The Coherence Time of Sunlight*

The discovery that quantum coherence might enhance biological processes such as photosynthesis is not only of fundamental importance but also leads to hopes of developing bio-inspired ‘green’ quantum technologies that mimic nature. A key question is how the time scale of coherent processes in molecular systems compare to that of the driving light source — the sun. Across the community, the coherence time quoted for sunlight spans at least two orders of magnitude, ranging from 0.6 to >10 of femtoseconds. This difference can potentially be significant in deciding whether the induced light-matter coherence is long enough to affect macro system behaviour. Here we revisit the historic calculations of sunlight coherence starting with the blackbody spectrum and then proceed to provide values for the more realistic case of atmospherically filtered light. We corroborate these values with interferometric measurements of atmospherically filtered sunlight in the visible spectrum, the wavelength range most relevant for photosynthetic organisms on Earth. This is the first step in our development of a novel sunlight emulator utilizing a train of ultrafast laser pulses.