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As conventional electronic materials approach their physical limits, the application of ultrafast optical fields to access transient states of matter cap- tures imagination. The inversion symmetry governs the optical parity selection rule, differentiating between accessible and inaccessible states of matter. To circumvent parity-forbidden transitions, the common practice is to break the inversion symmetry by material design or external fields. Here we report how the application of femtosecond ultraviolet pulses can energize a parity-forbidden dark exciton state in black phosphorus while maintaining its intrinsic material symmetry. Unlike its conventional bandgap absorption in visible-to-infrared, femtosecond ultraviolet excitation turns on efficient Coulomb scattering, promoting carrier multiplication and electronic heating to ~3000 K, and consequently populating its parity-forbidden states. Interfero- metric time- and angle-resolved two-photon photoemission spectroscopy reveals dark exciton dynamics of black phosphorus on ~100 fs time scale and its anisotropic wavefunctions in energy-momentum space, illuminating its potential applications in optoelectronics and photochemistry under ultraviolet optical excitation. 

Molecular materials offer a boundless design palette for light absorption and charge transport in both natural photosynthesis and engineered photovoltaics. They function in combination as chromophores, donors, conductors, and acceptors, enabling the excitation and charge carrier transport through space and wire-like intramolecular pathways. Although quantum coher- ence is believed to enhance photoexcitation and photoinduced charge transfer, fluctuating and inhomogeneous environments accelerate decoherence. Here, we assemble a nanoporous medium consisting of a templated bipyridyl ethylene (BPE) molecule array on a Ag(111) surface that functions as an exceptional intermolecular nonnuclear quantum well conductor of coherent electron waves spanning over 20 Å length. Time-periodic driving of the Ag/BPE interface by femtosecond pulses promotes electrons into a ladder of Floquet quasi-energy donor states, where intermolecular quantum well states act as a resonant doorway for coherent electron transport into BPE/vacuum image potential acceptor states. The bifurcation of electron passage between the Floquet donor ladder and the charge transfer acceptor channel is recorded by projecting the active electrons into the photoemission continuum in an interferometric time- and angle-resolved multiphoton photoemission experiment. We find that exceptional decoupling of electrons from the metal substrate by the molecule- dressed vacuum preserves the coherence on the ∼150 fs time scale. This offers a new paradigm for quantum state design where a molecule-dressed vacuum mediates coherent electron transport in nanoporous molecular architectures. 

Molecular constructs define the elementary units in porous materials for efficient CO2 capture. The design of appropriate interpore and intermolecular space is crucial to stabilize CO2 molecules and maximize the capacity. While the molecular construct usually has a fixed dimension, whether its intermolecular space could be self-adjustable during CO2 capture and release, behaving as a balloon, has captured imagination. Here we report a flexible intermolecular space of the double chain structure of self-assembled 1,4-phenylene diisocyanide (PDI) molecules on Ag(110) surface, which dynamically broadens and recovers during the CO2 capture and release. The incipient PDI double chains organize along the [001] direction of Ag(110), in which individual PDI molecules stand up in a zigzag order with the interchain width defined by twice the Ag lattice distance along [11¯0] direction (2α[11¯0]). When CO2 molecules are introduced, they assemble to occupy the interchain spaces, expanding the interchain width to 3α[11¯0], 4α[11¯0] and 5α[11¯0]. Warming up the sample leads to the thermally-driven CO2 desorption that recovers the original interchain space. High-resolution scanning tunneling microscopy (STM) jointly with density functional theory (DFT) calculations determine the structural and electronic interactions of CO2 molecules with the dynamical PDI structures, providing a molecular-level perspective for the design of a self-adjustable metal-organic construct for reversible gas capture and release. 

Abstract Quantum Key Distribution allows two parties to establish a secret key that is secure against computationally unbounded adversaries. To extend the distance between parties, quantum networks are vital. Typically, security in such scenarios assumes the absolute worst case: namely, an adversary has complete control over all repeaters and fiber links in a network and is able to replace them with perfect devices, thus allowing her to hide her attack within the expected natural noise. In a large-scale network, however, such a powerful attack may be infeasible. In this paper, we analyze the case where the adversary can only corrupt a subset of the repeater network connecting Alice and Bob, while some portion of the network near Alice and Bob may be considered safe from attack (though still noisy). We derive a rigorous finite key proof of security assuming this attack model, and show that improved performance and noise tolerances are possible. Our proof methods may be useful to other researchers investigating partially corrupted quantum networks, and our main result may be beneficial to future network operators. 

Light is a preeminent spectroscopic tool for investigating the electronic structure of surfaces. Time-resolved photoelectron spectroscopy has mainly been developed in the last 30 years. It is therefore not surprising that the topic was hardly mentioned in the issue on ‘‘The first thirty years’’ of surface science. In the second thirty years, however, we have seen tremendous progress in the development of time-resolved photoelectron spectroscopy on surfaces. Femtosecond light pulses and advanced photoelectron detection schemes are increasingly being used to study the electronic structure and dynamics of occupied and unoccupied electronic states and dynamic processes such as the energy and momentum relaxation of electrons, charge transfer at interfaces and collective processes such as plasmonic excitation and optical field screening. Using spin- and time-resolved photoelectron spectroscopy, we were able to study ultrafast spin dynamics, electron–magnon scattering and spin structures in magnetic and topological materials. Light also provides photon energy as well as electric and magnetic fields that can influence molecular surface processes to steer surface photochemistry and hot-electron-driven catalysis. In addition, we can consider light as a chemical reagent that can alter the properties of matter by creating non-equilibrium states and ultrafast phase transitions in correlated materials through the coupling of electrons, phonons and spins. Electric fields have also been used to temporarily change the electronic structure. This opened up new methods and areas such as high harmonic generation, light wave electronics and attosecond physics. This overview certainly cannot cover all these interesting topics. But also as a testimony to the cohesion and constructive exchange in our ultrafast community, a number of colleagues have come together to share their expertise and views on the very vital field of dynamics at surfaces. 

We present BGP-iSec, an enhancement of the BGPsec protocol for securing BGP, the Internet’s inter-domain routing protocol. BGP-iSec ensures additional and stronger security properties, compared to BGPsec, without significant extra overhead. The main improvements are: (i) Security for partial adoption: BGP-iSec provides significant security benefits for early adopters, in contrast to BGPsec, which requires universal adoption. (ii) Defense against route leakage: BGP-iSec defends against route leakage, a common cause of misrouting that is not prevented by BGPsec. (iii) Integrity of attributes: BGP-iSec ensures the integrity of integrity-protected attributes, thereby preventing announcement manipulation attacks not prevented by BGPsec. We argue that BGP-iSec achieves these goals using extensive simulations as well as security analysis. The BGP-iSec design conforms, where possible, with the BGPsec design, modifying it only where necessary to improve security or ease deployment. By providing stronger security guarantees, especially for partial adoption, we hope BGP-iSec will be a step towards finally protecting interdomain routing, which remains, for many years, a vulnerability of the Internet’s infrastructure.