1,133 research outputs found
Antiferromagnetism in NiO Observed by Transmission Electron Diffraction
Neutron diffraction has been used to investigate antiferromagnetism since
1949. Here we show that antiferromagnetic reflections can also be seen in
transmission electron diffraction patterns from NiO. The diffraction patterns
taken here came from regions as small as 10.5 nm and such patterns could be
used to form an image of the antiferromagnetic structure with a nanometre
resolution.Comment: 10 pages, 7 figures. Typos corrected. To appear in Physical Review
Letter
All-Optical Switching Using the Quantum Zeno Effect and Two-Photon Absorption
We have previously shown that the quantum Zeno effect can be used to
implement quantum logic gates for quantum computing applications, where the
Zeno effect was produced using a strong two-photon absorbing medium. Here we
show that the Zeno effect can also be used to implement classical logic gates
whose inputs and outputs are high-intensity fields (coherent states). The
operation of the devices can be understood using a quasi-static analysis, and
their switching times are calculated using a dynamic approach. The two-photon
absorption coefficient of rubidium vapor is shown to allow operation of these
devices at relatively low power levels.Comment: 21 pages, 11 figures. Submitted to Phys. Rev.
Measurement of the ac Stark shift with a guided matter-wave interferometer
We demonstrate the effectiveness of a guided-wave Bose-Einstein condensate
interferometer for practical measurements. Taking advantage of the large arm
separations obtainable in our interferometer, the energy levels of the 87Rb
atoms in one arm of the interferometer are shifted by a calibrated laser beam.
The resulting phase shifts are used to determine the ac polarizability at a
range of frequencies near and at the atomic resonance. The measured values are
in good agreement with theoretical expectations. However, we observe a
broadening of the transition near the resonance, an indication of collective
light scattering effects. This nonlinearity may prove useful for the production
and control of squeezed quantum states.Comment: 5 pages, three figure
Nonlinear Interferometry via Fock State Projection
We use a photon-number resolving detector to monitor the photon number
distribution of the output of an interferometer, as a function of phase delay.
As inputs we use coherent states with mean photon number up to seven. The
postselection of a specific Fock (photon-number) state effectively induces
high-order optical non-linearities. Following a scheme by Bentley and Boyd
[S.J. Bentley and R.W. Boyd, Optics Express 12, 5735 (2004)] we explore this
effect to demonstrate interference patterns a factor of five smaller than the
Rayleigh limit.Comment: 4 pages, 5 figure
A molecular theory for two-photon and three-photon fluorescence polarization
In the analysis of molecular structure and local order in heterogeneous samples, multiphoton excitation of fluorescence affords chemically specific information and high-resolution imaging. This report presents the results of an investigation that secures a detailed theoretical representation of the fluorescence polarization produced by one-, two-, and three-photon excitations, with orientational averaging procedures being deployed to deliver the fully disordered limits. The equations determining multiphoton fluorescence response prove to be expressible in a relatively simple, generic form, and graphs exhibit the functional form of the multiphoton fluorescence polarization. Amongst other features, the results lead to the identification of a condition under which the fluorescence produced through the concerted absorption of any number of photons becomes completely unpolarized. It is also shown that the angular variation of fluorescence intensities is reliable indicator of orientational disorder
An atom interferometer enabled by spontaneous decay
We investigate the question whether Michelson type interferometry is possible
if the role of the beam splitter is played by a spontaneous process. This
question arises from an inspection of trajectories of atoms bouncing
inelastically from an evanescent-wave (EW) mirror. Each final velocity can be
reached via two possible paths, with a {\it spontaneous} Raman transition
occurring either during the ingoing or the outgoing part of the trajectory. At
first sight, one might expect that the spontaneous character of the Raman
transfer would destroy the coherence and thus the interference. We investigated
this problem by numerically solving the Schr\"odinger equation and applying a
Monte-Carlo wave-function approach. We find interference fringes in velocity
space, even when random photon recoils are taken into account.Comment: 6 pages, 5 figures, we clarified the semiclassical interpretation of
Fig.
Quantum rings as electron spin beam splitters
Quantum interference and spin-orbit interaction in a one-dimensional
mesoscopic semiconductor ring with one input and two output leads can act as a
spin beam splitter. Different polarization can be achieved in the two output
channels from an originally totally unpolarized incoming spin state, very much
like in a Stern-Gerlach apparatus. We determine the relevant parameters such
that the device has unit efficiency.Comment: 4 pages, 3 figures; minor change
Quantum limits in interferometric measurements
Quantum noise limits the sensitivity of interferometric measurements. It is
generally admitted that it leads to an ultimate sensitivity, the ``standard
quantum limit''. Using a semi-classical analysis of quantum noise, we show that
a judicious use of squeezed states allows one in principle to push the
sensitivity beyond this limit. This general method could be applied to large
scale interferometers designed for gravitational wave detection.Comment: 4 page
Jump-like unravelings for non-Markovian open quantum systems
Non-Markovian evolution of an open quantum system can be `unraveled' into
pure state trajectories generated by a non-Markovian stochastic (diffusive)
Schr\"odinger equation, as introduced by Di\'osi, Gisin, and Strunz. Recently
we have shown that such equations can be derived using the modal (hidden
variable) interpretation of quantum mechanics. In this paper we generalize this
theory to treat jump-like unravelings. To illustrate the jump-like behavior we
consider a simple system: A classically driven (at Rabi frequency )
two-level atom coupled linearly to a three mode optical bath, with a central
frequency equal to the frequency of the atom, , and the two side
bands have frequencies . In the large limit we
observed that the jump-like behavior is similar to that observed in this system
with a Markovian (broad band) bath. This is expected as in the Markovian limit
the fluorescence spectrum for a strongly driven two level atom takes the form
of a Mollow triplet. However the length of time for which the Markovian-like
behaviour persists depends upon {\em which} jump-like unraveling is used.Comment: 11 pages, 5 figure
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