Generation of Cluster States in Cavity QED

We propose two schemes for the generation of cluster states in the context of cavity quantum electrodynamics （QED）. In the first scheme, we prepare multi-cavity cluster states with information encoded in the coherent states. The second scheme is to generate multi-atom cluster states, where qubits axe represented by the states of cascade Rydberg atoms. Both the schemes axe based on the atom-cavity interaction and the atomic spontaneous radiation can be efficiently reduced since the cavity frequency is largely detuned from the atomic transition frequency.     

Quantum computation based on a quantum switch in cavity QED

A feasible scheme for constructing quantum logic gates is proposed on the basis of quantum switches in cavity QED. It is shown that the light field which is fed into the cavity due to the passage of an atom in a certain state can be used to manipulate the conditioned quantum logical gate. In our scheme, the quantum information is encoded in the states of Rydberg atoms and the cavity mode is not used as logical qubits or as a communicating “bus”; thus, the effect of atomic spontaneous emission can be neglected and the strict requirements for the cavity can be relaxed.     

The anomalous magnetic moment of the electron

The value of the electron's magnetic moment is a fundamental quantity in physics. Its deviation from the value expected from Dirac theory has given enormous impetus to the field of quantum theory and especially to quantum electrodynamics (QED) as the relativistic quantum field theory of electrodynamics. In fact, the measured values both for free and for bound electrons are explained by corresponding QED calculations on the part per trillion and part per billion level of accuracy, respectively. This agreement is amongst the best known in physics today. In turn, it allows highly precise determinations of related fundamental constants like the fine structure constant α or the electron mass. The present article discusses the application of the continuous Stern–Gerlach effect to the precise measurement of magnetic moments, especially of the electron bound in highly charged ions and possible tests of calculations in the framework of QED of bound states. Also, a test of QED in a more general approach by the comparison of values for the fine structure constant derived from different measurements, will be discussed.     

Semiclassical calculation of the recurrence spectra of He Rydberg atom in perpendicular electric and magnetic fields

Closed orbit theory is a semiclassical technique for explaining the spectra of Rydberg atoms in external fields. By developing the closed orbit theory from two degrees of freedom to three non-separable degrees of freedom, we calculated the recurrence spectra of He Rydberg atom in perpendicular electric and magnetic fields. The closed orbits in the corresponding classical system have also been obtained. Fourier transformed spectra of He atoms have allowed direct comparison between the resonance peaks and the scaled action values of closed orbits, whereas the nonhydrogenic resonance can be explained in terms of the new orbits created by the core scattering. The semiclassical result is in good agreement with the quantum spectra, which suggests that our method is correct.     

Quantum-electrodynamics without renormalization : IV. Comparison with renormalized quantum-electrodynamics

The generalised function method is shown to be equivalent up to a finite renormalization with standard quantum-electrodynamics (QED). The result is a theory which is complete in a physical sense. A comparison with standard renormalisation theory is given.     

Scheme for splitting quantum information via W states in cavity QED systems

Assisted by multipartite entanglement, Quantum information may be split so that the original qubit can be reconstructed if and only if the recipients cooperate. This paper proposes an experimentally feasible scheme for splitting quantum information via W-type entangled states in cavity QED systems, where three-level Rydberg atoms interact with nonresonant cavities. Since W-type states are used as the quantum channel and the cavities are only virtually excited, the scheme is easy to implement and robust against decoherence, and the dependence on the quality factor of the cavities is greatly reduced.     

Scheme for Remote Implementation of Partially Unknown Quantum Operation of Two Qubits in Cavity QED

By constructing the recovery operations of the protocol of remote implementation of partially unknown quantum operation of two qubits [An-Min Wang: Phys. Rev. A 74 (2006) 032317] with two-qubit Cnot gate and single qubit logic gates, we present a scheme to implement it in cavity QED. Long-lived Rydberg atoms are used as qubits, and the interaction between the atoms and the field of cavity is a nonresonant one. Finally, we analyze the experimental feasibility of this scheme.     

