879 research outputs found
Complementarity and Young's interference fringes from two atoms
The interference pattern of the resonance fluorescence from a J=1/2 to J=1/2
transition of two identical atoms confined in a three-dimensional harmonic
potential is calculated. Thermal motion of the atoms is included. Agreement is
obtained with experiments [Eichmann et al., Phys. Rev. Lett. 70, 2359 (1993)].
Contrary to some theoretical predictions, but in agreement with the present
calculations, a fringe visibility greater than 50% can be observed with
polarization-selective detection. The dependence of the fringe visibility on
polarization has a simple interpretation, based on whether or not it is
possible in principle to determine which atom emitted the photon.Comment: 12 pages, including 7 EPS figures, RevTex. Submitted to Phys. Rev.
Testing the stability of fundamental constants with the 199Hg+ single-ion optical clock
Over a two-year duration, we have compared the frequency of the 199Hg+ 5d106s
2S 1/2 (F=0) 5d9 6s2 2D 5/2 (F=2) electric-quadrupole transition at 282 nm
with the frequency of the ground-state hyperfine splitting in neutral 133Cs.
These measurements show that any fractional time variation of the ratio
nu(Cs)/nu(Hg) between the two frequencies is smaller than +/- 7 10^-15 / yr (1
sigma uncertainty). According to recent atomic structure calculations, this
sets an upper limit to a possible fractional time variation of g(Cs) m_e / m_p
alpha^6.0 at the same level.Comment: 4 pages with 3 figures. RevTeX 4, Submitted to Phys. Rev. Let
Effects of Herbage Species on the Spatial Heterogeneity of Biomass in Grazed Pasture: Kentucky Bluegrass vs. White Clover
Terahertz frequency standard based on three-photon coherent population trapping
A scheme for a THz frequency standard based on three-photon coherent
population trapping in stored ions is proposed. Assuming the propagation
directions of the three lasers obey the phase matching condition, we show that
stability of few 10 at one second can be reached with a precision
limited by power broadening to in the less favorable case. The
referenced THz signal can be propagated over long distances, the useful
information being carried by the relative frequency of the three optical
photons.Comment: article soumis a PRL le 21 mars 2007, accepte le 10 mai, version 2
(24/05/2007
'Designer atoms' for quantum metrology
Entanglement is recognized as a key resource for quantum computation and
quantum cryptography. For quantum metrology, the use of entangled states has
been discussed and demonstrated as a means of improving the signal-to-noise
ratio. In addition, entangled states have been used in experiments for
efficient quantum state detection and for the measurement of scattering
lengths. In quantum information processing, manipulation of individual quantum
bits allows for the tailored design of specific states that are insensitive to
the detrimental influences of an environment. Such 'decoherence-free subspaces'
protect quantum information and yield significantly enhanced coherence times.
Here we use a decoherence-free subspace with specifically designed entangled
states to demonstrate precision spectroscopy of a pair of trapped Ca+ ions; we
obtain the electric quadrupole moment, which is of use for frequency standard
applications. We find that entangled states are not only useful for enhancing
the signal-to-noise ratio in frequency measurements - a suitably designed pair
of atoms also allows clock measurements in the presence of strong technical
noise. Our technique makes explicit use of non-locality as an entanglement
property and provides an approach for 'designed' quantum metrology
Dynamics of Quantum Collapse in Energy Measurements
The influence of continuous measurements of energy with a finite accuracy is
studied in various quantum systems through a restriction of the Feynman
path-integrals around the measurement result. The method, which is equivalent
to consider an effective Schr\"odinger equation with a non-Hermitian
Hamiltonian, allows one to study the dynamics of the wavefunction collapse. A
numerical algorithm for solving the effective Schr\"odinger equation is
developed and checked in the case of a harmonic oscillator. The situations, of
physical interest, of a two-level system and of a metastable quantum-well are
then discussed. In the first case the Zeno inhibition observed in quantum
optics experiments is recovered and extended to nonresonant transitions, in the
second one we propose to observe inhibition of spontaneous decay in mesoscopic
heterostructures. In all the considered examples the effect of the continuous
measurement of energy is a freezing of the evolution of the system proportional
to the accuracy of the measurement itself.Comment: 20 pages with figures, compressed and uuencoded ps fil
Doppler cooling of a Coulomb crystal
We study theoretically Doppler laser-cooling of a cluster of 2-level atoms
confined in a linear ion trap. Using several consecutive steps of averaging we
derive, from the full quantum mechanical master equation, an equation for the
total mechanical energy of the one dimensional crystal, defined on a
coarse-grained energy scale whose grid size is smaller than the linewidth of
the electronic transition. This equation describes the cooling dynamics for an
arbitrary number of ions and in the quantum regime. We discuss the validity of
the ergodic assumption (i.e. that the phase space distribution is only a
function of energy). From our equation we derive the semiclassical limit (i.e.
when the mechanical motion can be treated classically) and the Lamb-Dicke limit
(i.e. when the size of the mechanical wave function is much smaller than the
laser wavelength). We find a Fokker-Planck equation for the total mechanical
energy of the system, whose solution is in agreement with previous analytical
calculations which were based on different assumptions and valid only in their
specific regimes. Finally, in the classical limit we derive an analytic
expression for the average coupling, by light scattering, between motional
states at different energies.Comment: 19 pages, 3 figure
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