抗原变异可使得多种致病微生物易于逃避宿主免疫应答。表达在感染红细胞表面的恶性疟原虫红细胞表面蛋白1（PfPMP1）与感染红细胞、内皮细胞、树突状细胞以及胎盘的单个或多个受体作用，在黏附及免疫逃避中起关键的作用。每个单倍体基因组var基因家族编码约60种成员，通过启动转录不同的var基因变异体为抗原变异提供了分子基础。

疟原虫var基因转换速率变化导致抗原变异[英]／Paul H, Robert P, Christodoulou Z, et al//Proc Natl Acad Sci U S A

Spintronics, or spin electronics, involves the study of active control and manipulation of spin degrees of freedom in solid-state systems. This article reviews the current status of this subject, including both recent advances and well-established results. The primary focus is on the basic physical principles underlying the generation of carrier spin polarization, spin dynamics, and spin-polarized transport in semiconductors and metals. Spin transport differs from charge transport in that spin is a nonconserved quantity in solids due to spin-orbit and hyperfine coupling. The authors discuss in detail spin decoherence mechanisms in metals and semiconductors. Various theories of spin injection and spin-polarized transport are applied to hybrid structures relevant to spin-based devices and fundamental studies of materials properties. Experimental work is reviewed with the emphasis on projected applications, in which external electric and magnetic fields and illumination by light will be used to control spin and charge dynamics to create new functionalities not feasible or ineffective with conventional electronics.

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Spintronics: Fundamentals and applications

This article reviews recent theoretical and experimental advances in the fundamental understanding and active control of quantum fluids of light in nonlinear optical systems. In the presence of effective photon-photon interactions induced by the optical nonlinearity of the medium, a many-photon system can behave collectively as a quantum fluid with a number of novel features stemming from its intrinsically nonequilibrium nature. A rich variety of recently observed photon hydrodynamical effects is presented, from the superfluid flow around a defect at low speeds, to the appearance of a Mach-Cherenkov cone in a supersonic flow, to the hydrodynamic formation of topological excitations such as quantized vortices and dark solitons at the surface of large impenetrable obstacles. While the review is mostly focused on a specific class of semiconductor systems that have been extensively studied in recent years (planar semiconductor microcavities in the strong light-matter coupling regime having cavity polaritons as elementary excitations), the very concept of quantum fluids of light applies to a broad spectrum of systems, ranging from bulk nonlinear crystals, to atomic clouds embedded in optical fibers and cavities, to photonic crystal cavities, to superconducting quantum circuits based on Josephson junctions. The conclusive part of the article is devoted to a review of the future perspectives in the direction of strongly correlated photon gases and of artificial gauge fields for photons. In particular, several mechanisms to obtain efficient photon blockade are presented, together with their application to the generation of novel quantum phases.

Quantum fluids of light

In the past decade, a two-dimensional matter-light system called the microcavity exciton-polariton has emerged as a new promising candidate of Bose-Einstein condensation BEC in solids. Many pieces of important evidence of polariton BEC have been established recently in GaAs and CdTe microcavities at the liquid helium temperature, opening a door to rich many-body physics inaccessible in experiments before. Technological progress also made polariton BEC at room temperatures promising. In parallel with experimental progresses, theoretical frameworks and numerical simulations are developed, and our understanding of the system has greatly advanced. In this article, recent experiments and corresponding theoretical pictures based on the Gross-Pitaevskii equations and the Boltzmann kinetic simulations for a finite-size BEC of polaritons are reviewed.

