  The intense research activity on Twin-Field (TF) quantum key distribution
(QKD) is motivated by the fact that two users can establish a secret key by
relying on single-photon interference in an untrusted node. Thanks to this
feature, variants of the protocol have been proven to beat the point-to-point
private capacity of a lossy quantum channel. Here we generalize the main idea
of the TF-QKD protocol introduced by Curty et al. to the multipartite scenario,
by devising a conference key agreement (CKA) where the users simultaneously
distill a secret conference key through single-photon interference. The new CKA
is better suited to high-loss scenarios than previous multipartite QKD schemes
and it employs for the first time a W-class state as its entanglement resource.
We prove the protocol's security in the finite-key regime and under general
attacks. We also compare its performance with the iterative use of bipartite
QKD protocols and show that our truly multipartite scheme can be advantageous,
depending on the loss and on the state preparation.
