Scientists gathering at a meeting of the American Physical Society in Dallas, Texas are exploring a quantum physics theory of explaining how smell works. Current theories of human smell hold that molecules have certain shapes. When they enter our noses, they fit like a "key in a lock" with certain receptors in our sinus cavities.
However, new research from Dr. Luca Turin of MIT has suggested instead that the "vibrational" state of the molecule might have a bearing on smell. In experiments, flies were able to distinguish between similar molecules that had differing frequencies.
The BBC article above is very general, but if you'd like to read more specifically how quantum physics might intersect with smell, you can check out this paper in the Journal of Applied Physics by Dr. Andrew Horsfield of Imperial College London. Access will require a subscription.