We investigate qubit-qubit entanglement mediated by plasmons supported by one-dimensional waveguides. We explore both the situation of spontaneous formation of entanglement from an unentangled state and the emergence of driven steady-state entanglement under continuous pumping. In both cases, we show that large values for the concurrence are attainable for qubit-qubit distances larger than the operating wavelength by using plasmonic waveguides that are currently available.

Surface plasmon polaritons (SPPs) are localized surface electromagnetic waves that propagate along the interface between a metal and a dielectric. Owing to their inherent subwavelength confinement, SPPs have a strong potential to become building blocks of a type of photonic circuitry built up on 2D metal surfaces; however, SPPs are difficult to control on curved surfaces conformably and flexibly to produce advanced functional devices. Here we propose the concept of conformal surface plasmons (CSPs), surface plasmon waves that can propagate on ultrathin and flexible films to long distances in a wide broadband range from microwave to mid-infrared frequencies. We present the experimental realization of these CSPs in the microwave regime on paper-like dielectric films with a thickness 600-fold smaller than the operating wavelength. The flexible paper-like films can be bent, folded, and even twisted to mold the flow of CSPs. metamaterials | plasmonics | waveguiding S urface plasmon polaritons (SPPs) are highly localized surface waves (1) that propagate along the interface between two materials whose real parts of electric permittivity have opposite signs, and decay exponentially in the transverse direction. At optical frequencies, metals behave like plasma with negative permittivity, and thus SPPs exist on metal-air interfaces (2, 3). Owing to their ability to confine light in a subwavelength scale with high intensity, SPPs can be used to overcome the diffraction limit, miniaturize photonic components, and build highly integrated optical components and circuits. Thus, they have found (or have potential) applications in biomedical sensing, near-field microscopy, optoelectronics, photovoltaics, and nanophotonics (4-11).In the far-infrared, terahertz, and microwave frequency bands, metals behave akin to perfectly electrical conductors (PECs), and thus SPPs cannot be supported by a metal surface. Although some designs based on metal wires or strips are able to support surface leaky modes that have some degree of lateral confinement at terahertz frequencies (12, 13), the concept of plasmonic metamaterials has proven very useful in the production of highly confined surface electromagnetic (EM) waves at low frequencies (14-27). Early work in this area can be traced back to the 1950s and 1960s, when corrugated metal structures were used to generate surface EM waves at microwave frequencies (14, 15). Generally, plasmonic metamaterials consist of metal surfaces decorated with 1D arrays of subwavelength grooves, 2D arrays of subwavelength holes/dimples, or 3D metal wires in which a periodic array of radial grooves is drilled (16-26). Recently, an alternative "spoof" SPP structure using complementary split-ring resonators as the unit cell elements has been proposed theoretically (27). The surface EM modes decorated by all of these plasmonic metamaterials are called spoof SPPs, or designer SPPs, because their properties are very similar to those of SPPs at optical frequencies. An important advantage of this metamaterial approach ...

We show how both the subwavelength confinement associated with surface plasmons and the one-dimensional character of plasmonic waveguides can be exploited to enhance the coupling between quantum emitters. Resonance energy transfer and the phenomenon of superradiance are investigated in three different waveguiding schemes (wires, wedges, and channels) by means of the Finite Element Method. We also develop a simplified model that is able to capture the main features of the numerical results.

We study the generation of entanglement between two distant qubits mediated by the surface plasmons of a metallic waveguide. We show that a V-shaped channel milled in a flat metallic surface is much more efficient for this purpose than a metallic cylinder. The role of the misalignments of the dipole moments of the qubits, an aspect of great importance for experimental implementations, is also studied. A careful analysis of the quantum dynamics of the system by means of a master equation shows that two-qubit entanglement generation is essentially due to the dissipative part of the effective qubit-qubit coupling provided by the surface plasmons. The influence of a coherent external pumping, needed to achieve a steady-state entanglement, is discussed. Finally, we pay attention to the question of how to get information experimentally on the degree of entanglement achieved in the system.

A new approach for the spatial and temporal modulation of electromagnetic fields at terahertz frequencies is presented. The waveguiding elements are based on plasmonic and metamaterial notions and consist of an easy-to-manufacture periodic chain of metallic box-shaped elements protruding out of a metallic surface. It is shown that the dispersion relation of the corresponding electromagnetic modes is rather insensitive to the waveguide width, preserving tight confinement and reasonable absorption loss even when the waveguide transverse dimensions are well in the subwavelength regime. This property enables the simple implementation of key devices, such as tapers and power dividers. Additionally, directional couplers, waveguide bends, and ring resonators are characterized, demonstrating the flexibility of the proposed concept and the prospects for terahertz applications requiring high integration density.