In a quantum key distribution system, a method of autocalibrating the gating of a single-photon detector unit is disclosed. The uncoupling element includes a bellows made of metal and a ring-shaped damping element made of a lightweight composite. All of the edges of the foil sheet are then folded inwardly and the filler insert is finally rolled and secured to produce a cylindrical packet having a dense core. The pressing roller is mounted to the acting member.