Xi-Han Li

Improving the security of multiparty quantum secret sharing against Trojan horse attack

We analyzed the security of the multiparty quantum secret sharing (MQSS) protocol recently proposed by Zhang, Li, and Man [Phys. Rev. A 71, 044301 (2005)] and found that this protocol is secure for any other eavesdropper except for the agent Bob who prepares the quantum signals as he can attack the quantum communication with a Trojan horse. That is, Bob replaces the single-photon signal with a multiphoton one and the other agent Charlie cannot find this cheating as she does not measure the photons before they run back from the boss Alice, which reveals that this MQSS protocol is not secure for Bob. Finally, we present a possible improvement of the MQSS protocol security with two single-photon measurements and four unitary operations.

Multiparty quantum state sharing of an arbitrary two-particle state with Einstein-Podolsky-Rosen pairs

A scheme for multiparty quantum state sharing of an arbitrary two-particle state is presented with Einstein-Podolsky-Rosen pairs. Any one of the N agents has the access to regenerate the original state with two local unitary operations if he collaborates with the other agents, say the controllers. Moreover, each of the controllers is required to take only a product measurement {sigma}{sub x}x{sigma}{sub x} on his two particles, which makes this scheme more convenient for the agents in the applications on a network than others. As all the quantum source can be used to carry the useful information, the intrinsic efficiency of qubits approaches the maximal value. With a new notation for the multipartite entanglement, the sender need only publish two bits of classical information for each measurement, which reduces the information exchanged largely.

Efficient symmetric multiparty quantum state sharing of an arbitrary m-qubit state

We present a scheme for symmetric multiparty quantum state sharing of an arbitrary m-qubit state with m Greenberger–Horne–Zeilinger states following some ideas from the controlled teleportation (2005 Phys. Rev. A 72 02338). The sender Alice performs m Bell-state measurements on her 2m particles and the controllers need only take some single-photon product measurements on their photons independently, not multipartite entanglement measurements, which makes this scheme more convenient than the latter. Also it does not require the parties to perform a controlled-NOT gate on the photons for reconstructing the unknown m-qubit state and it is an optimal one as its efficiency for qubits approaches 100% in principle.

Quantum state sharing of an arbitrary two-qubit state with two-photon entanglements and Bell-state measurements

Abstract. Two schemes for sharing an arbitrary two-qubit state based on entanglement swapping are proposed with Bell-state measurements and local unitary operations. One is based on the quantum channel with four Einstein-Podolsky-Rosen (EPR) pairs shared in advance. The other is based on a circular topological structure, i.e., each user shares an EPR pair with his neighboring one. The advantage of the former is that the construction of the quantum channel between the agents is controlled by the sender Alice, which will improve the security of the scheme. The circular scheme reduces the quantum resource largely when the number of the agents is large. Both of those schemes have the property of high efficiency as almost all the instances can be used to split the quantum information. They are more convenient in application than the other schemes existing as they require only two-qubit entanglements and two-qubit joint measurements for sharing an arbitrary two-qubit state.

Quantum secure direct communication network with Einstein–Podolsky–Rosen pairs

Abstract We discuss the four requirements for a real point-to-point quantum secure direct communication (QSDC) first, and then present two efficient QSDC network schemes with an N ordered Einstein–Podolsky–Rosen pairs. Any one of the authorized users can communicate another one on the network securely and directly.

Efficient high-capacity quantum secret sharing with two-photon entanglement

An efficient high-capacity quantum secret sharing scheme is proposed following some ideas in quantum dense coding with two-photon entanglement. The message sender, Alice prepares and measures the two-photon entangled states, and the two agents, Bob and Charlie code their information on their photons with four local unitary operations, which makes this scheme more convenient for the agents than others. This scheme has a high intrinsic efficiency for qubits and a high capacity.

Faithful qubit transmission against collective noise without ancillary qubits

We present a faithful qubit transmission scheme with linear optics against collective noise, not resorting to ancillary qubits. Its setup is composed of three unbalanced polarization interferometers, based on a polarizing beam splitter, a beam splitter, and a half wave plate, which makes this scheme more feasible than others with present technology. The fidelity of successful transmission is 1, independent of the parameters of the collective noise, and the success probability for obtaining an uncorrupted state can be improved to 100% with some time delayers. Moreover, this scheme has some good applications in one-way quantum communication for rejecting the errors caused by the collective noise in quantum channel.

Multiparty-controlled teleportation of an arbitrary m-qudit state with a pure entangled quantum channel

We present a general scheme for multiparty-controlled teleportation of an arbitrary m-qudit (d-dimensional quantum system) state by using non-maximally entangled states as the quantum channel. The sender performs m generalized Bell-state measurements on her 2m particles, the controllers take some single-particle measurements with the measuring basis Xd and the receiver only needs to introduce one auxiliary two-level particle to extract quantum information probabilistically with the fidelity unit if he cooperates with all the controllers. All the parties can use some decoy photons to set up their quantum channel securely, which will forbid a dishonest party to eavesdrop freely. This scheme is optimal as the probability that the receiver obtains the originally unknown m-qudit state equals the entanglement of the quantum channel.

Multiparty quantum secret splitting and quantum state sharing

Abstract A protocol for multiparty quantum secret splitting is proposed with an ordered N Einstein–Podolsky–Rosen (EPR) pairs and Bell state measurements. It is secure and has the high intrinsic efficiency and source capacity as almost all the instances are useful and each EPR pair carries two bits of message securely. Moreover, we modify it for multiparty quantum state sharing of an arbitrary m-particle entangled state based on quantum teleportation with only Bell state measurements and local unitary operations which make this protocol more convenient in a practical application than others.

Fault tolerant quantum key distribution based on quantum dense coding with collective noise

We present two robust quantum key distribution protocols against two kinds of collective noise, following some ideas in quantum dense coding. Three-qubit entangled states are used as quantum information carriers, two of which form the logical qubit, which is invariant with a special type of collective noise. The information is encoded on logical qubits with four unitary operations, which can be read out faithfully with Bell-state analysis on two physical qubits and a single-photon measurement on the other physical qubit, not three-photon joint measurements. Two bits of information are exchanged faithfully and securely by transmitting two physical qubits through a noisy channel. When the losses in the noisy channel is low, these protocols can be used to transmit a secret message directly in principle.