Experimental Quantum Secret Sharing and Third-Man Quantum Cryptography

Quantum secret sharing (QSS) and third-man quantum cryptography (TQC) are essential for advanced quantum communication; however, the low intensity and fragility of the multiphoton entanglement source in previous experiments have made their realization an extreme experimental challenge. Here, we develop and exploit an ultrastable high intensity source of four-photon entanglement to report an experimental realization of QSS and TQC. The technology developed in our experiment will be important for future multiparty quantum communication.

Figure 1

Schematic drawing of the QSS (a) and TQC (b) schemes.

Figure 2

Experimental setup for QSS and TQC. (a) The UV pulse generated by up-conversion inside the LBO crystal passes through the BBO crystal twice to generate two pair of polarization-entangled photons to prepare the desired three-photon GHZ state. The UV laser with a central wavelength of 394 nm has a pulse duration of 200 fs, a repetition rate of 76 MHz, and an average pump power of 430 mw. By optimizing the collection efficiency, we are able to observe about 2 fourfold coincidence per second behind 3.6 nm filters (F) of central wavelength 788 nm. (b) Each of Alice, Bob, and Charlie randomly performs a projection measurement on her/his own photon. The BS is used to select basis randomly to measure the photons. The HWP ($\lambda /2$) and QWP ($\lambda /4$) are used to perform $x_{+}/x_{-}$ and $y_{+}/y_{-}$ measurements. If and only if there is one click registered at each of the receivers and trigger, they individually record the measurement result.

Figure 3

Sharing a secret image. Charlie encrypts the image of the “Great Wall” via bitwise xor operation with his key and transmits the encrypted image to Alice and Bob via the computer network. Neither Alice nor Bob can decrypt the image with her or his own key, but they can cooperate to decrypt the image. The final image cooperatively obtained by Alice and Bob shows only a few errors due to the remaining bit errors in the keys (0.35%).