Odors almost always represent mixtures of many different components in various ratios. The scent of a rose, for example, is made up of a mix of 275 components, with only a few contributing to the smell we perceive.

   Quantum molecular affects also suggest that the vibration state of a molecule bound to the olfactory receptors may be an added level of discrimination. In quantum terms a "tunneling" electron tuned to a specific virbrational frequency of the odor molecule may aid in delivering the smell signal to the receptor.

   In an experiment where substituting deuterium [the isotope of hydrogen with a nucleus having 1 proton and one neutron - hydrogen has no neutron] altered the vibrational state of an odor molecule, but not its chemistry, and resulted in a change in smell sensation of the modified odor molecule.