arXiv subjects

K. -A. Suominen

Publications and source records attributed to K. -A. Suominen.

52 records · Page 3Linked to original sources

arXiv2001

Atomic collision dynamics in optical lattices

We simulate collisions between two atoms, which move in an optical lattice under the dipole-dipole interaction. The model describes simultaneously the two basic dynamical processes, namely the Sisyphus cooling of single atoms, and the light-induced inelastic collisions between them. We consider the J=1/2 -> J=3/2 laser cooling transition for Cs, Rb and Na. We find that the hotter atoms in a thermal sample are selectively lost or heated by the collisions, which modifies the steady state distribution of atomic velocities, reminiscent of the evaporative cooling process.

physics.atom-ph↗

arXiv2001

Generation and evolution of vortex-antivortex pairs in Bose-Einstein condensates

We propose a method for generating and controlling a spatially separated vortex--antivortex pair in a Bose-Einstein condensate trapped in a toroidal potential. Our simulations of the time dependent Gross-Pitaevskii equation show that in toroidal condensates vortex dynamics are different from the dynamics in the homogeneous case. Our numerical results agree well with analytical calculations using the image method. Our proposal offers an effective example of coherent generation and control of vortex dynamics in atomic condensates.

cond-mat↗

arXiv2001

Comment on "Bose-Einstein condensation with magnetic dipole-dipole forces"

The ground state solutions of a dilute Bose condensate with contact and magnetic dipole-dipole interactions are examined. By lowering the value of the scattering length, Goral et al. [cond-mat/9907308 and Phys. Rev. A {\bf 61}, 051601 (2000)] numerically predict a region of unstable solutions, accompanied by a neighborhood where the ground-state wave functions have internal structure. On the contrary, we find that the dipolar condensate has an intuitively-located stability region, and ground-state solutions near the instability threshold which are absent any unusual structure.

cond-mat.stat-mech↗

arXiv2001

Calculations of collisions between cold alkaline earth atoms in a weak laser field

We calculate the light-induced collisional loss of laser-cooled and trapped magnesium atoms for detunings up to 50 atomic linewidths to the red of the ^1S_0-^1P_1 cooling transition. We evaluate loss rate coefficients due to both radiative and nonradiative state-changing mechanisms for temperatures at and below the Doppler cooling temperature. We solve the Schrodinger equation with a complex potential to represent spontaneous decay, but also give analytic models for various limits. Vibrational structure due to molecular photoassociation is present in the trap loss spectrum. Relatively broad structure due to absorption to the Mg_2 ^1Sigma_u state occurs for detunings larger than about 10 atomic linewidths. Much sharper structure, especially evident at low temperature, occurs even at smaller detunings due to of Mg_2 ^1Pi_g absorption, which is weakly allowed due to relativistic retardation corrections to the forbidden dipole transition strength. We also perform model studies for the other alkaline earth species Ca, Sr, and Ba and for Yb, and find similar qualitative behavior as for Mg.

physics.atom-ph↗

arXiv2000

Cold collisions between atoms in optical lattices

We have simulated binary collisions between atoms in optical lattices during Sisyphus cooling. Our Monte Carlo Wave Function simulations show that the collisions selectively accelerate mainly the hotter atoms in the thermal ensemble, and thus affect the steady state which one would normally expect to reach in Sisyphus cooling without collisions.

physics.atom-ph↗

arXiv2000

Tailoring of vibrational state populations with light-induced potentials in molecules

We propose a method for achieving highly efficient transfer between the vibrational states in a diatomic molecule. The process is mediated by strong laser pulses and can be understood in terms of light-induced potentials. In addition to describing a specific molecular system, our results show how, in general, one can manipulate the populations of the different quantum states in double well systems.

quant-ph↗

arXiv1999

Atomic dynamics in evaporative cooling of trapped alkali atoms in strong magnetic fields

We investigate how the nonlinearity of the Zeeman shift for strong magnetic fields affects the dynamics of rf field induced evaporative cooling in magnetic traps. We demonstrate for the 87-Rb and 23-Na F=2 trapping states with wave packet simulations how the cooling stops when the rf field frequency goes below a certain limit (for the 85-Rb F=2 trapping state the problem does not appear). We examine the applicability of semiclassical models for the strong field case as an extension of our previous work [Phys. Rev. A 58, 3983 (1998)]. Our results verify many of the aspects observed in a recent $^{87}$Rb experiment [Phys. Rev. A 60, R1759 (1999)].

physics.atom-ph↗

arXiv1999

The validity of the Landau-Zener model for output coupling of Bose condensates

We investigate the validity of the Landau-Zener model in describing the output coupling of Bose condensates from magnetic traps by a chirped radiofrequency field. The predictions of the model are compared with the numerical solutions of the Gross-Pitaevskii equation. We find a dependence on the chirp direction, and also quantify the role of gravitation.

cond-mat↗

arXiv1998

Creation of coherent atomic superpositions by fractional STIRAP

We discuss a simple scheme for preparing atoms and molecules in an arbitrary preselected coherent superposition of quantum states. The technique, which we call fractional stimulated Raman adiabatic passage ({\it f-STIRAP}), is based upon (incomplete) adiabatic population transfer between an initial state $ψ_1$ and state $ψ_3$ through an intermediate state $ψ_2$. As in STIRAP, the Stokes pulse arrives before the pump pulse, but unlike STIRAP, the two pulses terminate simultaneously while maintaining a constant ratio of amplitudes. The independence of f-STIRAP from details of pulse shape and pulse area makes it the analog of conventional STIRAP in the creation of coherent superpositions of states. We suggest a smooth realization of f-STIRAP which requires only two laser pulses (which can be derived from a single laser) and at the same time ensures the automatic fulfillment of the asymptotic conditions at early and late times. Furthermore, we provide simple analytic estimates of the robustness of f-STIRAP against variations in the pulse intensity, the pulse delay, and the intermediate-state detuning, and discuss its possible extension to multistate systems.

