516 research outputs found
Selective coherent destruction of tunneling in a quantum-dot array
The coherent manipulation of quantum states is one of the main tasks required
in quantum computation. In this paper we demonstrate that it is possible to
control coherently the electronic position of a particle in a quantum-dot
array. By tuning an external ac electric field we can selectively suppress the
tunneling between dots, trapping the particle in a determined region of the
array. The problem is treated non-perturbatively by a time-dependent
Hamiltonian in the effective mass approximation and using Floquet theory. We
find that the quasienergy spectrum exhibits crossings at certain field
intensities that result in the selective suppression of tunneling.Comment: 4 pages, 5 figures, submitted to PRB Rapid Com
Bilinear and quadratic Hamiltonians in two-mode cavity quantum electrodynamics
In this work we show how to engineer bilinear and quadratic Hamiltonians in
cavity quantum electrodynamics (QED) through the interaction of a single driven
two-level atom with cavity modes. The validity of the engineered Hamiltonians
is numerically analyzed even considering the effects of both dissipative
mechanisms, the cavity field and the atom. The present scheme can be used, in
both optical and microwave regimes, for quantum state preparation, the
implementation of quantum logical operations, and fundamental tests of quantum
theory.Comment: 11 pages, 3 figure
Sensitivity of electromagnetically induced transparency to light-mediated interactions
Here we present a microscopic model that describes the Electromagnetically
Induced Transparency (EIT) phenomenon in the multiple scattering regime. We
consider an ensemble of cold three-level atoms, in a configuration,
scattering a probe and a control field to the vacuum modes of the
electromagnetic field. By first considering a scalar description of the
scattering, we show that the light-mediated long-range interactions that emerge
between the dipoles narrow the EIT transparency window for increasing densities
and sample sizes. For a vectorial description, we demonstrate that near-field
interacting terms can critically affect the atomic population transfer in the
Stimulated Raman Adiabatic Passage (STIRAP). This result points out that
standard STIRAP-based quantum memories in cold atomic ensembles would not reach
high enough efficiencies for quantum information processing applications even
in dilute regimes.Comment: 9 pages, 5 figure
Nonadiabatic coherent evolution of two-level systems under spontaneous decay
In this paper we extend current perspectives in engineering reservoirs by
producing a time-dependent master equation leading to a nonstationary
superposition equilibrium state that can be nonadiabatically controlled by the
system-reservoir parameters. Working with an ion trapped inside a nonindeal
cavity we first engineer effective Hamiltonians that couple the electronic
states of the ion with the cavity mode. Subsequently, two classes of
decoherence-free evolution of the superposition of the ground and decaying
excited levels are achieved: those with time-dependent azimuthal or polar
angle. As an application, we generalise the purpose of an earlier study [Phys.
Rev. Lett. 96, 150403 (2006)], showing how to observe the geometric phases
acquired by the protected nonstationary states even under a nonadiabatic
evolution.Comment: 5 pages, no figure
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