Engineering noon states in cavity QED

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.

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.

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.

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 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.

Dense coding of multi-atom entangled states in cavity QED is studied. Ifthe quantum channel is generalized GHZ states, dense coding can bedirectly realized in a simply way. As for the partially entangled purestates, we propose a feasible protocol for entanglement concentration,and the efficiency of information transmitted is calculated. The schemesare insensitive to the cavity decay and the field state, due to the factthat the interaction here is a large-detuned one between atoms and thecavity.

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.

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.

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

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.

Generation of high fidelity photonic non-Gaussian states is a crucial\ningredient for universal quantum computation using continous-variable\nplatforms, yet it remains a challenge to do so efficiently. We present a\ngeneral framework for a probabilistic production of multimode non-Gaussian\nstates by measuring few modes of multimode Gaussian states via\nphoton-number-resolving detectors. We use Gaussian elements consisting of\nsqueezed displaced vacuum states and interferometers, the only non-Gaussian\nelements consisting of photon-number-resolving detectors. We derive analytic\nexpressions for the output Wigner function, and the probability of generating\nthe states in terms of the mean and the covariance matrix of the Gaussian state\nand the photon detection pattern. We find that the output states can be written\nas a Fock basis superposition state followed by a Gaussian gate, and we derive\nexplicit expressions for these parameters. These analytic expressions show\nexactly what non-Gaussian states can be generated by this probabilistic scheme.\nFurther, it provides a method to search for the Gaussian circuit and\nmeasurement pattern that produces a target non-Gaussian state with optimal\nfidelity and success probability. We present specific examples such as the\ngeneration of cat states, ON states, Gottesman-Kitaev-Preskill states, NOON\nstates and bosonic code states. The proposed framework has potential\nfar-reaching implications for the generation of bosonic error-correction codes\nthat require non-Gaussian states, resource states for the implementation of\nnon-Gaussian gates needed for universal quantum computation, among other\napplications requiring non-Gaussianity. The tools developed here could also\nprove useful for the quantum resource theory of non-Gaussianity.\n

A scheme for approximately conditional teleportation of an arbitrary two-particle state without Bell-state measurement in cavity QED is presented. The scheme involves four two-level atoms and two single-mode cavities. The detection of atoms 3 and 4 collapses the atoms 1 and 2 to the initial state of atoms 3 and 4, while the cavity fields left in the vacuum state. The probability of success and the fidelity of our scheme are 0.125 and 0.98, respectively.