417 research outputs found
Coherent Particle Transfer in an On-Demand Single-Electron Source
Coherent electron transfer from a localized state trapped in a quantum dot
into a ballistic conductor, taking place in on-demand electron sources, in
general may result in excitation of particle-hole pairs. We consider a simple
model for these effects, involving a resonance level with time-dependent
energy, and derive Floquet scattering matrix describing inelastic transitions
of particles in the Fermi sea. We find that, as the resonance level is driven
through the Fermi level, particle transfer may take place completely without
particle-hole excitations for certain driving protocols. In particular, such
noiseless transfer occurs when the level moves with constant rapidity, its
energy changing linearly with time. A detection scheme for studying the
coherence of particle transfer is proposed.Comment: 5 pages, 3 figures. Updated introduction, Fig. 1, and reference
Electron attachment to SF6 and lifetimes of SF6- negative ions
We study the process of low-energy electron capture by the SF6 molecule. Our
approach is based on the model of Gauyacq and Herzenberg [J. Phys. B 17, 1155
(1984)] in which the electron motion is coupled to the fully symmetric
vibrational mode through a weakly bound or virtual s state. By tuning the two
free parameters of the model, we achieve an accurate description of the
measured electron attachment cross section and good agreement with vibrational
excitation cross sections of the fully symmetric mode. An extension of the
model provides a limit on the characteristic time of intramolecular vibrational
relaxation in highly-excited SF6-. By evaluating the total vibrational spectrum
density of SF6-, we estimate the widths of the vibrational Feshbach resonances
of the long-lived negative ion. We also analyse the possible distribution of
the widths and its effect on the lifetime measurements, and investigate
nonexponential decay features in metastable SF6-.Comment: 22 pages, 10 figures, submitted to Phys. Rev.
Counterintuitive transitions in the multistate Landau-Zener problem with linear level crossings
We generalize the Brundobler-Elser hypothesis in the multistate Landau-Zener
problem to the case when instead of a state with the highest slope of the
diabatic energy level there is a band of states with an arbitrary number of
parallel levels having the same slope. We argue that the probabilities of
counterintuitive transitions among such states are exactly zero.Comment: 9 pages, 5 figure
Making Cold Molecules by Time-dependent Feshbach Resonances
Pairs of trapped atoms can be associated to make a diatomic molecule using a
time dependent magnetic field to ramp the energy of a scattering resonance
state from above to below the scattering threshold. A relatively simple model,
parameterized in terms of the background scattering length and resonance width
and magnetic moment, can be used to predict conversion probabilities from atoms
to molecules. The model and its Landau-Zener interpretation are described and
illustrated by specific calculations for Na, Rb, and Cs
resonances. The model can be readily adapted to Bose-Einstein condensates.
Comparison with full many-body calculations for the condensate case show that
the model is very useful for making simple estimates of molecule conversion
efficiencies.Comment: 11 pages, 11 figures; talk for Quantum Challenges Symposium, Warsaw,
Poland, September 4-7, 2003. Published in Journal of Modern Optics 51,
1787-1806 (2004). Typographical errors in Journal article correcte
Scalar and Spinor Particles with Low Binding Energy in the Strong Stationary Magnetic Field Studied by Means of Two-and Three-Dimensional Models
On the basis of analytic solutions of Schrodinger and Pauli equations for a
uniform magnetic field and a single attractive -potential the
equations for the bound one-active electron states are discussed. It is vary
important that ground electron states in the magnetic field essentially
different from the analog state of spin-0 particles that binding energy has
been intensively studied at more then forty years ago. We show that binding
energy equations for spin-1/2 particles can be obtained without using of a
well-known language of boundary conditions in the model of -potential
that has been developed in pioneering works. Obtained equations are used for
the analytically calculation of the energy level displacements, which
demonstrate nonlinear dependencies on field intensities. It is shown that in a
case of the weak intensity a magnetic field indeed plays a stabilizing role in
considering systems. However the strong magnetic field shows the opposite
action. We are expected that these properties can be of importance for real
quantum mechanical fermionic systems in two- and three-dimensional cases.Comment: 18 page
Non-divergent pseudo-potential treatment of spin-polarized fermions under 1D and 3D harmonic confinement
Atom-atom scattering of bosonic one-dimensional (1D) atoms has been modeled
successfully using a zero-range delta-function potential, while that of bosonic
3D atoms has been modeled successfully using Fermi-Huang's regularized s-wave
pseudo-potential. Here, we derive the eigenenergies of two spin-polarized 1D
fermions under external harmonic confinement interacting through a zero-range
potential, which only acts on odd-parity wave functions, analytically. We also
present a divergent-free zero-range potential treatment of two spin-polarized
3D fermions under harmonic confinement. Our pseudo-potential treatments are
verified through numerical calculations for short-range model potentials.Comment: 9 pages, 4 figures (subm. to PRA on 03/15/2004
Quantum theory of an atom laser originating from a Bose-Einstein condensate or a Fermi gas in the presence of gravity
We present a 3D quantum mechanical theory of radio-frequency outcoupled atom
lasers from trapped atomic gases in the presence of the gravitational force.