Absorption and Recurrence Spectra of Li Rydberg Atom in Perpendicular Electric and Magnetic Fields

We develop the semi-closed orbit theory from two degrees of freedom to three non-separable degrees of freedom and put forward a new model potential for the Li Rydberg atom, which reduces the study of the system to an effective one-particle problem. Using this model potential and the closed orbit theory for three degrees of freedom, we calculate the recurrence spectra of Li Rydberg atom in perpendicular electric and magnetic fields. The closed orbits in the corresponding classical system have also been obtained. The Fourier transformed spectra of Li atom have allowed direct comparison between the resonance peaks and the scaled action values of closed orbits, whereas the nonhydrogenic resonance can be explained in terms of the new orbits created by the core scattering. Our result is in good agreement with the quantum spectra, which suggests that our calculation is correct.     

Absorption and Recurrence Spectra of Li Rydberg Atom in Perpendicular Electric and Magnetic Fields

We develop the semi-closed orbit theory from two degrees of freedom to three non-separable degrees of freedom and put forward a new model potential for the Li Rydberg atom, which reduces the study of the system to an effective one-particle problem. Using this model potential and the closed orbit theory for three degrees of freedom, we caiculate the recurrence spectra of Li Rydberg atom in perpendicular electric and magnetic fields. The closed orbits in the corresponding classicai system have also been obtained. The Fourier transformed spectra of Li atom have ailowed direct comparison between the resonance peaks and the scaied action values of closed orbits, whereas the nonhydrogenic resonance can be explained in terms of the new orbits created by the core scattering. Our result is in good agreement with the quantum spectra, which suggests that our calculation is correct.     

Semiclassical calculation of Rydberg He2^＋ molecular of recurrence spectra ion in a magnetic field

Making use of the molecular closed-orbit theory and a new model potential for the Rydberg molecule, we have calculated the recurrence spectra of He^2＋ molecular ion in a magnetic field for different quantum defects. The Fourier transform spectra of He^2＋ molecular ion may be used to perform a direct comparison between peaks in the spectra and the scaled action values of closed orbits of the excited electron in external fields. We find that the spectral modulations can be analysed in terms of the scattering of the excited electron on the molecular core. Unlike the case of the Rydberg atom where the elastic scattering is predominant, modulations produced by inelastic scattering are also vital to the photoabsorption spectrum of the Rydberg molecule. Our results are in good agreement with the quantum results, which suggests that our method is correct.     

Rydberg spectroscopy in an optical lattice: blackbody thermometry for atomic clocks

We show that optical spectroscopy of Rydberg states can provide accurate in situ thermometry at room temperature. Transitions from a metastable state to Rydberg states with principal quantum numbers of 25-30 have 200 times larger fractional frequency sensitivities to blackbody radiation than the strontium clock transition. We demonstrate that magic-wavelength lattices exist for both strontium and ytterbium transitions between the metastable and Rydberg states. Frequency measurements of Rydberg transitions with 10(-16) accuracy provide 10 mK resolution and yield a blackbody uncertainty for the clock transition of 10(-18).     

The covariant-evolution-operator method in bound-state QED

The methods of quantum-electrodynamical (QED) calculations on bound atomic systems are reviewed with emphasis on the newly developed covariant-evolution-operator method. The aim is to compare that method with other available methods and also to point out possibilities to combine that with standard many-body perturbation theory (MBPT) in order to perform accurate numerical QED calculations, including quasi-degeneracy, also for light elements, where the electron correlation is relatively strong.As a background, the time-independent many-body perturbation theory (MBPT) is briefly reviewed, particularly the method with extended model space. Time-dependent perturbation theory is discussed in some detail, introducing the time-evolution operator and the Gell–Mann–Low relation, generalized to an arbitrary model space. Three methods of treating the bound-state QED problem are discussed. The standard S-matrix formulation, which is restricted to a degenerate model space, is discussed only briefly. Two methods applicable also to the quasi-degenerate problem are treated in more detail, the two-times Green's-function and the covariant-evolution-operator techniques. The treatment is concentrated on the latter technique, which has been developed more recently and which has not been discussed in more detail before. A comparison of the two-times Green's-function and the covariant-evolution-operator techniques, which have great similarities, is performed. In the appendix a simple procedure is derived for expressing the evolution-operator diagrams of arbitrary order.The possibilities of merging QED in the covariant evolution-operator formulation with MBPT in a systematic way is indicated. With such a technique it might be feasible to perform accurate QED calculations also on light elements, which is presently not possible with the techniques available.     

