The nonreciprocal propagation of light at the single-photon level is essential for building a quantum network. Bulk optical schemes are lossy and difficulty to integrate onto a chip. We propose a single-photon optical diode and a three-port circulator without a magnetic field by coupling an unbalanced quantum impurity to a passive, linear optical waveguide or a whispering-gallery-mode microresonator which supports a locally or globally circularly polarized photon. Thanks to the unbalanced quantum Jaynes-Cummings coupling, the optical nonreciprocal propagation of single photons can be achieved without an external magnetic field. In particular, the three-port single-photon circulator can be accomplished using the existing experimental technology. The optical isolation can be reversed by selectively populating the initial state of the quantum impurity. Moreover, by using an ensemble of identical atoms filled in a hollow-core microbottle resonator, nonreciprocal propagation of weak light pulses can be achieved.

To reconcile conflicting reports on the role of CD40 signaling in germinal center (GC) formation, we examined the earliest stages of murine GC B cell differentiation. Peri-follicular GC precursors first expressed intermediate levels of BCL6 while co-expressing the transcription factors RelB and IRF4, the latter known to repress Bcl6 transcription. Transition of GC precursors to the BCL6hi follicular state was associated with cell division, although the number of required cell divisions was immunogen dose dependent. Potentiating T cell help or CD40 signaling in these GC precursors actively repressed GC B cell maturation and diverted their fate towards plasmablast differentiation, whereas depletion of CD4+ T cells promoted this initial transition. Thus while CD40 signaling in B cells is necessary to generate the immediate precursors of GC B cells, transition to the BCL6hi follicular state is promoted by a regional and transient diminution of T cell help.DOI: http://dx.doi.org/10.7554/eLife.19552.001

Anti-Stokes one-photon luminescence from a single gold nanorod is experimentally investigated. The anti-Stokes emission of gold nanorods is enhanced and strongly modulated by localized surface plasmon resonance (LSPR). It is found that the polarization dependence of the anti-Stokes emission is in strong correlation with that of the Stokes emission. Further experiments provide evidence that LSPR significantly enhanced both excitation and emission processes. Moreover, the line shape of the anti-Stokes emission is dependent on the surface temperature, which is related to the distribution of free electrons near the Fermi level. This discovery provides an effective method in principle to probe localized temperature at nanoscale dimension. Here, the reported results about the anti-Stokes emission provide more understanding for the photoemission process from the plasmonic nanostructures.

Gold nanoparticles have excellent optical and plasmon properties and have enormous research potential when they combine with different ligands. Functionalized gold nanoparticles are widely used in drug delivery, biological detection, sensors, and even clinical treatment. At present, with the rapid development of functionalized gold nanoparticles, we think it is necessary to sort out the recent research results. In this paper, the synthesis, characteristics, and biological applications of the three aspects are introduced. Firstly, the synthetic methods of functionalized gold nanoparticles are reviewed, and the improvement of the commonly used synthetic methods is summarized. Then, the catalytic properties, oxidation resistance, and chirality optical properties are summarized in detail. Finally, we summarize the research progress in the field of biology in recent years.