The id201, released by id Quantique SA, is a single-photon detector with preset detection-probability values of 10%, 15%, 20% or 25%. Calibration is performed at 1550 nm, and the bias voltage can be adjusted to control detection efficiency. The device, based on an InGaAs/InP avalanche photodiode, covers the spectrum from 900 to 1700 nm and works in gated mode. Users can choose among gate widths of 2.5, 5, 20, 50 and 100 ns. Maximum trigger frequency is 10 MHz. The instrument operates from 10 to 30 °C, measures 300 × 250 × 150 mm, runs on a 110- to 230-VAC supply and has an RS-232 interface that controls parameters and performs data acquisition. It is available with single- or multimode fiber optical input.