EPR Pair and Measurement

H. F. Chau, Qinan Wang, Cardythy Wong

Given a shared EPR pair, two cooperative parties can obtain one bit of shared secret key by separately measuring the pair along the same basis. Apparently, the Holevo bound suggests that they cannot obtain more than one secret bit per shared EPR pair. Here we show that they can do so by independently putting each of their EPR pairs in a Hilbert subspace which is unknown to the eavesdropper. We also prove the unconditional security of a practical prepare-and-measure quantum key distribution scheme based on this principle and derive its key rate in the one-way communication setting.

Tao Fang

To transmit secure message, a controlled quantum secure direct communication protocol is proposed based on five-particle cluster state and EPR pair entanglement swapping. The sender and the controller own their EPR pair sequences respectively. The sender sends one photon of the EPR pair he owns to controller. The controller performs local unitary operation randomly on one photon of the EPR pair he owns and sends it to the receiver as his permission. The sender encodes his secret message on the other photon of the EPR pair he owns and sends it to the receiver. Because of EPR pair entanglement swapping, the two photons in the receiver's hand entangled. Under the permission of the controller, the controller and the receiver measure their new EPR pairs in the Bell basis in turn. By comparing the measurement result of the receiver and the original state, the receiver can get the secret message. To ensure the security of the message transmitting, five photons of five-particle cluster state are all mixed in sending sequence to detect eavesdropping which can bring per qubit 88% eavesdropping detection probability. Compared with taking three photons of five-particle cluster state as detection photons, five photons in this paper will decrease the number of five-particle cluster state for detection greatly. The security of the present protocol is discussed: the detection probability of this protocol is more than 88%, and intercept-measure-resend attacks and correlation-elicitation (CE) attack can be defended effectively.

Tore Wessel-Berg

Tapan Kumar Ghosh

Fengli Yan, Ting Gao

We present a scheme for quantum communication, where a set of EPR pairs, initially shared by the sender Alice and the receiver Bob, functions as a quantum channel. After insuring the safety of the quantum channel, Alice applies local measurement on her particles of the EPR pairs and informs Bob the encoding classical information publicly. According to Alice's classical information and his measurement outcomes on the EPR pairs Bob can infer the secret messages directly. In this scheme, to transmit one bit secret message, the sender Alice only needs to send one bit classical information to the receiver Bob. We also show that this scheme is completely secure if perfect quantum channel is used.