4,139 research outputs found
Polariton condensation in photonic crystals with high molecular orientation
We study Frenkel exciton-polariton Bose-Einstein condensation in a
two-dimensional defect-free triangular photonic crystal with an organic
semiconductor active medium containing bound excitons with dipole moments
oriented perpendicular to the layers. We find photonic Bloch modes of the
structure and consider their strong coupling regime with the excitonic
component. Using the Gross- Pitaevskii equation for exciton polaritons and the
Boltzmann equation for the external exciton reservoir, we demonstrate the
formation of condensate at the points in reciprocal space where photon group
velocity equals zero. Further, we demonstrate condensation at non-zero momentum
states for TM-polarized photons in the case of a system with incoherent
pumping, and show that the condensation threshold varies for different points
in the reciprocal space, controlled by detuning.Comment: 6 pages, 4 figure
Quantum Zeno and anti-Zeno effects in the Friedrichs model
We analyze the short-time behavior of the survival probability in the frame
of the Friedrichs model for different formfactors. We have shown that this
probability is not necessary analytic at the time origin. The time when the
quantum Zeno effect could be observed is found to be much smaller than usually
estimated. We have also studied the anti-Zeno era and have estimated its
duration.Comment: References added. Appendix B shortened. Discussions extende
Causality, delocalization and positivity of energy
In a series of interesting papers G. C. Hegerfeldt has shown that quantum
systems with positive energy initially localized in a finite region,
immediately develop infinite tails. In our paper Hegerfeldt's theorem is
analysed using quantum and classical wave packets. We show that Hegerfeldt's
conclusion remains valid in classical physics. No violation of Einstein's
causality is ever involved. Using only positive frequencies, complex wave
packets are constructed which at are real and finitely localized and
which, furthemore, are superpositions of two nonlocal wave packets. The
nonlocality is initially cancelled by destructive interference. However this
cancellation becomes incomplete at arbitrary times immediately afterwards. In
agreement with relativity the two nonlocal wave packets move with the velocity
of light, in opposite directions.Comment: 14 pages, 5 figure
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