In this paper, by introducing the additional classical information and taking advantage of the property of quantum entanglement swapping of two photon pairs instead of the property of quantum entanglement of a photon pair, the Bostrom–Felbinger protocol [Phys. Rev. Lett.89, 187902 (2002)] is improved. In the improved protocol, when the photon travels from the message receiver's site to the message sender's site, any attack can be detected in the control mode. Moreover, it is deterministically successful to transmit the secret messages in the message mode. The security of the present improved protocol against the eavesdropping is ensured by the Bostrom–Felbinger protocol, and besides, the attacks proposed by Cai [Phys. Rev. Lett.90, 109801 (2003)] to attack the Bostrom–Felbinger protocol without eavesdropping can be prevented.
We show that the capacity of the Bostr\"om-Felbinger (ping-pong) protocol [Phys. Rev. Lett. 89, 187902 (2002)] can be doubled by introducing two additional unitary operations. In message mode, Alice can use one of the four unitary operations to encode two classical bits of information. Bob can decode Alice's information by using a Bell-basis measurement. Security against eavesdropping attack can be ensured by using two conjugate measurement bases in control mode. A message authentification method can protect this communication against a man-in-the-middle attack without eavesdropping.
The eavesdropping scheme proposed by Wójcik [Phys. Rev. Lett. {\bf 90},157901(2003)] on the quantum communication protocol of Boström and Felbinger [Phys. Rev. Lett. {\bf 89}, 187902(2002)] is improved by constituting a new set of attack operations. The improved scheme only induces half of the eavesdropping losses in Wójcik's scheme, therefore, in a larger domain of the quantum channel transmission efficiency
η, i.e., [0,75%], the eavesdropper Eve can attack all the transmitted bits. Comparing to Wójcik's scheme, in the improved scheme the eavesdropping (legitimate) information gain does not vary in the
η domain of [0, 50%], while in the
η domain of (50%, 75%] the less eavesdropping losses induce more eavesdropping information gains, for Eve can attack {\it all} the transmitted bits and accordingly eavesdropping information gains do {\it not} decrease. Moreover, for the Boström-Felbinger protocol, the insecurity upper bound of
η presented by Wójcik is pushed up in the this paper, that is, according to Wójcik's eavesdropping scheme, the Boström-Felbinger protocol is not secure for transmission efficiencies lower than almost 60%, while according to the improved scheme, it is not secure for transmission efficiencies lower than almost 80%.
We introduce an attack scheme for eavesdropping freely the ping-pong quantum communication protocol proposed by Boström and Felbinger [ Phys. Rev. Lett. 89 , 187902 (2002)] in a noise channel. The vicious eavesdropper, Eve, intercepts and measures the travel photon transmitted between the sender and the receiver. Then she replaces the quantum signal with a multi-photon signal in the same state, and measures the returned photons with the measuring basis, with which Eve prepares the fake signal except for one photon. This attack increases neither the quantum channel losses nor the error rate in the sampling instances for eavesdropping check. It works for eavesdropping the secret message transmitted with the ping-pong protocol. Finally, we propose a way for improving the security of the ping-pong protocol.
Security of the "ping-pong" quantum communication protocol recently proposed by Boström and Felbinger [Phys. Rev. Lett. 89, 187902 (2002)]] is analyzed in the case of considerable quantum channel losses. The eavesdropping scheme is presented, which reveals that the ping-pong protocol is not secure for transmission efficiencies lower than 60%. Our scheme induces 50% losses, which, however, can be hidden in the channel losses if one replaces the original lossy channel with a less lossy one. Finally, a possible improvement of the ping-pong protocol security is proposed.
We propose a direct communication protocol using entanglement swapping. The safety of this protocol is guaranteed by the basic properties of entanglement swapping. This protocol is efficient to transmit two classical bits of information per one session. This efficiency is better than that of ping-pong protocol suggested by Bostrom and Felbinger. Even if an eavesdropper intervenes in midway, the eavesdropper will be detected with the probability of in the verification process of one bit. Therefore the perfect security is guaranteed if we use enough amount of bits for the verification process.
A deterministic quantum communication protocol, called the ping-pong protocol, has been represented by Boström and Felbinger [Phys. Rev. Lett. 89 (2002) 187902]. Based on an entangled pair of photons, it allows asymptotically secure key distribution and quasi-secure direct communication. However, it was concluded from our previous research that the experiment realization of this protocol requires two optic paths strictly equivalent for interferometry. Here we propose an improved protocol, which is more flexible and secure for conceivable applications. Its feasibility and security are also discussed.