(PhysicsWorld) A “quantum secret sharing” scheme that allows 10 parties to share information securely – the highest number so far – has been developed and demonstrated by researchers at the University of Witwatersrand. The protocol involves each party performing quantum operations on the photon without measuring its state and the team says it could help increase both the rate at which data is shared on secure quantum networks and how many parties can be involved in the sharing.
While commercial QKD systems are available, the protocol has its shortcomings. One is that photon polarization has only two orthogonal states. These are traditionally used to represent 1 and 0, as in a conventional bitstream. Given the technical difficulty of sending and detecting isolated single photons, however, it is highly desirable to pack more information into each photon. In the new research, therefore, Andrew Forbes and colleagues at the University of Witwatersrand in Johannesburg encoded the data not in the polarization of the photon, but in its orbital angular momentum (OAM) –which can, in principle, be infinitely large. “Polarization has only two possibilities, just like a coin which can only be heads or tails,” explains Forbes, “OAM is like an infinite-sided dice.”
Forbes and colleagues have created a “pass-the-parcel” scheme by which photons in a superposition of 11 possible OAM states could be sent sequentially between 10 parties, eventually arriving back at the original sender. Each party performs one or more of a set of possible predefined operations on the OAM of the photon, but none of them measures its state. Only after the photon has completed its circuit does the party that originally emitted the photon (“the distributor”) measure its final state and compare it to the state of the photon originally transmitted.

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