Perfect state transfer and efficient quantum routing: a discrete-time quantum walk approach

We show a perfect state transfer of an arbitrary unknown two-qubit state can be achieved via a discrete-time quantum walk with various settings of coin flippings, and extend this method to distribution of an arbitrary unknown multi-qubit entangled state between every pair of sites in the multi-dimensional network. Furthermore, we study the routing of quantum information on this network in a quantum walk architecture, which can be used as quantum information processors to communicate between separated qubits.Comment: 6 pages, 2 figure

Electrically Tunable Energy Bandgap in Dual-Gated Ultra-Thin Black Phosphorus Field Effect Transistors

The energy bandgap is an intrinsic character of semiconductors, which largely determines their properties. The ability to continuously and reversibly tune the bandgap of a single device during real time operation is of great importance not only to device physics but also to technological applications. Here we demonstrate a widely tunable bandgap of few-layer black phosphorus (BP) by the application of vertical electric field in dual-gated BP field-effect transistors. A total bandgap reduction of 124 meV is observed when the electrical displacement field is increased from 0.10V/nm to 0.83V/nm. Our results suggest appealing potential for few-layer BP as a tunable bandgap material in infrared optoelectronics, thermoelectric power generation and thermal imaging.Comment: 5 pages, 4 figure

High-energy-density and superhard nitrogen-rich B-N compounds

The pressure-induced transformation of diatomic nitrogen into non-molecular polymeric phases may produce potentially useful high-energy-density materials. We combine first-principles calculations with structure searching to predict a new class of nitrogen-rich boron nitrides with a stoichiometry of B3N5 that are stable or metastable relative to solid N2 and h-BN at ambient pressure. The most stable phase at ambient pressure has a layered structure (h-B3N5) containing hexagonal B3N3 layers sandwiched with intercalated freely rotating N2 molecules. At 15 GPa, a three-dimensional C2221 structure with single N-N bonds becomes the most stable. This pressure is much lower than that required for triple-to-single bond transformation in pure solid nitrogen (110 GPa). More importantly, C2221-B3N5 is metastable, and can be recovered under ambient conditions. Its energy density of 3.44 kJ/g makes it a potential high-energy-density material. In addition, stress-strain calculations estimate a Vickers hardness of 44 GPa. Structure searching reveals a new clathrate sodalite-like BN structure that is metastable under ambient conditions.Comment: 16 pages, 5 figures, accepted by PR

Plasmonic excitations in quantum-sized sodium nanoparticles studied by time-dependent density functional calculations

The plasmonic properties of sphere-like bcc Na nanoclusters ranging from Na$_{15}$ to Na$_{331}$ have been studied by real-time time-dependent local density approximation calculations. The optical absorption spectrum, density response function and static polarizability are evaluated. It is shown that the effect of the ionic background (ionic species and lattice) of the clusters accounts for the remaining discrepancy in the principal (surface plasmon) absorption peak energy between the experiments and previous calculations based on a jellium background model. The ionic background effect also pushes the critical cluster size where the maximum width of the principal peak occurs from Na$_{40}$ predicted by the previous jellium model calculations to Na$_{65}$. In the volume mode clusters (Na$_{27}$, Na$_{51}$, Na$_{65}$, Na$_{89}$ and Na$_{113}$) in which the density response function is dominated by an intense volume mode, a multiple absorption peak structure also appears next to the principal peak. In contrast, the surface mode clusters of greater size (Na$_{169}$, Na$_{229}$, Na$_{283}$ and Na$_{331}$) exhibit a smoother and narrower principal absorption peak because their surface plasmon energy is located well within that of the unperturbed electron-hole transitions, and their density responses already bear resemblance to that of classical Mie theory. Moreover, it is found that the volume plasmon that exist only in finite size particles, gives rise to the long absorption tail in the UV region. This volume plasmon manifests itself in the absorption spectrum even for clusters as large as Na$_{331}$ with an effective diameter of $\sim$3.0 nm