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.