7,905 research outputs found
Multiplication theory for dynamically biased avalanche photodiodes: new limits for gain bandwidth product
Novel theory is developed for the avalanche multiplication process in avalanche photodiodes (APDs) under time-varying reverse-biasing conditions. Integral equations are derived characterizing the statistics of the multiplication factor and the impulse-response function of APDs, as well as their breakdown probability, all under the assumption that the electric field driving the avalanche process is time varying and spatially nonuniform. Numerical calculations generated by the model predict that by using a bit-synchronous sinusoidal biasing scheme to operate the APD in an optical receiver, the pulse-integrated gain-bandwidth product can be improved by a factor of 5 compared to the same APD operating under the conventional static biasing. The bit-synchronized periodic modulation of the electric field in the multiplication region serves to (1) produce large avalanche multiplication factors with suppressed avalanche durations for photons arriving in the early phase of each optical pulse; and (2) generate low avalanche gains and very short avalanche durations for photons arriving in the latter part of each optical pulse. These two factors can work together to reduce intersymbol interference in optical receivers without sacrificing sensitivity
Clumping and quantum order: Quantum gravitational dynamics of NUT charge
Gravitational instantons with NUT charge are magnetic monopoles upon
dimensional reduction. We determine whether NUT charge can proliferate via the
Polyakov mechanism and partially screen gravitational interactions. In
semiclassical Einstein gravity, Taub-NUT instantons experience a universal
attractive force in the path integral that prevents proliferation. This
attraction further leads to semiclassical clumping instabilities, similar to
the known instabilities of hot flat space and the Kaluza-Klein vacuum. Beyond
pure Einstein gravity, NUT proliferation depends on the following question: is
the mass of a gravitational instanton in the theory always greater than its NUT
charge? Using spinorial methods we show that the answer to this question is
`yes' if all matter fields obey a natural Euclidean energy condition.
Therefore, the attractive force between instantons in the path integral wins
out and gravity is dynamically protected against screening. Semiclassical
gravity with a compactified circle can be self-consistently quantum ordered, at
the cost of suffering from clumping instabilities
Direct CP violation in decays as a probe for new physics
In this paper we investigate CP violation in charged decays of meson.
Particularly, we study the direct CP asymmetry of the Cabibbo favored
non-leptonic and the doubly Cabibbo-suppressed
decay mode within standard model, two Higgs doublet
model with generic Yukawa structure and left right symmetric models. In the
standard model, we first derive the contributions from box and di-penguin
diagrams contributing to their amplitudes which are relevant to the generation
of the weak phases essential for non-vanishing direct CP violation. Then, we
show that these phases are so tiny leading to a direct CP asymmetry of order
in both decay modes. Regarding the two Higgs doublet model with
generic Yukawa structure and after taking into account all constraints on the
parameter space of the model, we show that the enhanced direct CP asymmetries
can be 6 and 7 orders of magnitudes larger than the standard model prediction
for and
respectively. Finally, within left right symmetric models, we find that sizable
direct CP asymmetry of can be obtained for the decay
mode after respecting all relevant
constraints.Comment: 20 pages, 2 figures. arXiv admin note: text overlap with
arXiv:1710.0041
Spin-nematic order in the frustrated pyrochlore-lattice quantum rotor model
As an example of ordering due to quantum fluctuations, we examine the
nearest-neighbor antiferromagnetic quantum O(n) rotor model on the pyrochlore
lattice. Classically, this system remains disordered even at zero temperature;
we find that adding quantum fluctuations induces an ordered phase that survives
to positive temperature, and we determine how its phase diagram scales with the
coupling constant and the number of spin components. We demonstrate, using
quantum Monte Carlo simulations, that this phase has long-range spin-nematic
order, and that the phase transition into it appears to be first order.Comment: 10 pages, 8 figure
New Perspective on Passively Quenched Single Photon Avalanche Diodes: Effect of Feedback on Impact Ionization
Single-photon avalanche diodes (SPADs) are primary devices in photon counting systems used in quantum cryptography, time resolved spectroscopy and photon counting optical communication. SPADs convert each photo-generated electron hole pair to a measurable current via an avalanche of impact ionizations. In this paper, a stochastically self-regulating avalanche model for passively quenched SPADs is presented. The model predicts, in qualitative agreement with experiments, three important phenomena that traditional models are unable to predict. These are: (1) an oscillatory behavior of the persistent avalanche current; (2) an exponential (memoryless) decay of the probability density function of the stochastic quenching time of the persistent avalanche current; and (3) a fast collapse of the avalanche current, under strong feedback conditions, preventing the development of a persistent avalanche current. The model specifically captures the effect of the load’s feedback on the stochastic avalanche multiplication, an effect believed to be key in breaking today’s counting rate barrier in the 1.55–μm detection window
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