According to new research led by David Kielpinski and Erik Streed of Griffith University, Brisbane, the answer is one. Granted, the researchers used ytterbium, a rather large atom with an atomic weight of 174. Still, this remarkable result has far-reaching implications for many fields, including biology.
More specifically, the scientists measured the absorption of photons by single ytterbium ions. They did this by first trapping the individual ions in a supercooled vacuum and then bombarding them with light of precisely 369.5 nm in wavelength. Beyond making a nice shadow, this allowed the physicists to calculate the maximum signal extraction per photon, a useful bit of information to say the least.
As Streed explains:
Because we are able to predict how dark a single atom should be, as in how much light it should absorb in forming a shadow, we can measure if the microscope is achieving the maximum contrast allowed by physics. This is important if you want to look at very small and fragile biological samples such as DNA strands where exposure to too much UV light or x-rays will harm the material. We can now predict how much light is needed to observe processes within cells, under optimum microscopy conditions, without crossing the threshold and destroying them.No news yet on how many atoms it takes to make shadow puppets.