839 research outputs found
Level crossings in a cavity QED model
In this paper I study the dynamics of a two-level atom interacting with a
standing wave field. When the atom is subjected to a weak linear force, the
problem can be turned into a time dependent one, and the evolution is
understood from the band structure of the spectrum. The presence of level
crossings in the spectrum gives rise to Bloch oscillations of the atomic
motion. Here I investigate the effects of the atom-field detuning parameter. A
variety of different level crossings are obtained by changing the magnitude of
the detuning, and the behaviour of the atomic motion is strongly affected due
to this. I also consider the situation in which the detuning is oscillating in
time and its impact on the atomic motion. Wave packet simulations of the full
problem are treated numerically and the results are compared with analytical
solutions given by the standard Landau-Zener and the three-level Landau-Zener
models.Comment: 12 pages, 10 figure
New spin squeezing and other entanglement tests for two mode systems of identical bosons
For any quantum state representing a physical system of identical particles, the density operator must satisfy the symmetrization principle (SP) and conform to super-selection rules (SSR) that prohibit coherences between differing total particle numbers. Here we consider bi-partitite states for massive bosons, where both the system and sub-systems are modes (or sets of modes) and particle numbers for quantum states are determined from the mode occupancies. Defining non-entangled or separable states as those prepared via local operations (on the sub-systems) and classical communication processes, the sub-system density operators are also required to satisfy the SP and conform to the SSR, in contrast to some other approaches. Whilst in the presence of this additional constraint the previously obtained sufficiency criteria for entanglement, such as the sum of the ˆSx and ˆSy variances for the Schwinger spin components being less than half the mean boson number, and the strong correlation test of |haˆm (bˆ†)ni|2 being greater than h(aˆ†)maˆm (bˆ†)nbˆni(m, n = 1, 2, . . .) are still valid, new tests are obtained in our work. We show that the presence of spin squeezing in at least one of the spin components ˆSx , ˆSy and ˆSz is a sufficient criterion for the presence of entanglement and a simple correlation test can be constructed of |haˆm (bˆ†)ni|2 merely being greater than zero.We show that for the case of relative phase eigenstates, the new spin squeezing test for entanglement is satisfied (for the principle spin operators), whilst the test involving the sum of the ˆSx and ˆSy variances is not. However, another spin squeezing entanglement test for Bose–Einstein condensates involving the variance in ˆSz being less than the sum of the squared mean values for ˆSx and ˆSy divided by the boson number was based on a concept of entanglement inconsistent with the SP, and here we present a revised treatment which again leads to spin squeezing as an entanglement test
Evaporative cooling in a radio-frequency trap
A theoretical investigation for implementing a scheme of forced evaporative
cooling in radio-frequency (rf) adiabatic potentials is presented. Supposing
the atoms to be trapped by a rf field RF1, the cooling procedure is facilitated
using a second rf source RF2. This second rf field produces a controlled
coupling between the spin states dressed by RF1. The evaporation is then
possible in a pulsed or continuous mode. In the pulsed case, atoms with a given
energy are transferred into untrapped dressed states by abruptly switching off
the interaction. In the continuous case, it is possible for energetic atoms to
adiabatically follow the doubly-dressed states and escape out of the trap. Our
results also show that when the frequencies of the fields RF1 and RF2 are
separated by at least the Rabi frequency associated with RF1, additional
evaporation zones appear which can make this process more efficient.Comment: 12 pages, 11 figure
Control of atomic decay rates via manipulation of reservoir mode frequencies
We analyse the problem of a two-level atom interacting with a time-dependent
dissipative environment modelled by a bath of reservoir modes. In the model of
this paper the principal features of the reservoir structure remain constant in
time, but the microscopic structure does not. In the context of an atom in a
leaky cavity this corresponds to a fixed cavity and a time-dependent external
bath. In this situation we show that by chirping the reservoir modes
sufficiently fast it is possible to inhibit, or dramatically enhance the decay
of the atomic system, even though the gross reservoir structure is fixed. Thus
it is possible to extract energy from a cavity-atom system faster than the
empty cavity rate. Similar, but less dramatic effects are possible for moderate
chirps where partial trapping of atomic population is also possible.Comment: 12 pages, 9 figure
Two-dimensional atom trapping in field-induced adiabatic potentials
We show how to create a novel two-dimensional trap for ultracold atoms from a conventional magnetic trap. We achieve this by utilizing rf-induced adiabatic potentials to enhance the trapping potential in one direction. We demonstrate the loading process and discuss the experimental conditions under which it might be possible to prepare a 2D Bose condensate. A scheme for the preparation of coherent matterwave bubbles is also discussed
Sudden death and sudden birth of entanglement in common structured reservoirs
We study the exact entanglement dynamics of two qubits in a common structured
reservoir. We demonstrate that, for certain classes of entangled states,
entanglement sudden death occurs, while for certain initially factorized
states, entanglement sudden birth takes place. The backaction of the
non-Markovian reservoir is responsible for revivals of entanglement after
sudden death has occurred, and also for periods of disentanglement following
entanglement sudden birth.Comment: 4 pages, 2 figure
Observing the spin of a free electron
Long ago, Bohr, Pauli, and Mott argued that it is not, in principle, possible to measure the spin components of a free electron. One can try to use a Stern-Gerlach type of device, but the finite size of the beam results in an uncertainty of the splitting force that is comparable with the gradient force. The result is that no definite spin measurement can be made. Recently there has been a revival of interest in this problem, and we will present our own analysis and quantum-mechanical wave-packet calculations which suggest that a spin measurement is possible for a careful choice of initial conditions
Molecular heat pump for rotational states
In this work we investigate the theory for three different uni-directional
population transfer schemes in trapped multilevel systems which can be utilized
to cool molecular ions. The approach we use exploits the laser-induced coupling
between the internal and motional degrees of freedom so that the internal state
of a molecule can be mapped onto the motion of that molecule in an external
trapping potential. By sympathetically cooling the translational motion back
into its ground state the mapping process can be employed as part of a cooling
scheme for molecular rotational levels. This step is achieved through a common
mode involving a laser-cooled atom trapped alongside the molecule. For the
coherent mapping we will focus on adiabatic passage techniques which may be
expected to provide robust and efficient population transfers. By applying
far-detuned chirped adiabatic rapid passage pulses we are able to achieve an
efficiency of better than 98% for realistic parameters and including
spontaneous emission. Even though our main focus is on cooling molecular
states, the analysis of the different adiabatic methods has general features
which can be applied to atomic systems
Quantum metrology at the Heisenberg limit with ion traps
Sub-Planck phase-space structures in the Wigner function of the motional
degree of freedom of a trapped ion can be used to perform weak force
measurements with Heisenberg-limited sensitivity. We propose methods to
engineer the Hamiltonian of the trapped ion to generate states with such small
scale structures, and we show how to use them in quantum metrology
applications.Comment: 10 pages, 6 figure
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