Rosen--Zener Transition in a Nonlinear System for Two-Component Bose--Einstein Condensates in Optical Lattices

We study the Rosen-Zener transition （RZT） in a nonlinear system for two-component Bose-Einstein condensates in optical lattices. It is found that the percentage of the components could affect the quantum transition dramatically. For the component with large percentage it is equivalent that the effect of the nonlinearity is stronger, whereas for the component with small percentage the effect is weaker. We also find that the nonlinearity c11 can affect the quantum transition dramatically. This is similar to that reported from Ref. [14]. Compared with one-component systems, however, the effect of the nonlinearity is decreased due to the two components of the BEGs in optical lattices. Furthermore, the effect of the coupling nonlinearity between two components c12 is studied. The component with large percentage is more affected by the nonlinearity than that with small-percentage component.     

Dynamics of Bose-Einstein condensates with atomic pumping and dissipative processes

In the dynamics of condensed systems described by a nonconservative non-linear Schrödinger equation, we investigate numerically the role of atomic feeding and inelastic collisions where dipolar relaxation is the dominant process. By analyzing the range of possible values of the nonconservative parameters to have stable (or unstable) solutions, we verify that spatio-temporal chaos patterns, known for attractive two-body systems due to collapsing effects, can also occur in the repulsive case when the term corresponding to the linear atomic feeding dominates the nonconservative contributions.     

Optimized temperature measurement with time-of-flight method

Driven by resonance probe light background Rubidium gas can emit fluorescence, which interacts with the falling atomic cloud in the temperature measurement with the time-of-flight (TOF) method and influence the experimental results. The dependencies of the acceleration and the temperature of the atomic cloud on the detuning of the probe beams were studied. We propose that the deviation of the acceleration from the gravity acceleration can be taken as a criterion of the accuracy of the temperature measurement using TOF method. Moreover, using the principle of the radiation force on the cold atomic cloud, one may measure the considerably weak intensity of the fluorescence.     

Next-nearest-neighbor-tunneling-induced symmetry breaking of Hofstadter's butterfly spectrum for ultracold atoms on the honeycomb lattice

We study the spectrum of ultracold atoms on the honeycomb lattice under a constant effective magnetic field. In the tight-binding approximation, we derived the generalized Harper's equations and numerically calculate the spectrum, which has a fractal structure. For the cases with and without the next-nearest-neighbor tunneling, the graphs of spectra have different symmetries. Comparing the symmetries of the graphs of spectra, we find that next-nearest-neighbor tunneling induces symmetry breaking of the graph of spectrum.     

Arbitrary quantum superposition state for three-level system using oscillating dark states

An method for adiabatic population transfer and the preparation of an arbitrary quantum superposition state using the oscillating dark states (ODS) in atomic system is presented. Quantum state of a three-level Λ configuration atomic system finally evolves into the same time-dependent state, and oscillates periodically between two ground levels under evolving adiabatic conditions when two pairs of classical detuning laser fields drive the system into the ODS forcedly, whatever the initial states of the system are. The decoherence of the ODS evolution is greatly suppressed and the oscillation is very stable, therefore adiabatic population transfer and the preparation of an arbitrary quantum superposition state of atomic system can be completed accurately and conveniently.     

Cooling and Trapping 88Sr Atoms with 461nm Laser

We report the experimental realization of a ^88Sr magneto-optical trap （MOT） operating at the wavelength of 461 nm. The MOT is loaded via a 32 cm long spin-flip type Zeeman slower which enhances the MOT population by a factor of 22. The total laser power available in our experiment is about 300mW. We have trapped 1.6 × 10^8 ^88 Sr atoms with a 679nm and 707nm repumping laser. The two repumping lasers enhance the trap population and trap lifetime by factors of 11 and 7, respectively. The ^88 Sr cloud has a temperature of about 2.3 mK, measured by recording the time evolution of the absorption signal.     

Three-Dimensional Array for 40Ca+ Ion Trapping

We present a three-dimensional scalable linear ion trap scheme for ion trapping and discuss its applications for the optical frequency standard and scalable quantum information processing with its parallel strings of trapped 40Ca＋ ions. The geometry here contains nine equal-distance parallel rods driven by rf, which form trapping potentials for radial confinement and two end ring electrodes biased at a few volts for axial confinement. Its feasibility is calculated by using the finite element analysis method.     

