Quantum key distribution (QKD) is a key application in quantum communication,
enabling secure key exchange between parties using quantum states. Twin-field
(TF) QKD offers a promising solution that surpasses the repeaterless limits,
and its measurement-device-independent nature makes it suitable for star-type
network architectures. In this work, we propose and demonstrate a fully
connected TF-QKD network architecture, where users prepare quantum signals and
send them to network nodes. These nodes use splitters to route the signals to
measurement units, enabling secure key distribution among arbitrary users. A
proof-of-principle demonstration with three users successfully achieved secure
key sharing over simulated link losses of up to 30 dB, with an average rate of
$5.01 \times 10^{-7}$ bit per pulse. Additionally, simulations show that the
architecture can support up to $32$ users with a secure key rate of 280.90
bits/s under typical fiber loss conditions. This approach represents a
significant advancement in the topology of untrusted-node QKD networks and
holds promise for practical, large-scale applications in secure communication.

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