482 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
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
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
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
Quantum Zeno effect and parametric resonance in mesoscopic physics
As a realization of the quantum Zeno effect, we consider electron tunneling
between two quantum dots with one of the dots coupled to a quantum point
contact detector. The coupling leads to decoherence and to the suppression of
tunneling. When the detector is driven with an ac voltage, a parametric
resonance occurs which strongly counteracts decoherence. We propose a novel
experiment with which it is possible to observe both the quantum Zeno effect
and the parametric resonance in electric transport.Comment: 4 pages, 2 figure
Quantum damping of position due to energy measurements
Quantum theory for measurements of energy is introduced and its consequences
for the average position of monitored dynamical systems are analyzed. It turns
out that energy measurements lead to a localization of the expectation values
of other observables. This is manifested, in the case of position, as a damping
of the motion without classical analogue. Quantum damping of position for an
atom bouncing on a reflecting surface in presence of a homogeneous
gravitational field is dealt in detail and the connection with an experiment
already performed in the classical regime is studied. We show that quantum
damping is testable provided that the same measurement strength obtained in the
experimental verification of the quantum Zeno effect in atomic spectroscopy [W.
M. Itano et al., Phys. Rev. A {\bf 41}, 2295 (1990)] is made available.Comment: 19 pages + 4 figures available upon request; Plain REVTeX; To be
published in Phys. Rev.
Absolute Frequency Measurements of the Hg^+ and Ca Optical Clock Transitions with a Femtosecond Laser
The frequency comb created by a femtosecond mode-locked laser and a
microstructured fiber is used to phase coherently measure the frequencies of
both the Hg^+ and Ca optical standards with respect to the SI second as
realized at NIST. We find the transition frequencies to be f_Hg=1 064 721 609
899 143(10) Hz and f_Ca=455 986 240 494 158(26) Hz, respectively. In addition
to the unprecedented precision demonstrated here, this work is the precursor to
all-optical atomic clocks based on the Hg^+ and Ca standards. Furthermore, when
combined with previous measurements, we find no time variations of these atomic
frequencies within the uncertainties of |(df_Ca/dt)/f_Ca| < 8 x 10^{-14}
yr^{-1}, and |(df_Hg/dt)/f_Hg|< 30 x 10^{-14} yr^{-1}.Comment: 6 pages, including 4 figures. RevTex 4. Submitted to Phys. Rev. Let
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