Study of Dark-Hollow Beams Generated with Different Multimode Fibres

A dark-hollow beam （DHB） is generated by a coupfing of a single fundamental mode He-Ne laser beam with a misalignment multimode fibre （MMF） in a special way. Effects of the misalignment angle, diameter and length of the MMF are studied. The generated DHBs can be used for guiding and trapping of atoms, manipulating particles, or as optical tweezers.     

Light-Induced Hofstadter＇s Butterfly Spectrum of Ultracold Atoms on the Two-Dimensional Kagomé Lattice

We investigate the energy spectrum of ultracold atoms on the two-dimensional Kagomé optical lattice under an effective magnetic field, which can be realized with laser beams. We derive the generalized Harper＇s equations from the Schr？dinger equation. The energy spectrum with a fractal band structure is obtained by numerically solving the generalized Harper＇s equations. We analyze the properties of the Hofstadter＇s butterfly spectrum and discuss its observability.     

Hofstadter's butterfly energy spectrum of ultracold fermions on the two-dimensional triangular optical lattice

We study the energy spectrum of ultracold fermionic atoms on the two-dimensional triangular optical lattice subjected to a perpendicular effective magnetic field, which can be realized with laser beams. We derive the generalized Harper's equations and numerically solve them, then we obtain the Hofstadter's butterfly-like energy spectrum, which has a novel fractal structure. The observability of the Hofstadter's butterfly spectrum is also discussed.     

Simplified approach to generate controlled-NOT gates with single trapped ions for arbitrary Lamb-Dicke parameters

For certain specific (or “magic”) Lamb-Dicke (LD) parameters, [Phys. Rev. A 55 (1997) R2489] showed that a two-qubit quantum operation, between the external and internal degrees of freedom of a single trapped ion, could be implemented by applying a single carrier laser pulse. Here, we further show that, such a two-qubit operation (which is equivalent to the standard CNOT gate, only apart from certain phase factors) could also be significantly-well realized for arbitrarily selected LD parameters. Instead of the so-called “π-pulses” used in the previous demonstrations, the durations of the pulses applied in the present proposal are required to be accurately set within the decoherence times of the ion. We also propose a simple approach by using only one off-resonant (e.g., blue-sideband) laser pulse to eliminate the unwanted phase factors existed in the above two-qubit operations for generating the standard CNOT gates.     

Sub-Half-Wavelength Atom Localization Based on Phase-Dependent Electromagnetically Induced Transparency

We present a realistic and efficient scheme for sub-half-wavelength atom localization. This scheme is based on the phase-dependent electromagnetically induced transparency in a four-level system in the double-A configuration. We use a strong bichromatic field （one component of which is standing-wave field） as the driving components, and a weak bichromatic field as the probe components. By choosing the collective phase of the four applied components, the atom is localized in either of the two half-wavelength regions with 50% detecting probability when the absorption to the probe fields is detected.     

Validity of Lamb-Dicke Approximations in Ion-Trap Systems

The Lamb-Dicke （LD） approximation （LDA） is usually utilized to simplify the treatments for the dynamics of ion-trap systems, where the so-called LD parameters should be sufficiently small. In this Letter, based on the quantum dynamics of a single trapped ion beyond the LDA, we discuss the fidelities of the control-NOT （CNOT） gate generated by performing the usual LDA. It is shown that the fidelity of the generated CNOT gate under the LDA is sufficiently high for the current LD experiments, e.g., it reaches 99.9% for η=0.20. The validity of the LDA is also discussed by calculating these fidelities for slightly larger LD parameters, e.g., η=0.4, etc.     

Photo-induced dynamics of Bose-Einstein condensates in optical superlattice

The photo-induced dynamics of cold atoms in a one-dimensional optical superlattice is observed. Steady state distribution of the probability amplitudes and the site population in a one-dimensional optical superlattice is found. It is shown that this solution of the equations, which describes the temporal behavior of a Bose-Einstein condensate in a superlattice, is unstable at the sufficiently high level of boson density. The expression for the increment of modulational instability is obtained on the basis of the linear stability analysis. The numerical examples of non-stationary solutions for boson density in a superlattice for the general model are discussed as applied to both the attraction and repulsion potentials of boson interaction.     

