  Quantum secret sharing (QSS) is a protocol to split a message into several
parts so that no subset of parts is sufficient to read the message, but the
entire set is. In the scheme, three parties Alice, Bob and Charlie first share
a three-photon entangled state, Charlie can then force Alice and Bob to
cooperate to be able to establish the secret key with him by performing proper
polarization measurements on his photon and announcing which polarization basis
he has chosen. In a similar manner, in third-man quantum cryptography (TQC) the
third-man, Charlie, can control whether Alice and Bob can communicate in a
secure way while he has no access whatsoever on the content of the
communication between Alice and Bob. Although QSS and TQC are essential for
advanced quantum communication, the low intensity multi-photon entanglement
source has made their realization an extreme experimental challenge. Here,
exploiting a high intensity four-photon entanglement source we report an
experimental realization of QSS and TQC . In the experiment, a key of low
quantum bit error rate (QBER) 0.35% is obtained using a simple error reduction
scheme.

Quantum Physics