They made a detector that can convert one photon into more than one electron (about 1.3 electrons per photon on average). Note that this is not energy efficiency as in the 2nd Law of Thermodynamics; they haven't created a system that generates more energy out than it receives. The electrons have lower energy than the original photons. This "efficiency" is purely about the number of particles involved.
More electrons per photon means more accurate detection of low amounts of light and also lower power requirements for light sensors in general. That would include, for example, the cells that make up a digital camera's sensor grid. Economics permitting, this could lead to better low-light performance or more efficient optical equipment (e.g. fiber optic transceivers).