Bose-Einstein Condensates in a One-Dimensional Optical Lattice: from Superfluidity to Number-Squeezed States

We study the phase coherence property of Bose-Einstein condensates confined in a one-dimensional optical lattice formed by a standing-wave laser field. The lattice depth is determined using a method of Kapitza-Dirac scattering between a condensate and a short pulse lattice potential. Condensates are then adiabatically loaded into the optical lattice. The phase coherence property of the confined condensates is reflected by the interference patterns of the expanded atomic cloud released from the optical lattice. For weak lattice, nearly all of the atoms stay in a superfluid state. However, as the lattice depth is increased, the phase coherence of the whole condensate sample is gradually lost, which confirms that the sub-condensates in each lattice well have evolved into number-squeezed states.     

Rescattering and vibrations in homonuclear diatomic molecules in a strong electromagnetic field

The electron of a driven by a strong electromagnetic field induces molecular vibrations. Numerical and analytical results show that the molecule behaves as a parametric oscillator and can be treated as a kicked oscillator. The results are discussed from the point of view of the electron's periodic dressing and undressing processes.     

Quantum computing with trapped ions

Quantum computers hold the promise of solving certain computational tasks much more efficiently than classical computers. We review recent experimental advances towards a quantum computer with trapped ions. In particular, various implementations of qubits, quantum gates and some key experiments are discussed. Furthermore, we review some implementations of quantum algorithms such as a deterministic teleportation of quantum information and an error correction scheme.     

Stable Narrow Linewidth 689nm Diode Laser for the Second Stage Cooling and Trapping of Strontium Atoms

We report stable narrow linewidth laser systems based on self-developed Littman configuration external cavity diode lasers （ECDLs）. The frequency of the ECDL is stabilized to a high fineness ultralow-expansion glass reference cavity with the Pound-Drever-Hall technique. By heterodyne beating of two identical systems, we conclude that the linewidth 4.3× 10^-14 at an averaging measurement time. of each ECDL is reduced to lower than 150 Hz and its frequency stability reaches time of 1 s, the averaged long-term frequency drift is less than 0.2 Hz/s over 30 h     

Experimental study of vapor-cell magneto-optical traps for efficient trapping of radioactive atoms

We have studied magneto-optical traps (MOTs) for efficient on-line trapping of radioactive atoms. After discussing a model of the trapping process in a vapor cell and its efficiency, we present the results of detailed experimental studies on Rb MOTs. Three spherical cells of different sizes were used. These cells can be easily replaced, while keeping the rest of the apparatus unchanged: atomic sources, vacuum conditions, magnetic field gradients, sizes and power of the laser beams, detection system. By direct comparison, we find that the trapping efficiency only weakly depends on the MOT cell size. It is also found that the trapping efficiency of the MOT with the smallest cell, whose diameter is equal to the diameter of the trapping beams, is about 40% smaller than the efficiency of larger cells. Furthermore, we also demonstrate the importance of two factors: a long coated tube at the entrance of the MOT cell, used instead of a diaphragm; and the passivation with an alkali vapor of the coating on the cell walls, in order to minimize the losses of trappable atoms. These results guided us in the construction of an efficient large-diameter cell, which has been successfully employed for on-line trapping of Fr isotopes at INFN’s national laboratories in Legnaro, Italy.     

Expansion dynamics of an array of high density Bose-Einstein condensates produced in a 1D CO<Subscript>2</Subscript> laser optical lattice

We discuss the expansion dynamics under mean-field repulsion of an array of ⁸⁷Rb Bose-Einstein condensates produced in an all-optical scheme involving 1D lattice with nearly 10⁵ atoms, after fast evaporative cooling of just about 1 s. Single site occupation exceeds 2 × 10⁴ in our experiments. The possibility of transition to two-dimensionality was also investigated. The expansion behavior of the high density multiple micro-condensates produced directly in the CO₂ laser 1D optical lattice, with a lattice spacing of 5.3 μm, agrees well with a numerical simulation based on the mean-field theory.