A measurement-device-independent quantum key distribution (MDI-QKD) network includes a plurality of user nodes connected to untrusted relay node that performs Bell-state measurements on qubits transmitted by the user nodes. The relay node contains a calibration laser that serves as a wavelength reference for the user nodes. The output of the calibration laser is split into two wavelength-calibration signals, which are transmitted to a pair of user nodes via optical fiber. At each user node, a laser diode used to generate weak coherent pulses is injection-locked with the wavelength calibration-signal, thereby ensuring that the user nodes generate photonic qubits with the same wavelength. The embodiments may be implemented with any encoding scheme compatible with MDI-QKD, such as polarization encoding and time-bin phase-encoding. No auxiliary connections between the user nodes are needed, allowing the MDI-QKD network to be scaled up to many users.