A protocol for quantum secure direct communication using blocks of EPR pairs is proposed. A set of ordered N EPR pairs is used as a data block for sending secret message directly. The ordered N EPR set is divided into two particle sequences, a checking sequence and a message-coding sequence. After transmitting the checking sequence, the two parties of communication check eavesdropping by measuring a fraction of particles randomly chosen, with random choice of two sets of measuring bases. After insuring the security of the quantum channel , the sender, Alice encodes the secret message directly on the message-coding sequence and send them to Bob. By combining the checking and message-coding sequences together, Bob is able to read out the encoded messages directly. The scheme is secure because an eavesdropper cannot get both sequences simultaneously. We also discuss issues in a noisy channel.

Quantum secure direct communication is the direct communication of secret messages without first producing a shared secret key. It maybe used in some urgent circumstances. Here we propose a quantum secure direct communication protocol using single photons. The protocol uses batches of single photons prepared randomly in one of four different states. These single photons serve as a one-time-pad which are used directly to encode the secret messages in one communication process. We also show that it is unconditionally secure. The protocol is feasible with present-day technique.PACS numbers: 03.67. Hk, 03.65.Ud Published in Physical Review A 69, 052319 (2004) Quantum key distribution(QKD) provides a novel way for two legitimate parties to establish a common secret key over a long distance. Its ultimate advantage is its unconditional security, the feat in cryptography. Combining with the one-time-pad scheme in which the private key is as long as the messages, secret messages can be communicated safely from one place to another place. QKD has progressed quickly [1] since Bennett and Brassard designed the original QKD protocol [2].Recently, a novel concept, quantum secure direct communication (QSDC) is proposed and pursued [3,4,5,6]. Different from QKD whose object is to establish a common random key between the two remote parties of communication, QSDC is to transmit the secret message directly without first creating a key to encrypt them. In 2002, Beige et al. presented a QSDC scheme based on single-photon two-qubit states [3]. In this scheme the message can be read after a transmission of an additional classical information for each qubit, which is similar to a QKD scheme as each bit of key can represent one bit of secret message with an additional classical information, i.e, retaining or flipping the bit value in the key according to the secret message. Boström and Felbinger put forward a Ping-Pong QSDC scheme[4] using Einstein-Podolsky-Rosen (EPR) pairs [7] as quantum information carriers. It is secure for key distribution, but is only quasi-secure for direct secret communication if perfect quantum channel is used. However it is insecure even for QKD if it is operated in a noisy quantum channel, as shown by Wójcik[8]. The Ping-Pong protocol can be modified for secure QKD by taking into account of the procedures proposed by Wójcik [8]. Cai found that the Ping-Pong scheme can be attacked without eavesdropping [9]. Meanwhile, we proposed a two-step secure QSDC protocol with EPR pairs transmitted in blocks [5] by modifying a QKD protocol based on EPR pairs [10]. In Ref.[6], Cai modifies the Ping-Pong scheme by replacing the entangled photons with single photons in mixed state. However it is unsafe in a noisy channel, and is vulnerable to the opaque attack [11].QSDC maybe important in some applications. For instance when the transmission time is urgent, or the transmission maybe subject to the danger of destruction. Furthermore, as the technologies for quantum information improves, the efficiency of quantum transmi...

In this work, we generalize the quantum secret sharing scheme of Hillary, Bužek and Berthiaume[Phys. Rev. A59, 1829(1999)] into arbitrary multi-parties. Explicit expressions for the shared secret bit is given. It is shown that in the Hillery-Bužek-Berthiaume quantum secret sharing scheme the secret information is shared in the parity of binary strings formed by the measured outcomes of the participants. In addition, we have increased the efficiency of the quantum secret sharing scheme by generalizing two techniques from quantum key distribution. The favored-measuring-basis Quantum secret sharing scheme is developed from the Lo-Chau-Ardehali technique[H. K. Lo, H. F. Chau and M. Ardehali, quant-ph/0011056] where all the participants choose their measuring-basis asymmetrically, and the measuring-basis-encrypted Quantum secret sharing scheme is developed from the Hwang-Koh-Han technique [W. Y. Hwang, I. G. Koh and Y. D. Han, Phys. Lett. A244, 489 (1998)] where all participants choose their measuring-basis according to a control key. Both schemes are asymptotically 100% in efficiency, hence nearly all the GHZ-states in a quantum secret sharing process are used to generate shared secret information.