Dense Coding with Multi-Atom Entanglement Channel in CavityQED

The utility of a five-qubit entangled state for quantum teleportation, quantum state sharing and superdense coding is investigated. The state can be utilized for perfect teleportation and quantum state sharing of an arbitrary single- and two-qubit state. The capacity of superdense coding of the state reaches the "Holevo bound", which means that five classical bits can be transmitted by sending three qubits. The preparation of the five-qubit state and detection of the multipartite states in cavity QED are discussed. The distinct advantage of the feasible cavity QED technology that we use is insensitive to the thermal field and the cavity decay.

An experimentally feasible scheme for generalized quantum state sharing of an arbitrary unknown single-qubit state in cavity QED is presented. Using a generalized Greenberger–Horne–Zeilinger (GHZ) state as the quantum channel among the three parties, the quantum information (i.e. the single-qubit state) from the sender can be split in such a way that the information can be recovered if and only if both receivers collaborate. Moreover, the scheme is insensitive to both the effects of thermal field and cavity decay.

We demonstrate that a genuine six-atom entangled state can be used to realize the deterministic quantum state sharing of an arbitrary two-atom state in cavity QED. In our scheme, any one of the two agents is sufficient to reconstruct the original state only under the condition that he/she obtains the help of the other one, but one of them obtains no information. The presented protocol is showed to be secure against certain eavesdropping attacks.

We use a quantum electrodynamics model, to study the evolution of maximally entangled bipartite states (Bell states), as well as a maximally entangled tripartite states as a multipartite system. Furthermore, we study the entanglement behaviour of these output states in cavity QED as function of interaction time and the coupling strength. The present study discusses the separability and the entanglement limit of such states after interaction with a cavity QED.

We propose a scheme to prepare many two-mode cavities intoone-dimensional cluster states in the context of cavity QED. Theleft-circularly polarized state and right-circularly polarized state ofthe cavity are encoded as the logic zero and one of the qubits. In thescheme, the atomic spontaneous emission is suppressed, and the fidelityis unaffected by the cavity decay on the assumption that the detectionefficiencies of all the photondetectors are 1.

It is shown that strong competition between the absorption and emission of the cavity photons and the incoherent processes like atomic spontaneous emission and cavity decay effects result in new nonequilibrium states in cavity quantum electrodynamics. Such states can exhibit saturation, nonclassical character, and strong atom-cavity correlation.

Based on presently available experimental advancements, we present a theoretical technique to generate single-mode electromagnetic-field entangled NOON states between two resonators. A two-level atom interacts dispersively with a standing wave field, which results in Bragg diffraction and thus a superposition of atomic external degrees of freedom. Later, a resonant interaction of the atom with two different resonators in the presence of a Ramsey field and a Stark field entangles the two resonators in their NOON state. We further discuss the experimental parameters necessary to realize the scheme in the laboratory and calculate the corresponding success probability.

We present a scheme for conditionally teleporting an unknown atomic state in cavity QED, which requires two atoms and one cavity mode. The translational degrees of freedom of the atoms are taken into account using the optical Stern-Gerlach model. We show that successful teleportation with probability 1/2 can be achieved through local measurements of the cavity photon number and atomic positions. Neither direct projection onto highly entangled states nor holonomous interaction-time constraints are required.

We generate highly non-classical entangled two-mode field states of the type by utilizing an atomic analogue of the Mach–Zehnder interferometer, where quantized fields in the high-Q cavities act as beam splitters and mirrors. We discuss that the probability for the production of the desired states may approach a value close to unity under presently available experimental conditions.

A simple scheme is proposed to generate an $n$-qubit $W$ state in cavity QED. Conditioned on no photon leakage from the cavity, the $n$-qubit $W$ state can be generated by resonant interaction between atoms and the cavity if the cavity is initially prepared in the single-photon state and all the atoms are in the ground states. We check the time evolution of the system involving decay, and show that, since the required interaction time is very short, with present cavity QED techniques, the success probability of our scheme is almost unity.

We propose a scheme to generate a 3×3-dimensional maximallyentangled state of two particles. Two three-level atoms interact with astrongly detuned cavity so that the cavity is only virtually excited andefficient decoherence time of the cavity is greatly prolonged. Compared toother protocols, this protocol is simpler and has a higher fidelity.

A method to prepare multi-particle entanglement of Greenberger-Horne-Zeilinger-type and W-type is proposed through the interaction between the cavity and many atoms simultaneously. After suitable interaction, the selected measurements on the cavity field can create the entangled atomic states. These entangled states can be useful for quantum information manipulation and fundamental study of quantum theory. These states that violate the Bell's inequality are proven.

We propose a scheme to perform basic gates of quantum computing and prepare\nentangled states in a system with cold trapped ions located in a single mode\noptical cavity. General quantum computing can be made with both motional state\nof the trapped ion and cavity state being qubits. We can also generate\ndifferent kinds of entangled states in such a system without state reduction,\nand can transfer quantum states from the ion in one trap to the ion in another\ntrap. Experimental requirement for achieving our scheme is discussed.\n

An experimentally feasible protocol for realizing dense coding by using a class of W-state in cavity quantum electrodynamics (QED) is proposed in this paper. The prominent advantage of our scheme is that the successful probability of the dense coding with a W-class state can reach 1. In addition, the scheme can be implemented by the present cavity QED techniques.

Quantum communication provides an enormous advantage over its classical counterpart: security of communications based on the very principles of quantum mechanics. Researchers have proposed several approaches for user identity authentication via entanglement. Unfortunately, these protocols fail because an attacker can capture some of the particles in a transmitted sequence and send what is left to the receiver through a quantum channel. Subsequently, the attacker can restore some of the confidential messages, giving rise to the possibility of information leakage. Here we present a new robust General N user authentication protocol based on N-particle Greenberger–Horne–Zeilinger (GHZ) states, which makes eavesdropping detection more effective and secure, as compared to some current authentication protocols. The security analysis of our protocol for various kinds of attacks verifies that it is unconditionally secure, and that an attacker will not obtain any information about the transmitted key. Moreover, as the number of transferred key bits N becomes larger, while the number of users for transmitting the information is increased, the probability of effectively obtaining the transmitted authentication keys is reduced to zero.