3,325 research outputs found
Demonstration of negative group delays in a simple electronic circuit
We present a simple electronic circuit which produces negative group delays
for base-band pulses. When a band-limited pulse is applied as the input, a
forwarded pulse appears at the output. The negative group delays in lumped
systems share the same mechanism with the superluminal light propagation, which
is recently demonstrated in an absorption-free, anomalous dispersive medium
[Wang et al., Nature 406, 277 (2000)]. In this circuit, the advance time more
than twenty percent of the pulse width can easily be achieved. The time
constants, which can be in the order of seconds, is slow enough to be observed
with the naked eye by looking at the lamps driven by the pulses.Comment: 6pages,8 figure
Generation of photon pairs using polarization-dependent two-photon absorption
We propose a new method for generating photon pairs from coherent light using
polarization-dependent two-photon absorption. We study the photon statistics of
two orthogonally polarized modes by solving a master equation, and show that
when we prepare a coherent state in one polarization mode, photon pairs are
created in the other mode. The photon pairs have the same frequency as that of
the incident light.Comment: 4 pages, 3 figures, submitted to PR
Simulation of Slow Light with Electronics Circuits
We present an electronic circuit which simulates wave propagation in
dispersive media. The circuit is an array of phase shifter composed of
operational amplifiers and can be described with a discretized version of
one-dimensional wave equation for envelopes. The group velocity can be changed
both spatially and temporarily. It is used to emulate slow light or stopped
light, which has been realized in a medium with electromagnetically induced
transparency (EIT). The group-velocity control of optical pulses is expected to
be a useful tool in the field of quantum information and communication.Comment: The following article has been submitted to the American Journal of
Physics. After it is published, it will be found at
http://scitation.aip.org/ajp (7 pages, 7 figures
Absorption-free optical control of spin systems:the quantum Zeno effect in optical pumping
We show that atomic spin motion can be controlled by circularly polarized
light without light absorption in the strong pumping limit. In this limit, the
pumping light, which drives the empty spin state, destroys the Zeeman coherence
effectively and freezes the coherent transition via the quantum Zeno effect. It
is verified experimentally that the amount of light absorption decreases
asymptotically to zero as the incident light intensity is increased.Comment: 4 pages with 4 figure
Supersymmetry, Naturalness, and Signatures at the LHC
Weak scale supersymmetry is often said to be fine-tuned, especially if the
matter content is minimal. This is not true if there is a large A term for the
top squarks. We present a systematic study on fine-tuning in minimal
supersymmetric theories and identify low energy spectra that do not lead to
severe fine-tuning. Characteristic features of these spectra are: a large A
term for the top squarks, small top squark masses, moderately large tan\beta,
and a small \mu parameter. There are classes of theories leading to these
features, which are discussed. In one class, which allows a complete
elimination of fine-tuning, the Higgsinos are the lightest among all the
superpartners of the standard model particles, leading to three nearly
degenerate neutralino/chargino states. This gives interesting signals at the
LHC -- the dilepton invariant mass distribution has a very small endpoint and
shows a particular shape determined by the Higgsino nature of the two lightest
neutralinos. We demonstrate that these signals are indeed useful in realistic
analyses by performing Monte Carlo simulations, including detector simulations
and background estimations. We also present a method that allows the
determination of all the relevant superparticle masses without using input from
particular models, despite the limited kinematical information due to short
cascades. This allows us to test various possible models, which is demonstrated
in the case of a model with mixed moduli-anomaly mediation. We also give a
simple derivation of special renormalization group properties associated with
moduli mediated supersymmetry breaking, which are relevant in a model without
fine-tuning.Comment: 56 pages, 24 figure
Nonlinear behavior of geometric phases induced by photon pairs
In this study, we observe the nonlinear behavior of the two-photon geometric
phase for polarization states using time-correlated photons pairs. This phase
manifests as a shift of two-photon interference fringes. Under certain
arrangements, the geometric phase can vary nonlinearly and become very
sensitive to a change in the polarization state. Moreover, it is known that the
geometric phase for identically polarized photons is times larger than
that for one photon. Thus, the geometric phase for two photons can become two
times more sensitive to a state change. This high sensitivity to a change in
the polarization can be exploited for precision measurement of small
polarization variation. We evaluate the signal-to-noise ratio of the
measurement scheme using the nonlinear behavior of the geometric phase under
technical noise and highlight the practical advantages of this scheme.Comment: 10 pages, 10 figure
Observation of Brewster's effect for transverse-electric electromagnetic waves in metamaterials: Experiment and theory
We have experimentally realized Brewster's effect for transverse-electric
waves with metamaterials. In dielectric media, Brewster's no-reflection effect
arises only for transverse-magnetic waves. However, it has been theoretically
predicted that Brewster's effect arises for TE waves under the condition that
the relative permeability r is not equal to unity. We have designed an array of
split-ring resonators as a metamaterial with mu_r 1 using a finite-difference
time-domain method. The reflection measurements were carried out in a 3-GHz
region and the disappearance of reflected waves at a particular incident angle
was confirmed.Comment: 4 pages, 5 figure
Suppression of the charge-density-wave state in Sr_14Cu_24O_41 by calcium doping
The charge response in the spin chain/ladder compound Sr_14-xCa_xCu_24O_41 is
characterized by DC resistivity, low-frequency dielectric spectroscopy and
optical spectroscopy. We identify a phase transition below which a
charge-density wave (CDW) develops in the ladder arrays. Calcium doping
suppresses this phase with the transition temperature decreasing from 210 K for
x=0 to 10 K for x=9, and the CDW gap from 130 meV down to 3 meV, respectively.
This suppression is due to the worsened nesting originating from the increase
of the inter-ladder tight-binding hopping integrals, as well as from disorder
introduced at the Sr sites. These results altogether speak in favor of
two-dimensional superconductivity under pressure.Comment: 4 pages, 4 figures, accepted for publication in PR
Search for Lepton Flavor-Violating "tau -> mu gamma" decay
We search for the lepton flavor-violating "tau -> mu gamma" decay using 29.7
million tau pairs accumulated by the Belle experiment. The main background
sources are found to be tau pairs with "tau -> mu nu nu" decay and radiative
dimuon events. One event is observed in the signal region, while 2.5 +- 0.6
background events are expected. A preliminary upper limit Br(tau -> mu gamma) <
6 x 10^{-7} at the 90% confidence limit is obtained.Comment: Invited talk at the Seventh International Workshop on Tau Lepton
Physics (TAU02), Santa Cruz, Ca, USA, Sept 2002, 6 pages, LaTeX, 13 eps
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