Jiang Li-Zhen

Improvement of Controlled Bidirectional Quantum Direct Communication Using a GHZ State

The security of controlled bidirectional quantum direct communication using a GHZ state [Chin. Phys. Lett. 23 (2006) 1680] is analyzed. It turns out that the MX protocol has the problem of definite information leakage, i.e., the first bit of a secret message from any communication party is always leaked out without any active attack after the controllers announcement of measurement results. We put forward two approaches to improve this. The first is to merely modify the encoding rule of the MX protocol, while the second is to use a Bell state as the quantum resource instead of a GHZ state. Both our approaches can ensure that all the bits of secret messages from two communication parties are not leaked out after the controllers announcement of measurement results. Moreover, the controlled bidirectional quantum secure direct communication protocol based on the second approach is more convenient to implement than the MX protocol, since it merely uses a Bell state as the quantum resource and only needs to perform the Bell-basis measurement.

Quantum steganography with a large payload based on dense coding and entanglement swapping of Greenberger—Horne—Zeilinger states

A quantum steganography protocol with a large payload is proposed based on the dense coding and the entanglement swapping of the Greenberger—Horne—Zeilinger (GHZ) states. Its super quantum channel is formed by building up a hidden channel within the original quantum secure direct communication (QSDC) scheme. Based on the original QSDC, secret messages are transmitted by integrating the dense coding and the entanglement swapping of the GHZ states. The capacity of the super quantum channel achieves six bits per round covert communication, much higher than the previous quantum steganography protocols. Its imperceptibility is good, since the information and the secret messages can be regarded to be random or pseudo-random. Moreover, its security is proved to be reliable.

Large payload quantum steganography based on cavity quantum electrodynamics

A large payload quantum steganography protocol based on cavity quantum electrodynamics (QED) is presented in this paper, which effectively uses the evolutionary law of atoms in cavity QED. The protocol builds up a hidden channel to transmit secret messages using entanglement swapping between one GHZ state and one Bell state in cavity QED together with the Hadamard operation. The quantum steganography protocol is insensitive to cavity decay and the thermal field. The capacity, imperceptibility and security against eavesdropping are analyzed in detail in the protocol. It turns out that the protocol not only has good imperceptibility but also possesses good security against eavesdropping. In addition, its capacity for a hidden channel achieves five bits, larger than most of the previous quantum steganography protocols.

Reply to the Comment on “Improvement of Controlled Bidirectional Quantum Direct Communication Using a GHZ State" [Chin. Phys. Lett. 30 (2013) 040305]

Most recently, Liu and Chen [Chin. Phys. Lett. 30 (2013) 079901] used the tool of information theory to analyze the security of our two improved versions of the MX protocol [Chin. Phys. Lett. 30 (2013) 040305], and they found out that our two improved versions still have the information leakage problem. After revisiting them with the tool of information theory again, with respect to the security, we draw the same conclusion as Liu and Chen. In addition, we point out that how to use a single quantum state as the sole quantum resource to design a bidirectional quantum secure direct communication protocol without information leakage is still a big challenge that needs to be solved.

Relative Entropy of Entanglement of a Kind of Two-Qubit Entangled States

We in this paper strictly prove that some block diagonalizable two qubit entangled state with six none zero elements reaches its quantum relative entropy entanglement by the a separable state having the same matrix structure. The entangled state comprises local filtering result state as a special case. PACS: 03.67.Mn;03.65.UdWe strictly prove that some block diagonalizable two-qubit entangled state with six non-zero elements reaches its quantum relative entropy entanglement by a separable state having the same matrix structure. The entangled state comprises local filtering result state as a special case.

Quantum separable criterion based on characteristic function

Entanglement is the most important feature of quantum information science and it is closely related to the foundation of quantum mechanics. To determine a given bipartite quantum state to be entangled or not is by no means easy. We propose a separable criterion for bipartite quantum states based on their characteristic function or characteristic matrix. We use local filtering transform and local rotation transform to simplify the characteristic matrix, while keeping the positivity and entanglement property of the quantum state invariant. The development of Heisenberg-Weyl matrix basis makes the transforms possible. The local transforms greatly reduce the number of none-zero elements of characteristic matrix. The new criterion of separability then is constructed on the none-zero elements. We compare the entanglement detecting properties of our criterion with realignment criterion and modified correlation matrix criterion on

Entanglement bound for multipartite pure states based on local measurements

3\times~3

Inseparability of Phase-Damped Bipartite Gaussian States

bound entangled states. It is shown that our criterion is better than realignment criterion for most of the states.

The separability of two-mode Gaussian state under amplification and symmetric damping

An entanglement bound based on local measurements is introduced for multipartite pure states. It is the upper bound of the geometric measure and the relative entropy of entanglement. It is the lower bound of the minimal-measurement entropy. For pure bipartite states, the bound is equal to the entanglement entropy. The bound is applied to pure tripartite qubit states and the exact tripartite relative entropy of entanglement is obtained for a wide class of states.An entanglement bound based on local measurements is introduced for multipartite pure states. It is the upper bound of the geometric measure and the relative entropy of entanglement. It is the lower bound of minimal measurement entropy. For pure bipartite states, the bound is equal to the entanglement entropy. The bound is applied to pure tripartite qubit states and the exact tripartite relative entropy of entanglement is obtained for a wide class of states.

Non-information leakage controlled quantum conversation protocol based on using entanglement exchange measurement correlation and reducing transmission efficiency

For bipartite non-Gaussian states which are prepared by applying the phase damping to Gaussian states, we use Fock subspace inseparable criterion and the Shchukin–Vogel inseparable criterion to analyze their inseparability. The lowest order of the two criteria is obtained. Fock subspace criterion is more efficient in detecting the entanglement of damped Gaussian state by numerical results. Two photon Fock subspace criterion does not improve the entanglement detecting condition.