  The technologies of heating, photovoltaics, water photocatalysis and
artificial photosynthesis depend on the absorption of light and novel
approaches such as coherent absorption from a standing wave promise total
dissipation of energy. Extending the control of absorption down to very low
light levels and eventually to the single photon regime is of great interest
yet remains largely unexplored. Here we demonstrate the coherent absorption of
single photons in a deeply sub-wavelength 50% absorber. We show that while
absorption of photons from a travelling wave is probabilistic, standing wave
absorption can be observed deterministically, with nearly unitary probability
of coupling a photon into a mode of the material, e.g. a localised plasmon when
this is a metamaterial excited at the plasmon resonance. These results bring a
better understanding of the coherent absorption process, which is of central
importance for light harvesting, detection, sensing and photonic data
processing applications.
