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Physical review. B./Physical review. B

Efficient entanglement between two long-distance quantum emitters mediated by dark-gap-plasmon waveguides

Hua Qiu, Yue You, Xi-Hua Guan, Xiao-Jing Du, Jun He, Zhong‐Jian Yang

Published Aug 29, 2024• 12 citations
Publisher

Abstract

We theoretically investigate the entanglement between two well-separated quantum emitters (QEs) mediated by dark-gap-plasmon waveguides. Both the plasmon-QE coupling strength and the energy transfer efficiency between QEs are crucial for entanglement based on analytical results. To overcome the low energy transfer efficiency brought by common plasmonic waveguides, we construct a simple dark-gap-plasmon waveguide. Such a structure consists of two parallel silver nanowires, where dark plasmon modes are formed in the gap between the two nanowires. The leaky (or radiative) feature of propagating plasmons is highly suppressed, which results in the quality factors of the dark plasmon modes reaching the material limit. Thus, the energy transfer efficiency can reach more than 94% with a distance even over ten working wavelengths ($>10\phantom{\rule{4pt}{0ex}}\textmu{}\mathrm{m}$). Meanwhile, by introducing a tip dimer structure in the nanowire gap, the plasmon-QE coupling can reach a strong coupling regime with realistic parameters. The influences from the geometric parameters of the system are also considered. The energy transfer efficiency is robust to the geometric variations due to the dark feature of the plasmon responses.

