Absolutely and asymptotically secure protocols for organizing an exam in a quantum way are proposed basing judiciously on multipartite entanglement. The protocols are shown to stand against common types of eavesdropping attack.
The quantum secure direct communication is a new-style quantum communication scheme. It arouses much interest recently. The ping-pong protocol proposed by Bostroem and Felbinger is one of relatively mature quantum secure direct communication protocols, which is asymptotically secure under Eve's eavesdropping attack. But it ignores the message authentication. Being aimed at this problem, we propose a novel ping-pong protocol with authentication. The former has two operation modes, or the message mode and the control mode, which are used to send message and detect Eve's attack, respectively. But it just detects Eve's eavesdropping attack and does not authenticate the sent message in the control mode. The latter has also the above two operation modes, which employs a braid-based key agreement protocol for both the parties participating in communication to share a secret, whose size and magnitude are undecided in advance. Using the shared secret, it not only detects Eve's eavesdropping attack, but also effectively performs the message authentication in the control mode. It is also asymptotically secure under Eve's eavesdropping attack. The security of the proposed protocol is based on the no-cloning theorem, the correlations of quantum entanglement and the fact that the p-th root finding problem on braid groups is intractable even on quantum computers, as well as classical computers. In addition different from the original ping-pong protocol, the communication was initiated by the sender Alice instead of by the receiver Bob. This corresponds to the way of common communications. The present protocol not only can resist the attacks known recently, but should be experimentally feasible using today's technology.
A continuous-variable measurement-device-independent (CV-MDI) multipartite quantum communication protocol is designed to realize multipartite communication based on the GHZ state analysis using Gaussian coherent states. It can remove detector side attack as the multi-mode measurement is blindly done in a suitable Black Box. The entanglement-based CV-MDI multipartite communication scheme and the equivalent prepare-and-measurement scheme are proposed to analyze the security and guide experiment, respectively. The general eavesdropping and coherent attack are considered for the security analysis. Subsequently, all the attacks are ascribed to coherent attack against imperfect links. The asymptotic key rate of the asymmetric configuration is also derived with the numeric simulations illustrating the performance of the proposed protocol.
In single-qubit quantum secret sharing, a secret is shared between N parties via manipulation and measurement of one qubit at a time. Each qubit is sent to all N parties in sequence; the secret is encoded in the first participant's preparation of the qubit state and the subsequent participants' choices of state rotation or measurement basis. We present a protocol for single-qubit quantum secret sharing using polarization entanglement of photon pairs produced in type-I spontaneous parametric downconversion. We investigate the protocol's security against eavesdropping attack under common experimental conditions: a lossy channel for photon transmission, and imperfect preparation of the initial qubit state. A protocol which exploits entanglement between photons, rather than simply polarization correlation, is more robustly secure. We implement the entanglement-based secret-sharing protocol with 87 secret-sharing fidelity, limited by the purity of the entangled state produced by our present apparatus. We demonstrate a photon-number splitting eavesdropping attack, which achieves no success against the entanglement-based protocol while showing the predicted rate of success against a correlation-based protocol.