A receiver in a quantum key distribution system includes a quantum optical unit configured to receive quantum signals from a transmitter of the QKD system, modulate the quantum signals based on a basis sequence, and output detection information by detecting the modulated quantum signals. It does this by moving the participant in the public situation to a quiet mode of communication. The first plenum is to structurally support and supply air for a removable module, and the second plenum is to structurally support and exhaust air from the removable module.