DOI
10.1103/physrevb.110.075432

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We theoretically investigate the entanglement between two well-separated quantum emitters (QEs) mediated by dark-gap-plasmon waveguides. Both the plasmon-QE coupling strength and the energy transfer efficiency between QEs are crucial for entanglement based on analytical results. To overcome the low energy transfer efficiency brought by common plasmonic waveguides, we construct a simple dark-gap-plasmon waveguide. Such a structure consists of two parallel silver nanowires, where dark plasmon modes are formed in the gap between the two nanowires. The leaky (or radiative) feature of propagating plasmons is highly suppressed, which results in the quality factors of the dark plasmon modes reaching the material limit. Thus, the energy transfer efficiency can reach more than 94% with a distance even over ten working wavelengths (>10\textmum>10\phantom{\rule{4pt}{0ex}}\textmu{}\mathrm{m}>10\textmum). Meanwhile, by introducing a tip dimer structure in the nanowire gap, the plasmon-QE coupling can reach a strong coupling regime with realistic parameters. The influences from the geometric parameters of the system are also considered. The energy transfer efficiency is robust to the geometric variations due to the dark feature of the plasmon responses.
The resonance energy transfer and entanglement between two-level quantum emitters are typically limited to sub-wavelength distances due to the inherently short-range nature of the dipole-dipole interactions. Moreover, the entanglement of quantum systems is hard to preserve for a long time period due to decoherence and dephasing mainly caused by radiative and nonradiative losses. In this work, we outperform the aforementioned limitations by presenting efficient long-range inter-emitter entanglement and large enhancement of resonance energy transfer between two optical qubits mediated by epsilon-near-zero (ENZ) and other plasmonic waveguide types, such as V-shaped grooves and cylindrical nanorods. More importantly, we explicitly demonstrate that the ENZ waveguide resonant energy transfer and entanglement performance drastically outperforms the other waveguide systems. Only the excited ENZ mode has an infinite phase velocity combined with a strong and homogeneous electric field distribution, which leads to a giant energy transfer and efficient entanglement independent of the emitters' separation distances and nanoscale positions in the ENZ nanowaveguide, an advantageous feature that can potentially accommodate multi-qubit entanglement. Moreover, the transient entanglement can be further improved and become almost independent of the detrimental decoherence effect when an optically active (gain) medium is embedded inside the ENZ waveguide. We also present that efficient steady-state entanglement can be achieved by using a coherent external pumping scheme. Finally, we report a practical way to detect the steady-state entanglement by computing the second-order correlation function. The presented findings stress the importance of plasmonic ENZ waveguides in the design of the envisioned on-chip quantum communication and information processing plasmonic nanodevices.
Multi-qubit entanglement is of paramount importance to perform truly secure quantum optical communication operations. In this work, we present efficient long-range entanglement mediated by plasmonic waveguides operating at their cut-off frequency, where they exhibit an effective epsilon-near-zero (ENZ) response. Only the excited ENZ mode at plasmonic waveguides has a strong and homogeneous electric field distribution compared to different modes excited at groove and rod waveguides, resulting in an efficient two- and multi-qubit entanglement that can persist over extended time periods and long distances. Efficient multipartite entanglement from three equidistantly arranged two-level quantum emitters (qubits) inside the ENZ waveguide is also demonstrated by using three different multi-qubit entanglement criterions, i.e., pairwise concurrence, negativity, and genuine multipartite entanglement. The findings of this work stress the importance of ENZ plasmonic waveguides as an ideal nanophotonic platform to support efficient quantum optical effects.
We investigate a system consisting of a single, as well as two emitters strongly coupled to surface plasmon modes of a nanowire using a Green's function approach. Explicit expressions are derived for the spontaneous decay rate into the plasmon modes and for the atom-plasmon coupling as well as a plasmon-mediated atom-atom coupling. Phenomena due to the presence of losses in the metal are discussed. In case of two atoms, we observe Dicke subradiance and superradiance resulting from their plasmon-mediated interaction. Based on this phenomenon, we propose a scheme for a deterministic two-qubit quantum gate. We also discuss a possible realization of interesting many-body Hamiltonians, such as the spin-boson model, using strong emitter-plasmon coupling.
Allowing the generation of effective interactions between distant quantum emitters (QEs) via flying photons, quantum interconnect (QI) is essentially a light-matter interface and acts as a building block in quantum technologies. A surface plasmon polariton (SPP) supported by a metallic waveguide provides an ideal interface to explore strong light-matter couplings and to realize QI. However, the loss of SPP in metal makes the mediated entanglement of the QEs damp with the increase of the distance and time, which hinders its applications. We propose a scheme of non-Markovian QI formed by the SPP of a metallic nanowire. A mechanism to make the generated entanglement of the QEs persistent is discovered. We find that, as long as bound states are formed in the energy spectrum of total QE-SPP system, the damping of the SPP-mediated entanglement is overcome even in the presence of the metal absorption to the SPP. Our finding enriches our understanding of light-matter couplings in absorptive medium and paves the way for using the SPP in designing QI.
Ultra-thin noble metal nanodisks facilitate high entanglement between distant quantum emitters (QEs). The ultra-thin noble metal nanodisk supports localized plasmon resonances; when these modes are excited high Purcell factors, of a single QE, and high coupling values, between a pair of QEs, are observed. We present that the entanglement time of the QEs surpasses the relaxation time of the individual QEs interacting with the metallic disk and reach the steady state faster than the case the QE are placed in free-space.
Ultra-thin metallic nanodisks, supporting localized plasmon (LP) modes, are used as a platform to facilitate high entanglement between distant quantum emitters (QEs). High Purcell factors, with values above 103, are probed for a QE placed near to an ultra-thin metallic nanodisk, composed of the noble metals Au, Ag, Al, and Cu. The disk supports two sets of localized plasmon modes, which can be excited by QEs with different transition dipole moment orientations. The two QEs are placed on opposite sides of the nanodisk, and their concurrence is used as a measure of the entanglement. We observe that the pair of QEs remains entangled for a duration that surpasses the relaxation time of the individual QE interacting with the metallic disk. Simultaneously, the QEs reach the entangled steady state faster than in the case where the QEs are in free space. Our results reveal a high concurrence value for a QES separation distance of 60 nm, and a transition energy of 0.8 eV (λ = 1550 nm). The robustness exhibited by this system under study paves the way for future quantum applications.