quant-ph↗

arXiv1998

Nonlinear level crossing models

We examine the effect of nonlinearity at a level crossing on the probability for nonadiabatic transitions $P$. By using the Dykhne-Davis-Pechukas formula, we derive simple analytic estimates for $P$ for two types of nonlinear crossings. In the first type, the nonlinearity in the detuning appears as a {\it perturbative} correction to the dominant linear time dependence. Then appreciable deviations from the Landau-Zener probability $P_{LZ}$ are found to appear for large couplings only, when $P$ is very small; this explains why the Landau-Zener model is often seen to provide more accurate results than expected. In the second type of nonlinearity, called {\it essential} nonlinearity, the detuning is proportional to an odd power of time. Then the nonadiabatic probability $P$ is qualitatively and quantitatively different from $P_{LZ}$ because on the one hand, it vanishes in an oscillatory manner as the coupling increases, and on the other, it is much larger than $P_{LZ}$. We suggest an experimental situation when this deviation can be observed.

quant-ph↗

arXiv1998

Nonadiabatic dynamics in evaporative cooling of trapped atoms by a radio frequency field

Magnetically trapped neutral atoms can be cooled with the evaporation technique. This is typically done by using a radiofrequency (rf) field that adiabatically couples trapped and untrapped internal atomic states for atoms with kinetic energies above a value set by the field frequency. The rf-field can also induce nonadiabatic changes of internal atomic spin states (F,M) that lead to heating and enhanced loss of atoms. In this paper we use wave packet simulations to show that the evaporation process can induce these nonadiabatic transitions which change the internal spin state of doubly spin-polarized (2,2) trapped atoms. We also verify the validity of a multistate Landau-Zener model in describing the nonadiabatic dynamics. In addition, we calculate exchange relaxation rate coefficients for collisions between atoms in the (2,M) states of 23-Na atoms. Large exchange relaxation coefficients for 23-Na as compared to 87-Rb F=2 suggest that evaporative cooling of (2,2) Na will be more difficult than for the corresponding state of Rb.

physics.atom-ph↗

arXiv1997

The wave packet - a universal quantum object

We summarize the theoretical description of wave packets on molecular energy levels. We review the various quantum mechanical effects which can be studied and the models that can be verified on this system. This justifies our claim that the wave packet constitutes a universal quantum object.

quant-ph↗

arXiv1997

Adiabatic passage by light-induced potentials in molecules

We present the APLIP process (Adiabatic Passage by Light Induced Potentials) for the adiabatic transfer of a wave packet from one molecular potential to the displaced ground vibrational state of another. The process uses an intermediate state, which is only slightly populated, and a counterintuitive sequence of light pulses to couple the three molecular states. APLIP shares many features with STIRAP (stimulated Raman adiabatic passage), such as high efficiency and insensitivity to pulse parameters. However, in APLIP there is no two-photon resonance, and the main mechanism for the transport of the wave packet is a light-induced potential. The APLIP process appears to violate the Franck-Condon principle, because of the displacement of the wave packet, but does in fact take place on timescales which are at least a little longer than a vibrational timescale.

physics.chem-ph↗

arXiv1997

Quantum and Semiclassical Calculations of Cold Atom Collisions in Light Fields

We derive and apply an optical Bloch equation (OBE) model for describing collisions of ground and excited laser cooled alkali atoms in the presence of near-resonant light. Typically these collisions lead to loss of atoms from traps. We compare the results obtained with a quantum mechanical complex potential treatment, semiclassical Landau-Zener models with decay, and a quantum time-dependent Monte-Carlo wave packet (MCWP) calculation. We formulate the OBE method in both adiabatic and diabatic representations. We calculate the laser intensity dependence of collision probabilities and find that the adiabatic OBE results agree quantitatively with those of the MCWP calculation, and qualitatively with the semiclassical Landau-Zener model with delayed decay, but that the complex potential method or the traditional Landau-Zener model fail in the saturation limit.

physics.atom-ph↗

arXiv1997

Wave packet dynamics with Bose-Einstein condensates

We study wave packet dynamics of a Bose condensate in a periodically shaken trap. Dichotomy, that is, dynamic splitting of the condensate, and dynamic stabilization are analyzed in analogy with similar phenomena in the domain of atoms in strong laser fields.

quant-ph↗

arXiv1997

Time-dependent control of ultracold atoms and Bose-Einstein condensates in magnetic traps

With radiofrequency fields one can control ultracold atoms in magnetic traps. These fields couple the atomic spin states, and are used in evaporative cooling which can lead to Bose-Einstein condensation in the atom cloud. Also, they can be used in controlled release of the condensate from the trap, thus providing output couplers for atom lasers. In this paper we show that the time-dependent multistate models for these processes have exact solutions which are polynomials of the solutions of the corresponding two-state models. This allows simple, in many cases analytic descriptions for the time-dependent control of the magnetically trapped ultracold atoms.

physics.atom-ph↗