When a single semiconductor quantum dot is placed a few nanometres from a chemically grown silver nanowire, its emission couples to the wire's guided surface plasmon polaritons, generating single, quantized plasmons that scatter off the wire ends (Akimov et al., Nature 450, 402, 2007). The source paper framed the open goal of this platform: exploiting the cavity-free, broadband emitter–wire coupling for quantum-functional devices, culminating in optical opacity of a single emitter to incident single plasmons — a single-plasmon transistor — with deterministic emitter placement, loss-managed propagation, and mode-matched in/out-coupling as the enabling challenges such a realization must address. Single-plasmon generation is established, deterministic placement and integrated circuits have progressed substantially, but no demonstration exists of a single emitter rendering a propagating plasmonic mode nonlinear at the single-plasmon level; the best single-quantum-dot–wire experiments (2023) still operate in the intermediate-coupling regime. The problem remains open.

Background

A surface plasmon polariton (SPP) is an electromagnetic surface wave bound to a metal–dielectric interface that can be confined to a cross section far below the diffraction limit. Chemically synthesized silver nanowires with diameters of order 100 nm support such guided modes at optical frequencies, at the price of ohmic dissipation — absorption in the metal that limits the distance a plasmon travels (typically micrometres at visible wavelengths). A quantum dot (QD) is a semiconductor nanocrystal whose discrete excitonic states act as an artificial atom, a two-level-like optical emitter. When a QD sits within ~10 nm of a nanowire, the tight plasmonic field confinement strongly modifies its spontaneous emission — the Purcell effect, an enhancement of the radiative rate caused by the increased local density of optical states. The fraction of emission funneled into the guided mode is the beta factor, and single-plasmon generation is certified by photon-correlation measurements: the normalized second-order correlation function g(2)(τ)g^{(2)}(\tau), which shows antibunching (g(2)(0)<0.5g^{(2)}(0) < 0.5) when emission events occur one at a time. In the 2007 experiment of Akimov, Mukherjee, Yu, Chang, Zibrov, Hemmer, Park and Lukin, single CdSe dots near silver nanowires produced single plasmons verified by non-classical correlations between dot and wire-end emission, with efficient coupling accompanied by more than 2.5-fold spontaneous-emission enhancement — all without any optical cavity, hence broadband. The same platform had been proposed by Chang, Sørensen, Demler and Lukin (Nature Physics 3, 807, 2007) as the basis for quantum nonlinear optics: in the formalism of waveguide quantum electrodynamics (QED), where a two-level emitter couples to a one-dimensional continuum of guided modes, the relevant figure of merit is the effective Purcell factor P=Γpl/Γ′P = \Gamma_{pl}/\Gamma', the ratio of the emitter's decay rate into the guided plasmon mode to its decay into all other channels. An emitter with PP substantially above 1 scatters a large fraction of a resonant incident photon, and for P≫1P \gg 1 it acts on resonance as a nearly perfect mirror — a single emitter making the waveguide nearly opaque — while for P≪1P \ll 1 the emitter has little influence on the incident field. Because a two-level emitter saturates after absorbing one quantum, a first (gate) plasmon can switch the transmission of subsequent (signal) plasmons: a single-photon transistor at the nanoscale, or 'optical opacity' of a single atom for individual light quanta. The dissipative nature of plasmonic modes helps reach large Purcell factors but also adds absorption loss, and the trade-off is governed by the ratio of emitter-mediated scattering to ohmic loss. Subsequent work established single-plasmon generation across emitter types (molecules, nitrogen-vacancy centers in diamond) and developed theory for strong coupling to propagating SPPs (González-Tudela et al., Phys. Rev. Lett. 110, 126801, 2013), while reviews of quantum plasmonics (Tame et al., Nature Physics 9, 329, 2013; Chang, Vuletić and Lukin, Nature Photonics 8, 685, 2014) identified photon–photon nonlinearities at the single-emitter level as the outstanding experimental target. Strong coupling of a single emitter to a plasmonic mode has since been achieved only in nanocavities (metallic nanogaps), not in the propagating-wire geometry of the source experiment, and single-emitter waveguide experiments remain in the intermediate-coupling regime (Seidel et al., Nano Letters, 2023). The unresolved target is the full quantum-nonlinear regime first sketched in the source paper's outlook — optical opacity and single-plasmon transistor action — for which deterministic emitter placement, loss-managed propagation, and mode-matched in/out-coupling are the enabling challenges a realization must contend with.

