  Measurement-device-independent quantum key distribution (MDI-QKD) removes all
detector side channels and enables secure QKD with an untrusted relay. It is
suitable for building a star-type quantum access network, where the complicated
and expensive measurement devices are placed in the central untrusted relay and
each user requires only a low-cost transmitter, such as an integrated photonic
chip. Here, we experimentally demonstrate a 1.25 GHz silicon photonic
chip-based MDI-QKD system using polarization encoding. The photonic chip
transmitters integrate the necessary encoding components for a standard QKD
source. We implement random modulations of polarization states and decoy
intensities, and demonstrate a finite-key secret rate of 31 bps over 36 dB
channel loss (or 180 km standard fiber). This key rate is higher than
state-of-the-art MDI-QKD experiments. The results show that silicon photonic
chip-based MDI-QKD, benefiting from miniaturization, low-cost manufacture and
compatibility with CMOS microelectronics, is a promising solution for future
quantum secure networks.
