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

Topological photonics is a rapidly emerging field of research in which geometrical and topological ideas are exploited to design and control the behavior of light. Drawing inspiration from the discovery of the quantum Hall effects and topological insulators in condensed matter, recent advances have shown how to engineer analogous effects also for photons, leading to remarkable phenomena such as the robust unidirectional propagation of light, which hold great promise for applications. Thanks to the flexibility and diversity of photonics systems, this field is also opening up new opportunities to realize exotic topological models and to probe and exploit topological effects in new ways. This article reviews experimental and theoretical developments in topological photonics across a wide range of experimental platforms, including photonic crystals, waveguides, metamaterials, cavities, optomechanics, silicon photonics, and circuit QED. A discussion of how changing the dimensionality and symmetries of photonics systems has allowed for the realization of different topological phases is offered, and progress in understanding the interplay of topology with non-Hermitian effects, such as dissipation, is reviewed. As an exciting perspective, topological photonics can be combined with optical nonlinearities, leading toward new collective phenomena and novel strongly correlated states of light, such as an analog of the fractional quantum Hall effect.

Topological Photonics

Microcavity physics and design will be reviewed. Following an overview of applications in quantum optics, communications and biosensing, recent advances in ultra-high-Q research will be presented.

Optical microcavities

http://ltl.tkk.fi/ice/Kevo/noisereview.pdf

Shot noise in mesoscopic conductors

It is now shown that coupled optical microcavities bear all the hallmarks of parity–time symmetry; that is, the system’s dynamics are unchanged by both time-reversal and mirror transformations. The resonant nature of microcavities results in unusual effects not seen in previous photonic analogues of parity–time-symmetric systems: for example, light travelling in one direction is resonantly enhanced but there are no resonance peaks going the other way.

Parity–time-symmetric whispering-gallery microcavities

Bound states in the continuum (BICs) are waves that remain localized even though they coexist with a continuous spectrum of radiating waves that can carry energy away. Their very existence defies conventional wisdom. Although BICs were first proposed in quantum mechanics, they are a general wave phenomenon and have since been identified in electromagnetic waves, acoustic waves in air, water waves and elastic waves in solids. These states have been studied in a wide range of material systems, such as piezoelectric materials, dielectric photonic crystals, optical waveguides and fibres, quantum dots, graphene and topological insulators. In this Review, we describe recent developments in this field with an emphasis on the physical mechanisms that lead to BICs across seemingly very different materials and types of waves. We also discuss experimental realizations, existing applications and directions for future work. The fascinating wave phenomenon of ‘bound states in the continuum’ spans different material and wave systems, including electron, electromagnetic and mechanical waves. In this Review, we focus on the common physical mechanisms underlying these bound states, whilst also discussing recent experimental realizations, current applications and future opportunities for research.

Bound states in the continuum

The conductance of any metallic sample is predicted to fluctuate as a function of chemical potential or magnetic field by an amount of order $\frac{{e}^{2}}{h}(\ensuremath{\simeq}4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}{\ensuremath{\Omega}}^{\ensuremath{-}1})$ independent of sample size and degree of disorder as long as the temperature is low enough so that $\mathrm{kT}$ and the inelastic-scattering rate are less than the inverse time to diffuse across the sample. The theory is shown to be in excellent agreement with numerical simulations and explains many features of experiments on small wires and rings.

Universal conductance fluctuations in metals.

Using a scattering matrix formalism, we derive the general scattering properties of optical structures that are symmetric under a combination of parity and time reversal (PT). We demonstrate the existence of a transition between PT-symmetric scattering eigenstates, which are norm preserving, and symmetry-broken pairs of eigenstates exhibiting net amplification and loss. The system proposed by Longhi [Phys. Rev. A 82, 031801 (2010).], which can act simultaneously as a laser and coherent perfect absorber, occurs at discrete points in the broken-symmetry phase, when a pole and zero of the S matrix coincide.

/pdf/pt-symmetry-breaking-and-laser-absorber-modes-in-optical-4khw8bpjqp.pdf

PT-symmetry breaking and laser-absorber modes in optical scattering systems.

In the time-reversed counterpart to laser emission, incident coherent optical fields are perfectly absorbed within a resonator that contains a loss medium instead of a gain medium. The incident fields and frequency must coincide with those of the corresponding laser with gain. We demonstrated this effect for two counterpropagating incident fields in a silicon cavity, showing that absorption can be enhanced by two orders of magnitude, the maximum predicted by theory for our experimental setup. In addition, we showed that absorption can be reduced substantially by varying the relative phase of the incident fields. The device, termed a “coherent perfect absorber,” functions as an absorptive interferometer, with potential practical applications in integrated optics.

https://science.sciencemag.org/content/sci/331/6019/889.full.pdf

Time-reversed lasing and interferometric control of absorption.

High-power and highly directional semiconductor microcylinder lasers based on an optical resonator with deformed cross section are reported. In the favorable directions of the far-field, a power increase of up to three orders of magnitude over the conventional circularly symmetric lasers was obtained. A “bow-tie”–shaped resonance is responsible for the improved performance of the lasers in the higher range of deformations, in contrast to “whispering-gallery”–type modes of circular and weakly deformed lasers. This resonator design, although demonstrated here in midinfrared quantum-cascade lasers, should be applicable to any laser based on semiconductors or other high–refractive index materials.

/pdf/high-power-directional-emission-from-microlasers-with-45msrqsfsh.pdf

A. Douglas Stone

Papers

Bound states in the continuum

Universal conductance fluctuations in metals.

PT-symmetry breaking and laser-absorber modes in optical scattering systems.

Time-reversed lasing and interferometric control of absorption.

High-Power Directional Emission from Microlasers with Chaotic Resonators