  We present a scalable star-shaped quantum key distribution (QKD) optical
fiber network. We use wavelength-division demultiplexing (WDM) of broadband
photon pairs to establish key exchange between multiple pairs of participants
simultaneously. Our QKD system is the first entanglement-based network of four
participants using BBM92 time-bin coding and the first network achieving timing
synchronization solely by clock recovery based on the photon arrival times. We
demonstrate simultaneous bipartite key exchange between any possible
combination of participants and show that the quantum bit error rate (QBER)
itself can be used to stabilize the phase in the interferometers by small
temperature adjustments. The key distribution is insensitive to polarization
fluctuations in the network, enabling key distribution using deployed fibers
even under challenging environmental conditions. We show that our network can
be readily extended to 34 participants by using a standard arrayed-waveguide
grating for WDM with 100 GHz channel spacing and that reconfigurable network
connections are possible with a wavelength-selective switch. In a field test we
demonstrate secure key rates of 6.3 bit/s with a QBER of 4.5% over a total
fiber length of 108 km with 26.8 km of deployed fiber between two participants
with high stability.
  Our system features a relatively simple design of the receiver modules and
enables scaling QKD networks without a trusted nodes to distances up to more
than 100 km and to more than 100 users. With such a network, a secure
communication infrastructure on a metropolitan scale can be established.
