  Twin-Field (TF) quantum key distribution (QKD) represents a novel QKD
approach whose principal merit is to beat the point-to-point private capacity
of a lossy quantum channel, thanks to performing single-photon interference in
an untrusted node. Indeed, recent security proofs of various TF-QKD type
protocols have confirmed that the secret key rate of these schemes scales
essentially as the square root of the transmittance of the channel. Here, we
focus on the TF-QKD protocol introduced by Curty et al, whose secret key rate
is nearly an order of magnitude higher than previous solutions. Its security
relies on the estimation of the detection probabilities associated to various
photon-number states through the decoy-state method. We derive analytical
bounds on these quantities assuming that each party uses either two, three or
four decoy intensity settings, and we investigate the protocol's performance in
this scenario. Our simulations show that two decoy intensity settings are
enough to beat the point-to-point private capacity of the channel, and that the
use of four decoys is already basically optimal, in the sense that it almost
reproduces the ideal scenario of infinite decoys. We also observe that the
protocol seems to be quite robust against intensity fluctuations of the optical
pulses prepared by the parties.