Problem Statement

Realize the quantum-nonlinear regime of a single two-level emitter coupled to a one-dimensional propagating plasmonic mode on a metallic nanowire — the regime envisioned in Akimov et al. (2007) and Chang et al. (2007) — and the integrated quantum devices that depend on it. The common scientific objective is that a single emitter controls the transport of individual plasmons in a cavity-free, broadband plasmonic waveguide. Concretely, the goal is to achieve and demonstrate: (i) optical opacity — a single quantum emitter acting as a nearly perfect mirror for resonant incident individual plasmons in the wire geometry, i.e., its spontaneous emission into the guided plasmon mode dominating its decay into all other channels (large effective Purcell factor P=Γpl/Γ′P = \Gamma_{pl}/\Gamma', in the notation of Chang et al., 2007), so that resonant extinction/reflection of incident individual plasmons approaches unity to within ∼1/P\sim 1/P, rather than merely exceeding the loss channels; and (ii) single-plasmon nonlinearity — conditional switching or transistor action in which one gate plasmon changes the transmission of subsequent signal plasmons, with the incident fields calibrated to the single-plasmon level rather than classical intensities. Deterministic emitter placement and orientation relative to the wire with a controlled spacer, propagation loss characterized and managed (materials, geometry) over the required distance, and mode-matched in/out-coupling (e.g., to optical fibers or engineered launch/collection sites) are the enabling challenges identified by the source papers that any realization must contend with; they are context for the demonstration rather than independent acceptance criteria. Single-plasmon generation itself, and standalone demonstrations of deterministic placement, fiber coupling, or circuit routing in the weak- or intermediate-coupling regime, are already established and do not by themselves answer the question, which requires (i) and (ii) jointly: near-unity resonant extinction (i) alone, without the demonstrated single-plasmon switching of (ii), is not sufficient.

