"Light passing through natural water systems experiences both absorption and scattering leading to important effects such as heating of the water, growth of plants through photosynthesis and generation of reflectance signals for remote sensing systems. One of the most common measures of the optical properties of a water body is the beam attenuation coefficient which is the sum of absorption and scattering. This is usually measured by recording the intensity of a beam of light after it has passed through a known length of water and comparing the signal with that obtained either in air or, more usually, in ultrapure water. It is usually assumed that any photons either absorbed or scattered do not make it to the detector and so the remaining signal is due entirely to directly transmitted photons. However, in reality, light is scattered in water in such a way that standard transmissometers accidentally collect a large and quite variable amount of forward scattered light. This means that the signal they generate has a large error that is actually a feature of the instrument design, and sensors with different optical layouts will provide substantially different values. It has long been thought that this was an inevitable feature of the measurement and most users simply ignore the problem. Indeed, current NASA measurement protocols for this parameter explicitly leave it to the end user of data to work out how to deal with this problem. This is an intolerable position for which we have recently found a new solution.
We are planning to build a new device to measure beam attenuation that exploits a recently developed understanding of a quantum property of photons called orbital angular momentum, OAM. We can control this quantum state of light and generate a beam of light with a defined OAM state. When such a beam of light experiences a scattering event, the OAM state changes by a defined, quantum amount that we can easily identify. We can use this change of quantum state to effectively label scattered photons and discriminate them from directly transmitted photons. This means we can measure the number of photons that make it across a volume of water without being absorbed or scattered, without being affected by the scattering collection error that causes problems for current instruments. Our device will then be significantly more accurate than what is currently available and will help researchers and other end-users make significantly better and consistent measurements of what is an extremely important optical property of natural water systems."