Rydberg quantum computation with nuclear spins in two-electron neutral atoms

Alkaline-earth-like (AEL) atoms with two valence electrons and a nonzero nuclear spin can be excited to Rydberg state for quantum computing. Typical AEL ground states possess no hyperfine splitting, but unfortunately a GHz-scale splitting seems necessary for Rydberg excitation. Though strong magnetic fields can induce a GHz-scale splitting, weak fields are desirable to avoid noise in experiments. Here, we provide two solutions to this outstanding challenge with realistic data of well-studied AEL isotopes. In the first theory, the two nuclear spin qubit states |0〉 and |1〉 are excited to Rydberg states |r〉 with detuning Δ and 0, respectively, where a MHz-scale detuning Δ arises from a weak magnetic field on the order of 1 G. With a proper ratio between Δ and Ω, the qubit state |1〉 can be fully excited to the Rydberg state while |0〉 remains there. In the second theory, we show that by choosing appropriate intermediate states a two-photon Rydberg excitation can proceed with only one nuclear spin qubit state. The second theory is applicable whatever the magnitude of the magnetic field is. These theories bring a versatile means for quantum computation by combining the broad applicability of Rydberg blockade and the incomparable advantages of nuclear-spin quantum memory in two-electron neutral atoms.     

铅原子J=2偶宇称Rydberg系列组态相互作用

通过实验获得了碳族原子铅的偶宇称J＝0和2的束缚Rydberg态能级，利用六通道摸型分析了铅原子J＝2的束缚Rydberg系列之间强烈的组态相互作用对电离光谱的影响。     

Light scattering,origin invariant multipole moments and molecular property tensors: the case of electric-field-gradient-induced birefringence

A comparison of semiclassical and quantum versions of molecular light scattering theory at finite temperatures is presented. A general formulation of the semiclassical radiation model is developed to the point where its relationship to the corresponding QED formalism can be established: the classical scattered electric field is proportional to the same R-matrix element as that obtained from QED for the photon scattering amplitude. The result is valid for non-resonant scattering at T = 0. The semiclassical theory conventionally also inherits aspects of a classical molecular model, principally origin-dependent molecular multipole moments. Origin independent multipoles, and corresponding response functions can be defined if the theory is cast in terms of centre-of-mass and translation invariant internal coordinates. Such a choice of coordinates brings molecular light scattering theory into line with the theory of the molecular Schrödinger equation. This is illustrated for the case of a diatomic molecule. A specific application of these results of current interest is electric-field-gradient induced birefringence (EFGB) for which there are four competing theories in the literature. In this paper we examine the treatment of finite temperature effects in two semiclassical accounts of EFGB in polar molecules and identify a likely source of the discrepancy between them revealed in a recent ab initio computational study.     

Can MBPT and QED be merged in a systematic way?

The possiblities of merging QED with the standard many-body perturbation theory (MBPT) for atomic systems in a rigorous and systematic way are analysed. Time-dependent MBPT, based on the time-evolution operator, a technique well developed particularly in nuclear theory, is used and somewhat reformulated to be consistent with the covariant QED formalism. An effective QED Hamiltonian, free from singularities, is constructed. The procedure can be applied to degenerate and quasi-degenerate systems (extended model space), which is not possible with the standard QED technique based upon the S-matrix formulation. To include in the model space closely lying energy levels, such as fine-structure levels, can have a dramatic effect on the convergence rate. The electron-electron interaction is investigated in detail, and it is shown that it can be separated into irreducible multi-photon interactions, which in principle can be iterated as in standard MBPT. Singularities do not appear, and a simple procedure for evaluating residual finite contributions is described. Comparison is made with the closely related Green's function technique. The procedure is presently being tested on the fine-structure levels of He-like ions.     

Theoretical Analysis of Rydberg and Autoionizing State Spectra of Antimony

We calculate the Rydberg and autoionization Rydbergspectra of antimony(Sb) from first principles by relativistic multichannel theory within the framework of multichannel quantum defect theory. Our calculation can be used to classify and assign the atomic states described in recently reported three Rydberg series and four autoionizing states. The perturbation effects on line intensity, variation and line profile are discussed. Assignments of the perturber states and autoionizing states are presented.     

Quantum electrodynamics as a supersymmetric theory of Loops

It is shown that the Wilson loop is not a renormalizable operator even in free QED. A modification is suggested and proven to be renormalizable. Feynman's worldline formulation of QED is generalized to include spinning particles and matter loops, using this modified Wilson loop. QED formulated this way is shown to have a supersymmetry. A superfield formalism is found that describes it. QED is rewritten as a second quantized field theory of loop functionals.