Stronger correlations than dense coding with the same quantum resources

Dense coding is the seminal example of how entanglement can boost quantum communication. By sharing an Einstein-Podolsky-Rosen (EPR) pair, dense coding allows one to transmit two bits of classical information while sending only a single qubit [1]. This doubling of the channel capacity is the largest allowed in quantum theory [2]. In this letter we show in both theory and experiment that same elementary resources, namely a shared EPR pair and qubit communication, are strictly more powerful than two classical bits in more general communication tasks. In contrast to dense coding experiments [3–8], we show that these advantages can be revealed using merely standard optical Bell state analysers [9, 10]. Our results reveal that the power of entanglement in enhancing quantum communications qualitatively goes beyond boosting channel capacities.

Bell Inequality from Holographic Gravity

We study a holographic model of an EPR pair at the boundary of bulk gravity, and use Bell inequality as a sharp test of entanglement. By revealing how Bell inequality is violated by gravity in the bulk, our study sheds light on the entanglement of the original ER=EPR conjecture.

A Quantum Repeater Based on Entanglement Purification and Entanglement Swapping

We discuss a long-distance quantum communication system based on entangled photon pairs, which apply entanglement as its fundamental resource. For distances longer than the coherence length of a counterpart noisy quantum channel, the fidelity of transmission is ordinarily so low that standard purification processes are not applicable. The quantum repeater stretches the length of the entangled photon pairs. And the high fidelity entanglement of photons between sender and receiver is obtained by entanglement purification and entanglement swapping. We compare the nested repeater with the common repeater and show that it outperforms the latter, which is built an EPR pair in less time.

Semi-loss-tolerant strong coin flipping protocol using EPR pairs

In this paper, we present a quantum strong coin flipping protocol. In this protocol, an EPR pair and a quantum memory storage are made use of, and losses in the quantum communication channel and quantum memory storage are all analyzed. We obtain the bias in the fair scenario as a function of $p$, where $p$ is the probability that the particle in Bob's quantum memory storage is lost, which means our bias varies as the degree of losses in the quantum memory storage changes. Therefore we call our protocol semi-loss-tolerant. We also show that the bias decreases with decreasing $p$. When $p$ approaches $0$, the bias approaches 0.3536, which is less than that of all the previous loss-tolerant protocols. Details of both parties' optimal cheating strategies are also given and analyzed. What's more, experimental feasibility is discussed and demonstrated. Compared with previous qubit-based loss-tolerant SCF protocols, we introduce the EPR pair to keep our protocol loss-tolerant while trying to push down the bias. In addition, a quantum memory storage is used and the losses in it has been taken into account. We obtain the bias in the fair scenario as a function of $p$, where $p$ is the probability that the particle in Bob's quantum memory storage is lost, which means our bias varies as the degree of losses in the quantum memory storage changes. We also show that the bias decreases with decreasing $p$. When $p$ approaches $0$, the bias approaches 0.3536, which is less than that of all the previous loss-tolerant protocols. Details of both parties' optimal cheating strategies are also given and analyzed. Besides, experimental feasibility is discussed and demonstrated.

QUANTUM SECURE DIRECT COMMUNICATION WITH A CONSTANT NUMBER OF EPR PAIRS

In this paper, we propose a quantum secure direct communication (QSDC) protocol based on Einstein–Podolsky–Rosen (EPR) pairs. Previous QSDC protocols usually consume one EPR pair to transmit a single qubit. If Alice wants to transmit an n-bit message, she needs at least n/2 EPR pairs when a dense coding scheme is used. In our protocol, if both Alice and Bob preshare 2c + 1 EPR pairs with the trusted server, where c is a constant, Alice can transmit an arbitrary number of qubits to Bob. The 2c EPR pairs are used by Alice and Bob to authenticate each other and the remaining EPR pair is used to encode and decode the message qubit. Thus the total number of EPR pairs used for one communication is a constant no matter how many bits will be transmitted. It is not necessary to transmit EPR pairs before transmitting the secret message except for the preshared constant number of EPR pairs. This reduces both the utilization of the quantum channel and the risk. In addition, after the authentication, the server is not involved in the message transmission. Thus we can prevent the server from knowing the message.