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Register a new userKlein Paradox for Optical Scattering from Excited Targets
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The well known Klein paradox for the relativistic Dirac wave equation consists in the computation of possible ``negative probabilities induced by certain potentials in some regimes of energy. The paradox may be resolved employing the notion of electron-positron pair production in which the number of electrons present in a process can increase. The Klein paradox also exists in Maxwells equations viewed as the wave equation for photons. In a medium containing ``inverted energy populations of excited atoms, e.g. in a LASER medium, one may again compute possible ``negative probabilities. The resolution of the electromagnetic Klein paradox is that when the atoms decay, the final state may contain more photons then were contained the initial state. The optical theorem total cross section for scattering photons from excited state atoms may then be computed as negative within a frequency band with matter induced amplification.
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We analyse a little known aspect of the Klein paradox. A Klein-Gordon boson appears to be able to cross a supercritical rectangular barrier without being reflected, while spending there a negative amount of time. The transmission mechanism is demonstrably acausal, yet an attempt to construct the corresponding causal solution of the Klein-Gordon equation fails. We relate the causal solution to a divergent multiple-reflections series, and show that the problem is remedied for a smooth barrier, where pair production at the energy equal to a half of the barriers height is enhanced yet remains finite.
We uncover a new quantum paradox, where a simple question about two identical quantum systems reveals unsettlingly paradoxical answers when weak measurements are considered. Our resolution of the paradox, from within the weak measurement framework, amounts to a proof of counterfactuality for our generalised protocol (2014)---the first to do so---for sending an unknown qubit without any particles travelling between the communicating parties, i.e. counterfactually. The paradox and its resolution are reproduced from a consistent-histories viewpoint. We go on to propose a novel, experimentally feasible implementation of this counterfactual disembodied transport that we call counterportation, based on cavity quantum electrodynamics, estimating resources for beating the no-cloning fidelity limit---except that unlike teleportation no previously-shared entanglement nor classical communication are required. Our approach is up to several orders of magnitude more efficient in terms of physical resources than previously proposed techniques and is remarkably tolerant to device imperfections. Surprisingly, while counterfactual communication is intuitively explained in terms of interaction-free measurement and the Zeno effect, we show based on our proposed scheme that neither is necessary, with implications in support of an underlying physical reality.
The Klein-Gordon equation in the presence of a spatially one-dimensional Hulthen potential is solved exactly and the scattering solutions are obtained in terms of hypergeometric functions. The transmission coefficient is derived by the matching conditions on the wavefunctions and the condition for the existence of transmission resonances are investigated. It is shown how the zero-reflection condition depends on the shape of the potential.
The EPR paradox and the meaning of the Bell inequality are discussed. It is shown that considering the quantum objects as carrying with them instruction kits telling them what to do when meeting a measurement apparatus any paradox disappears. In this view the quantum state is characterized by the prescribed behaviour rather than by the specific value a parameter assumes as a result of an interaction.
The Klein paradox refers to counterintuitive reflection or transmission of relativistic particles from a potential barrier, which is a natural consequence of relativistic quantum theory. The realization of this paradox using fundamental particles is nearly impossible because of the high energy barrier that needs to be overcome. Graphene, with emergent gapless fermion excitations, allows for the study of the fermionic Klein paradox. The test of this paradox for bosonic particles, however, remains a challenging problem. Here, we show that the bosonic Klein paradox can be tested in a driven-dissipative magnonic system. By carefully designing the strength of external drivings through spin-orbit torque and internal dissipation of the magnet, both positive-energy states (magnon) and negative energy states (antimagnon) can be dynamically stabilized. The reflection of incident magnons at a barrier can be amplified to be larger than one, accompanied by a backflow antimagnon current. Our findings may benefit the amplification of magnons in spintronic devices and further enable magnonic system as a platform to study relativistic physics.
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