  Secret sharing allows a trusted party (the dealer) to distribute a secret to
a group of players, who can only access the secret cooperatively. Quantum
secret sharing (QSS) protocols could provide unconditional security based on
fundamental laws in physics. While the general security proof has been
established recently in an entanglement-based QSS protocol, the tolerable
channel loss is unfortunately rather small. Here we propose a continuous
variable QSS protocol using conventional laser sources and homodyne detectors.
In this protocol, a Gaussian-modulated coherent state (GMCS) prepared by one
player passes through the secure stations of the other players sequentially,
and each of the other players injects a locally prepared, independent GMCS into
the circulating optical mode. Finally, the dealer measures both the amplitude
and phase quadratures of the receiving optical mode using double homodyne
detectors. Collectively, the players can use their encoded random numbers to
estimate the measurement results of the dealer and further generate a shared
key. Unlike the existing single photon based sequential QSS protocol, our
scheme is intrinsically immune to Trojan horse attacks. Furthermore, the
additional loss introduced by each player's system can be extremely small,
which makes the protocol scalable to a large number of players. We prove the
unconditional security of the proposed protocol against both eavesdroppers and
dishonest players in the presence of high channel loss, and discuss various
practical issues.
