Avalanche Photodiodes (APDs) are a semiconductor technology that converts a single photon into a single electron, which starts an amplification ‘avalanche’ creating thousands more electrons, and a measurable electrical current. To detect the smallest avalanches, and allow superfast operation, our research has led to the Self-Differencing APD, which compares electrical signals with, and without a photon, isolating the output due to light detection. This allows us to detect up to 1 billion individual photons per second.