Experimental Single Qubit Quantum Secret Sharing

We present a simple and practical protocol for the solution of a secure multiparty communication task, the secret sharing, and its proof-of-principle experimental realization. In this protocol, a secret is split among several parties in a way that its reconstruction requires the collaboration of the participating parties. In our scheme the parties solve the problem by sequential transformations on a single qubit. In contrast with recently proposed schemes involving multiparticle Greenberger-Horne-Zeilinger states, the approach demonstrated here is much easier to realize and scalable in practical applications.

Figure 1

Scheme for $N$ party single qubit secret sharing. A qubit is prepared in an initial state and sequentially communicated from party to party, each acting on it with a phase operator $\widehat{U_j}({\phi }_j)$, applying a randomly chosen phase ${\phi }_j$. The last recipient performs a measurement on the qubit leading to the result $\pm 1$. In half of the cases the phases add up such that the measurement result is deterministic. These instances can be used to achieve the aim of secret sharing.

Figure 2

Setup for single qubit secret sharing. Pairs of orthogonally polarized photons are generated via a type II SPDC process in a BBO crystal. The detection of one photon from the pair by $D_T$ heralds the existence of the other one used in the protocol. The initial polarization state is prepared by placing a polarizer in front of the trigger detector. Each of the recipients ($R_1\dots R_6$) introduces one out of four phase shifts, according to the output of a pseudorandom number generator (RNG), using half- and quarter-wave plates (${\mathrm{HWP}}_1$, QWP) or ${\mathrm{YVO}}_4$ crystals ($C_1\dots C_5$), respectively. The last party analyzes additionally the final polarization state of the photon by detecting it behind a half-wave plate (${\mathrm{HWP}}_2$) and a polarizing beam splitter (PBS).