Predictions for the total outcoupling rate as a function of the radio-frequency
and for the beam wave function are given. We establish a sum rule for the
energy integrated outcoupling, which leads to a separate determination of the
coupling strength between the atoms and the radiation field.
For a non-interacting Bose-Einstein condensate analytic solutions are derived
which are subsequently extended to include the effects of atomic interactions.
The interactions enhance interference effects in the beam profile and modify
the outcoupling rate of the atom laser. We provide a complete quantum
mechanical solution which is in line with experimental findings and allows to
determine the validity of commonly used approximative methods.
We also extend the formalism to a fermionic atom laser and analyze the effect
of superfluidity on the outcoupling of atoms.Comment: 13 pages, 8 figures, slightly expanded versio
Stable Heteronuclear Few-Atom Bound States in Mixed Dimensions
We study few-body problems in mixed dimensions with heavy atoms
trapped individually in parallel one-dimensional tubes or two-dimensional
disks, and a single light atom travels freely in three dimensions. By using the
Born-Oppenheimer approximation, we find three- and four-body bound states for a
broad region of heavy-light atom scattering length combinations. Specifically,
the existence of trimer and tetramer states persist to negative scattering
lengths regime, where no two-body bound state is present. These few-body bound
states are analogous to the Efimov states in three dimensions, but are stable
against three-body recombination due to geometric separation. In addition, we
find that the binding energy of the ground trimer and tetramer state reaches
its maximum value when the scattering lengths are comparable to the separation
between the low-dimensional traps. This resonant behavior is a unique feature
for the few-body bound states in mixed dimensions.Comment: Extended version with 14 pages and 14 figure
Mean-field dynamics of a Bose-Einstein condensate in a time-dependent triple-well trap: Nonlinear eigenstates, Landau-Zener models and STIRAP
We investigate the dynamics of a Bose--Einstein condensate (BEC) in a
triple-well trap in a three-level approximation. The inter-atomic interactions
are taken into account in a mean-field approximation (Gross-Pitaevskii
equation), leading to a nonlinear three-level model. New eigenstates emerge due
to the nonlinearity, depending on the system parameters. Adiabaticity breaks
down if such a nonlinear eigenstate disappears when the parameters are varied.
The dynamical implications of this loss of adiabaticity are analyzed for two
important special cases: A three level Landau-Zener model and the STIRAP
scheme. We discuss the emergence of looped levels for an equal-slope
Landau-Zener model. The Zener tunneling probability does not tend to zero in
the adiabatic limit and shows pronounced oscillations as a function of the
velocity of the parameter variation. Furthermore we generalize the STIRAP
scheme for adiabatic coherent population transfer between atomic states to the
nonlinear case. It is shown that STIRAP breaks down if the nonlinearity exceeds
the detuning.Comment: RevTex4, 7 pages, 11 figures, content extended and title/abstract
change
Degenerate Landau-Zener model: Exact analytical solution
The exact analytical solution of the degenerate Landau-Zener model, wherein
two bands of degenerate energies cross in time, is presented. The solution is
derived by using the Morris-Shore transformation, which reduces the fully
coupled system to a set of independent nondegenerate two-state systems and a
set of decoupled states. Due to the divergence of the phase of the off-diagonal
element of the propagator in the original Landau-Zener model, not all
transition probabilities exist for infinite time duration. In general, apart
from some special cases, only the transition probabilities between states
within the same degenerate set exist, but not between states of different sets.
An illustration is presented for the transition between the magnetic sublevels
of two atomic levels with total angular momenta J=2 and 1
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