Transmitter for Quantum Key Distribution, QKD, the transmitter comprising a signal generation unit configured to generate a QKD signal and to output it onto a transmission line , and an input configured to receive an optical signal having a different wavelength than the QKD signal , the optical signal being output onto the transmission line , wherein between the input and the transmission line the optical signal is passed through a part of the signal generation unit , which is transparent to the optical signal , wherein at least one of: the signal generation unit includes a wavelength-selective attenuator configured to attenuate the QKD signal , and the optical signal is passed through the attenuator , which is transparent to the optical signal ; and the signal generation unit includes an optical modulation unit configured to modulate a laser beam to generate the QKD signal , and the optical signal is passed through the optical modulation unit , which is transparent to the optical signal , wherein the optical modulation unit includes a wavelength-selective interferometer configured to pass the laser beam through both of its arms but the optical signal only through one of its arms, and/or wherein the optical modulation unit includes a phase modulator and a wavelength-selective coupler , and the wavelength-selective coupler is configured to direct a first part of the laser beam to the phase modulator and to pass a second part of the laser beam and the optical signal by the phase modulator . a signal generation unit configured to generate a QKD signal and to output it onto a transmission line , and an input configured to receive an optical signal having a different wavelength than the QKD signal , the optical signal being output onto the transmission line , wherein between the input and the transmission line the optical signal is passed through a part of the signal generation unit , which is transparent to the optical signal , wherein at least one of: the signal generation unit includes a wavelength-selective attenuator configured to attenuate the QKD signal , and the optical signal is passed through the attenuator , which is transparent to the optical signal ; and the signal generation unit includes an optical modulation unit configured to modulate a laser beam to generate the QKD signal , and the optical signal is passed through the optical modulation unit , which is transparent to the optical signal , wherein the optical modulation unit includes a wavelength-selective interferometer configured to pass the laser beam through both of its arms but the optical signal only through one of its arms, and/or wherein the optical modulation unit includes a phase modulator and a wavelength-selective coupler , and the wavelength-selective coupler is configured to direct a first part of the laser beam to the phase modulator and to pass a second part of the laser beam and the optical signal by the phase modulator . the signal generation unit includes a wavelength-selective attenuator configured to attenuate the QKD signal , and the optical signal is passed through the attenuator , which is transparent to the optical signal ; and the signal generation unit includes an optical modulation unit configured to modulate a laser beam to generate the QKD signal , and the optical signal is passed through the optical modulation unit , which is transparent to the optical signal , wherein the optical modulation unit includes a wavelength-selective interferometer configured to pass the laser beam through both of its arms but the optical signal only through one of its arms, and/or wherein the optical modulation unit includes a phase modulator and a wavelength-selective coupler , and the wavelength-selective coupler is configured to direct a first part of the laser beam to the phase modulator and to pass a second part of the laser beam and the optical signal by the phase modulator . <EOS>