Laser cooling with electromagnetically induced transparency: application to trapped samples of ions or neutral atoms

A novel method of ground-state laser cooling of trapped atoms utilizes the absorption profile of a three- (or multi-) level system that is tailored by a quantum interference. With cooling rates comparable to conventional sideband cooling, lower final temperatures may be achieved. The method was experimentally implemented to cool a single Ca⁺ ion to its vibrational ground state. Since a broad band of vibrational frequencies can be cooled simultaneously, the technique will be particularly useful for the cooling of larger ion strings, thereby being of great practical importance for initializing a quantum register based on trapped ions. We also discuss its application to different level schemes and for ground-state cooling of neutral atoms trapped by a far-detuned standing wave laser field. Received: 10 July 2001 / Published online: 23 November 2001     

Scheme for direct measurement of the Wigner characteristic function of traveling fields

We present a proposal to measure field states for traveling modes. The scheme leads, in a simple and direct way, to the characteristic function of the state, yielding the determination of the Wigner function without a demanding data analysis. We employ a Mach-Zehnder interferometer including an auxiliary nonlinear medium in one arm. Analogies with other proposals to reconstruct states of stationary fields and trapped atoms are discussed.     

Efficient Scheme for Greenberger-Horne-Zeilinger State and Cluster State with Trapped Ions

We propose a scheme to generate the Greenberger-Horne-Zeilinger （GHZ） states and the cluster states of many trapped ions. In the scheme, the ion is illuminated by a single laser tuned to the first lower vibrational sideband. The scheme only requires resonant interactions. Thus the scheme is very simple and the quantum dynamics operation can be realized at a high speed, which is important in view of decoherence.     

Variance squeezing and information entropy squeezing via Bloch coherent states in two-level nonlinear spin models

The nonclassical squeezing effect emerging from a nonlinear coupling model (generalized Jaynes–Cummings model) of a two-level atom interacting resonantly with a bimodal cavity field via two-photon transitions is investigated in the rotating wave approximation. Various Bloch coherent initial states (rotated states) for the atomic system are assumed, i.e., (i) ground state, (ii) excited state, and (iii) linear superposition of both states. Initially, the atomic system and the field are in a disentangled state, where the field modes are in Glauber coherent states via Poisson distribution. The model is numerically tested against simulations of time evolution of the based Heisenberg uncertainty relation variance and Shannon information entropy squeezing factors. The quantum state purity is computed for the three possible initial states and used as a criterion to get information about the entanglement of the components of the system. Analytical expression of the total density operator matrix elements at t > 0 shows, in fact, the present nonlinear model to be strongly entangled, where each of the definite initial Bloch coherent states is reduced to statistical mixtures. Thus, the present model does not preserve the modulus of the Bloch vector.     

Ground state laser cooling using electromagnetically induced transparency

A laser cooling method for trapped atoms is described which achieves ground state cooling by exploiting quantum interference in a driven Lambda-shaped arrangement of atomic levels. The scheme is technically simpler than existing methods of sideband cooling, yet it can be significantly more efficient, in particular when several motional modes are involved, and it does not impose restrictions on the transition linewidth. We study the full quantum mechanical model of the cooling process for one motional degree of freedom and show that a rate equation provides a good approximation.     

Experimental realization of deterministic entanglement transformations of bipartite pure states

We experimentally realize a kind of entanglement transformations of bipartite pure states with 100% efficiency. The protocol employs two-outcome positive operator-valued measures (POVMs) and can transform two-photon maximally entangled state to any two-photon entangled pure state with 100% efficiency. The average fidelity of all output states is 96%. Moreover, the scheme to implement arbitrary POVM on single-photon polarization state is also discussed. In principle, our setup can be applied to any kind of entanglement transformations of two-qubit entangled states to achieve optimal successful probability.     

Preparation of Entanglement of Atoms and of Photons via Cavity Input-Output Process

We propose a method to prepare multipartite entangled states such as cluster states and graph states based on the cavity input-output process and single photon measurement. Two quantum gates, a controlled phase gate and a fusion gate between two atoms trapped in respective cavities, are proposed to prepare atomic cluster states and graph states with one and two dimensions. We also introduce a scheme that can generate an arbitrary multipartite photon duster state which uses two coherent states as a qubit basis.     

Trapping State in a System of Single Three-level Trapped Ion Driven by External Fields

We investigate the interaction of a single three-level trapped ion with two laser beams. By applying a unitary transformation and a small rotating transformation, an exact solution to this quantum system is obtained without performing the Lamb-Dicke approximation, and the trapping state is observed.     

Teleporting an Unknown Two-qubit Entangled State via Ion-Trap Systems

We propose a scheme for teleportation of an unknown two-qubit entangled state via trapped ions. In this scheme, we use the GHZ state as a quantum channel and the success probability can reach 1. The distinct advantage of our scheme is insensitive to the heating of the vibrational mode. In addition, Bell-state measurement is not required.     

Three-photon interference spectra in four-level atomic systems

We have considered the interference spectra that occur at the three-photon generated frequency arising from the interaction of three laser fields with a four-level atom, where two of the laser fields are on two-photon resonance with the three levels forming a “λ” scheme while the third laser operates between the second ground and the second excited state of the atom. At low intensities of all three laser fields, the overall intensity of the peak at the three-photon generated frequency, describing the spectrum of an electron in the second excited state, depends on the strength of the combined field of the two laser fields that are on two-photon resonance and it takes negative values. This indicates that light amplification without population inversion is likely to occur at the three-photon generated frequency. The combined field of the three laser fields induces multiphoton excitations near the three-photon generated frequency, whose peaks are characterized by linewidths which are much less than the natural linewidths of the atoms. These excitations describe absorption or stimulating emission processes depending on the values of the detunings of the laser fields. The derived results are graphically presented and discussed. Received: 24 January 2001 / Published online: 8 June 2001