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Exciton-polariton Bose-Einstein condensation

We have created polaritons in a harmonic potential trap analogous to atoms in optical traps. The trap can be loaded by creating polaritons 50 micrometers from its center that are allowed to drift into the trap. When the density of polaritons exceeds a critical threshold, we observe a number of signatures of Bose-Einstein condensation: spectral and spatial narrowing, a peak at zero momentum in the momentum distribution, first-order coherence, and spontaneous linear polarization of the light emission. The polaritons, which are eigenstates of the light-matter system in a microcavity, remain in the strong coupling regime while going through this dynamical phase transition.

https://science.sciencemag.org/content/sci/316/5827/1007.full.pdf

Bose-Einstein Condensation of Microcavity Polaritons in a Trap

The spin injection into GaAs has been studied for the ferromagnetic metal MnAs. Evidence for preferential minority-spin injection is obtained from the circular polarization of the electroluminescence in GaAs/(In,Ga)As light-emitting diodes (LED). The spin-injection efficiency of 6% at the MnAs/GaAs interface is estimated on the basis of spin-relaxation times extracted from time-resolved photoluminescence measurements. This efficiency, as well as the preferential spin orientation, resembles very much the injection behavior found for epitaxial Fe layers. The results do not depend on the azimuthal orientation of the epitaxial MnAs injection layer.

Electrical spin injection from ferromagnetic MnAs metal layers into GaAs

We study the momentum distribution and relaxation dynamics of semiconductor microcavity polaritons by angle-resolved and time-resolved spectroscopy. Above a critical pump level, the thermalization time of polaritons at positive detunings becomes shorter than their lifetime, and the polaritons form a quantum degenerate Bose-Einstein distribution in thermal equilibrium with the lattice.

Quantum degenerate exciton-polaritons in thermal equilibrium.

Spin transport and manipulation in semiconductors have been studied intensively with the ultimate goal of realizing spintronic devices. Previous work in GaAs has focused on controlling the carrier density1, crystallographic orientation2 and dimensionality3,4 to limit the electron spin decoherence and allow transport over long distances4,5,6,7. Here, we introduce a new method for the coherent transport of spin-polarized electronic wave packets using dynamic quantum dots (DQDs) created by the piezoelectric field of coherent acoustic phonons8,9,10,11. Photogenerated spin carriers transported by the DQDs in undoped GaAs (001) quantum wells exhibit a spin coherence length exceeding 100 μm, which is attributed to the simultaneous control of the carrier density and the dimensionality12 by the DQDs during transport. In the absence of an applied magnetic field, we observe the precession of the electron spin induced by the internal magnetic field associated with the spin splitting of the conduction band (Dresselhaus term)13. The coherent manipulation of the precession frequency is also achieved by applying an external magnetic field.

Coherent spin transport through dynamic quantum dots.

We perform Young's double-slit experiment to study the spatial coherence properties of a two-dimensional dynamic condensate of semiconductor microcavity polaritons. The coherence length of the system is measured as a function of the pump rate, which confirms a spontaneous buildup of macroscopic coherence in the condensed phase. An independent measurement reveals that the position and momentum uncertainty product of the condensate is close to the Heisenberg limit. An experimental realization of such a minimum uncertainty wave packet of the polariton condensate opens a door to coherent matter-wave phenomena such as Josephson oscillation, superfluidity, and solitons in solid state condensate systems.

https://arxiv.org/pdf/cond-mat/0703508.pdf

Spatial Coherence of a Polariton Condensate

Femtosecond degenerate four-wave mixing is used to investigate collective excitonic effects in radiatively coupled semiconductor multiple-quantum-well Bragg and anti-Bragg structures. The experimental and theoretical analysis of Bragg structures shows that the light-induced coupling causes a faster signal decay for increasing quantum-well numbers. For anti-Bragg structures, the predicted splitting of the excitonic resonances is observed in the four-wave-mixing spectrum causing quantum beats in the time-integrated signal.

Rudolf Hey

Papers

Electrical spin injection from ferromagnetic MnAs metal layers into GaAs

Quantum degenerate exciton-polaritons in thermal equilibrium.

Coherent spin transport through dynamic quantum dots.

Spatial Coherence of a Polariton Condensate

Collective effects of excitons in multiple-quantum-well Bragg and anti-Bragg structures.