Fourier limit

Light from a single molecule or atom never has one perfectly sharp colour. How sharp the colour can be has a hard lower limit: the Fourier limit. It cannot get better than that, according to the laws of physics.

How does it work?

The limit depends on time. A molecule absorbs energy and after a short time gives it off again as light. The longer it holds on to that energy, the purer the colour of the emitted light can be.

Compare it to a tuning fork. A tuning fork that keeps vibrating for a long time gives a very pure tone. A tuning fork that you damp immediately sounds duller and less sharp. The Fourier limit is the purest colour that goes with the time of vibrating.

In a noisy environment, a molecule usually does not reach that limit. Vibrations and electrical noise nearby make the light less pure. A molecule that does reach the Fourier limit is therefore in an unusually quiet environment.

Why it matters

For quantum technology, such pure light sources are valuable: they are needed for future quantum computers and ultra-sensitive sensors. Until now this mostly worked with molecules deep inside a crystal, well shielded. Researchers at the Max Planck Institute in Erlangen got individual molecules on the surface of a crystal to reach the Fourier limit. On a surface you can look at them from very close by and steer them with a microscope needle. That points to quantum components that you can build molecule by molecule.