Effective Scheme for Generating Cluster States in Cavity QED

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.

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 propose a dissipative scheme to prepare a three-dimensional entangled state for two atoms trapped in separate coupled cavities. Our work shows that both atomic spontaneous emission and cavity decay, which are two typical obstacles in unitary-dynamics-based schemes, are no longer detrimental, but necessary for three-dimensional entangled state preparation without specifying initial state and controlling the evolution time precisely. Final numerical simulation with one group of experimental parameters indicates that the performance of our scheme could be better than the unitary-dynamics-based scheme.

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.

A scheme of fast synthesizing the Fredkin gate is proposed in cavity QED via quantum Zeno dynamics. Three atoms are trapped in three different but directly coupled cavities in this scheme. The strictly numerical simulations are given, and the influences of cavity decay and spontaneous emission on the gate operation are analyzed with master equation. The result shows that our scheme is robust against atomic spontaneous emission because of the large detuning. Since the atoms are separated in different cavities, it is easier to manipulate atoms experimentally.

A selective photon scheme is proposed to realize an N-qubit Greenberger–Horne–Zeilinger (GHZ) state with distant atoms trapped in spatially separated bimodal cavities coupled by optical fibres. The influence of deviations of some experimental parameters on our scheme is studied exactly and we prove that a highly reliable GHZ state is achievable. Moreover, we analyse the independence of fidelity on decoherence processes, such as atomic spontaneous emission, cavity decay and fibre losses. The results show that atomic spontaneous emission and fibre losses can be ignored in some special cases. With regards to the cavity decay, it indicates that a GHZ state with high fidelity may be realized in the current experiment.

We propose a scheme to implement a two-qubit controlled-phase gate for single atomic qubits, which works in principle with nearly ideal success probability and fidelity. Our scheme is based on the cavity input-output process and the single-photon polarization measurement. We show that, even with the practical imperfections such as atomic spontaneous emission, weak atom-cavity coupling, violation of the Lamb-Dicke condition, cavity photon loss, and detection inefficiency, the proposed gate is feasible for generation of a cluster state in that it meets the scalability criterion and it operates in a conclusive manner. We demonstrate a simple and efficient process to generate a cluster state with our high probabilistic entangling gate.

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.

We propose a scheme to synthesize atom---photon hybrid controlled-not (CNOT) gate by combining atomic single-qubit operations via stimulated Raman adiabatic passage and photonic Faraday rotation in cavity QED system. Benefiting from its hybrid characteristic, we utilize atom---photon CNOT gate to construct quantum CNOT gate for remote atoms and photons, respectively. As our scheme works in the bad cavity regime and only involves virtual excitation of atoms, it may be robust against both cavity decay and atomic spontaneous emission, thus can be realized with less demanding technology than that previously mentioned.

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.

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.

The scheme for preparing a multipartite entangled state and implementing 1→M economical phase-covariant quantum anti-cloning only by one step are proposed. By using the symmetric and asymmetric interactions between N atoms and the cavity, N-particle Greenberger–Horne–Zeilinger (GHZ) and W states are prepared, respectively. Furthermore, a scheme of 1→M anti-cloning is also proposed through preparing the W state. The decoherence from atomic spontaneous emission is negligible due to the excited state of the atom being adiabatically eliminated under large detuning. The scheme is insensitive to the cavity decay due to the cavity only being virtually excited. The experimental feasibility of the present scheme is also discussed.

A controlled-phase gate is constructed deterministically in a quantum dot (QD)-cavity system based on Faraday rotation via singly charged QDs in the strong-coupling regime. The fidelity of the gate can reach relatively high values even if cavity decay and leakage are considered. Furthermore, we present two schemes for implementing N-qubit cluster states for electron spins and photons, respectively, by exploiting this gate. The schemes are very simple and can be easily realized in the current experiment.

Preparation of W state among spatially separated atomic ensembles collectively controlled by a single atom via adiabatic passage