References

  1. A. Akimov, A. Mukherjee, C. Yu, D. Chang, A. Zibrov, P. Hemmer, H. Park, M. Lukin (2007). Generation of single optical plasmons in metallic nanowires coupled to quantum dots. Nature 450, 402-406. DOI: 10.1038/nature06230, https://doi.org/10.1038/nature06230
  2. D. E. Chang, A. S. Sørensen, E. A. Demler, M. D. Lukin (2007). A single-photon transistor using nanoscale surface plasmons. Nature Physics 3, 807-812. DOI: 10.1038/nphys708, https://doi.org/10.1038/nphys708
  3. R. Kolesov, B. Grotz, G. Balasubramanian, R. J. Stöhr, A. A. Nicolet, P. R. Hemmer, F. Jelezko, J. Wrachtrup (2009). Wave-particle duality of single surface plasmon polaritons. Nature Physics 5, 470-474. DOI: 10.1038/nphys1278, https://doi.org/10.1038/nphys1278
  4. A. Cuche, O. Mollet, A. Drezet, S. Huant (2010). “Deterministic” quantum plasmonics. Nano Letters 10, 4566-4570. DOI: 10.1021/nl102568m, https://doi.org/10.1021/nl102568m
  5. M. Celebrano, R. Lettow, P. Kukura, M. Agio, A. Renn, S. Götzinger, V. Sandoghdar (2010). Efficient coupling of single photons to single plasmons. Optics Express 18, 13829-13835. DOI: 10.1364/OE.18.013829, https://doi.org/10.1364/OE.18.013829
  6. González-Tudela, Huidobro, Martín-Moreno, Tejedor, García-Vidal (2013). Theory of strong coupling between quantum emitters and propagating surface plasmons. Physical Review Letters 110, 126801. DOI: 10.1103/PhysRevLett.110.126801, https://doi.org/10.1103/PhysRevLett.110.126801
  7. Tame, McEnery, Özdemir, Lee, Maier, Kim (2013). Quantum plasmonics. Nature Physics 9, 329-340. DOI: 10.1038/nphys2615, https://doi.org/10.1038/nphys2615
  8. D. E. Chang, V. Vuletić, M. D. Lukin (2014). Quantum nonlinear optics — photon by photon. Nature Photonics 8, 685-694. DOI: 10.1038/nphoton.2014.192, https://doi.org/10.1038/nphoton.2014.192
  9. S.-H. Gong, J.-H. Kim, Y.-H. Ko, C. Rodriguez, J. Shin, Y.-H. Lee, L. S. Dang, X. Zhang, Y.-H. Cho (2015). Self-aligned deterministic coupling of single quantum emitter to nanofocused plasmonic modes. Proceedings of the National Academy of Sciences 112, 5280-5285. DOI: 10.1073/pnas.1418049112, https://doi.org/10.1073/pnas.1418049112
  10. R. Chikkaraddy, B. de Nijs, F. Benz, S. J. Barrow, O. A. Scherman, E. Rosta, A. Demetriadou, P. Fox, O. Hess, J. J. Baumberg (2016). Single-molecule strong coupling at room temperature in plasmonic nanocavities. Nature 535, 127-130. DOI: 10.1038/nature17974, https://doi.org/10.1038/nature17974
  11. H. Siampour, S. Kumar, S. I. Bozhevolnyi (2017). Nanofabrication of plasmonic circuits containing single photon sources. ACS Photonics 4, 1879-1884. DOI: 10.1021/acsphotonics.7b00374, https://doi.org/10.1021/acsphotonics.7b00374
  12. J. D. Cox, F. J. García de Abajo (2019). Single-plasmon thermo-optical switching in graphene. Nano Letters 19, 3743-3750. DOI: 10.1021/acs.nanolett.9b00879, https://doi.org/10.1021/acs.nanolett.9b00879
  13. S. Toyouchi, M. Wolf, Y. Nakao, Y. Fujita, T. Inose, B. Fortuni, K. Hirai, J. Hofkens, S. De Feyter, J. Hutchison, H. Uji-i (2020). Controlled fabrication of optical signal input/output sites on plasmonic nanowires. Nano Letters 20, 2460-2467. DOI: 10.1021/acs.nanolett.9b05199, https://doi.org/10.1021/acs.nanolett.9b05199
  14. C. Schörner, M. Lippitz (2020). Single molecule nonlinearity in a plasmonic waveguide. Nano Letters 20, 2152-2156. DOI: 10.1021/acs.nanolett.0c00196, https://doi.org/10.1021/acs.nanolett.0c00196
  15. D. Ge, S. Marguet, A. Issa, S. Jradi, T. H. Nguyen, M. Nahra, J. Béal, R. Deturche, H. Chen, S. Blaize, J. Plain, C. Fiorini, L. Douillard, O. Soppera, X. Q. Dinh, C. Dang, X. Yang, T. Xu, B. Wei, X. W. Sun, C. Couteau, R. Bachelot (2020). Hybrid plasmonic nano-emitters with controlled single quantum emitter positioning on the local excitation field maximum. Nature Communications 11, 3414. DOI: 10.1038/s41467-020-17248-8, https://doi.org/10.1038/s41467-020-17248-8
  16. S. Kumar, S. I. Bozhevolnyi (2021). Single photon emitters coupled to plasmonic waveguides: a review. Advanced Quantum Technologies 4, 2100057. DOI: 10.1002/qute.202100057, https://doi.org/10.1002/qute.202100057
  17. C. You, M. Hong, N. Bhusal, J. Chen, M. A. Quiroz-Juárez, J. Fabre, F. Mostafavi, J. Guo, I. De Leon, R. de J. León-Montiel, O. S. Magaña-Loaiza (2021). Observation of the modification of quantum statistics of plasmonic systems. Nature Communications 12, 5161. DOI: 10.1038/s41467-021-25489-4, https://doi.org/10.1038/s41467-021-25489-4
  18. Sugawara, Xuan, Mitsumori, Edamatsu, Sadgrove (2022). Plasmon-enhanced single photon source directly coupled to an optical fiber. Physical Review Research 4, 043146. DOI: 10.1103/PhysRevResearch.4.043146, https://doi.org/10.1103/PhysRevResearch.4.043146
  19. M. Seidel, Y. Yang, T. Schumacher, Y. Huo, S. F. Covre da Silva, S. Rodt, A. Rastelli, S. Reitzenstein, M. Lippitz (2023). Intermediate field coupling of single epitaxial quantum dots to plasmonic waveguides. Nano Letters 23, 10532-10537. DOI: 10.1021/acs.nanolett.3c03442, https://doi.org/10.1021/acs.nanolett.3c03442
  20. S. Abdullah, E. J. C. Dias, J. Krpenský, V. Mkhitaryan, F. J. García de Abajo (2024). Toward complete optical coupling to confined surface polaritons. ACS Photonics 11, 2183-2193. DOI: 10.1021/acsphotonics.3c01742, https://doi.org/10.1021/acsphotonics.3c01742
  21. M. Hong, R. B. Dawkins, B. Bertoni, C. You, O. S. Magaña-Loaiza (2024). Nonclassical near-field dynamics of surface plasmons. Nature Physics 20, 830-835. DOI: 10.1038/s41567-024-02426-y, https://doi.org/10.1038/s41567-024-02426-y
  22. S. N. Gupta (2025). Single quantum emitter strong coupling with plasmonic cavities: a review. physica status solidi (b) 263, e202500491. DOI: 10.1002/pssb.202500491, https://doi.org/10.1002/pssb.202500491