At 20cm (8in) — the optimum distance is defined by the wavelength of the oscillating power — the quadcopter system can transmit 5.5 watts with an efficiency of 35%. This is more efficient than non-resonant inductive coupling (and works over a larger distance), and as an added bonus, magnetic resonance deals well with occlusions and other atmospheric interference (something that laser power transmission cannot surmount). The one problem with strongly coupled magnetic resonance, though — as you can see in the video below — is that the quadcopter needs to be almost perpendicular with the receiving device for power to be transmitted. According to IEEE Spectrum, the NIMBUS Lab team is currently working on ways of autonomously keeping the UAV exactly 20cm directly above the receiving device.