Note: Scalable multiphoton coincidence-counting electronics

We present a multichannel coincidence-counting module for use in quantum optics experiments. The circuit takes up to four transistor-transistor logic pulse inputs and counts either twofold, threefold, or fourfold coincidences, within a user-selected coincidence-time window as short as 12 ns. The module can accurately count eight sets of multichannel coincidences, for input rates of up to 84 MHz. Due to their low cost and small size, multiple modules can easily be combined to count arbitrary M-order coincidences among N inputs.