Light-matter interactions underlie many quantum technologies, yet whether quasiparticles formed from such interactions preserve the full quantum state of light remains unresolved. Surface plasmon polaritons (SPPs), a class of polaritons formed by interacting photons with free-electron oscillations at metal-dielectric interfaces, are prime candidates to explore this question. Here we demonstrate quantum interference between single photons and SPPs using an Au-SiN$_{\mathrm{x}}$ integrated photonic-plasmonic device. Our results reveal that SPPs retain the indistinguishability of their excitation photons, establishing SPP as a viable quantum information carrier and opening a potential route toward photonic-plasmonic quantum circuitry.
Light-matter interactions underlie many quantum technologies, yet whether quasiparticles formed from such interactions preserve the full quantum state of light remains unresolved. Surface plasmon polaritons (SPPs), a class of polaritons formed by interacting photons with free-electron oscillations at metal-dielectric interfaces, are prime candidates to explore this question. Here we demonstrate quantum interference between single photons and SPPs using an Au-SiNx integrated photonic-plasmonic device. Our results reveal that SPPs retain the indistinguishability of their excitation photons, establishing SPP as a viable quantum information carrier and opening a potential route toward photonic-plasmonic quantum circuitry.
Surface plasmon polaritons (SPPs) result from collective oscillations of free electrons coupled to an electromagnetic field at a plane interface between a metal and a dielectric medium. As photons, SPPs can be considered either as waves or as bosonic particles [1] and they can experience striking quantum interferences such as Hong-Ou-Mandel (HOM) effect [2]. We use in this paper a plasmonic platform (Fig.1) that can convert photons into plasmons, and separate or recombine them on a plasmonic beamsplitter [3]. Single SPPs are excited using single photons created by parametric down conversion into a periodically-poled KTP crystal. The platform is placed in a Mach-Zehnder interferometer in order to vary the path difference between SPPs before impinging on the beamsplitter. The setup allows us to revisit quantum optics experiments using SPP as carriers of the quantum of energy. Using the same device, we can test the wave-particle duality, two-particle (HOM) interference and non-local control of single plasmon interference using the entanglement between a photon and a SPP [4]. In the case of the HOM experiment, we demonstrate that the losses can be used as a new degree of freedom [5]. Playing with the dephasing between transmission and reflection coefficient of the beamsplitter, we can observe a peak instead of the usually expected dip in the HOM interference (Fig.2), i.e. an anti-coalescence effect of SPP. In this talk, we will introduce the plasmonic device main features as well as the different quantum plasmonics experiments that have been performed thanks to the SPP beamsplitter. These results are essential to understand the SPP quantum behavior or to develop hybrid plasmon-photon systems for potential future applications in quantum communication mediated by SPP. [1] JM Elson and R.H. Ritchie, Phys. Rev. B 4, 4129 (1971) [2] J.S. Fakonas et al., Nat. Phot. 8, 317 (2014) [3] M.C. Dheur et al. , Sc. Advances 2, e151574 (2016) [4] M.C. Dheur et al. ArXiv 1610.07493, B. Vest et al. ArXiv 1610.07479 [5] S. Barnett et al., Phys. Rev. A 57, 2134 (1998) Fig.1: Plasmonic platform used for the quantum optics experiment (B). It consists in two SPP launchers (A), a plasmonic beamsplitter, and two slits to convert SPP to photons in the glass substrate. The plasmonic interference can thus be probed using single photon counting modules. Fig2: HOM experiment on a lossy plasmonic beamsplitter, whose complex amplitude reflection and transmission factors are denoted r and t respectively. The usual HOM dip is observed when r = it (left), a peak in the coincidence rate is shown when r = t.
Surface plasmon polaritons (SPPs) are collective excitations of free electrons propagating along a metal-dielectric interface. Although some basic quantum properties of SPPs, such as the preservation of entanglement, the wave-particle duality of a single plasmon, the quantum interference of two plasmons, and the verification of entanglement generation, have been shown, more advanced quantum information protocols have yet to be demonstrated with SPPs. Here, we experimentally realize quantum state teleportation between single photons and SPPs. To achieve this, we use polarization-entangled photon pairs, coherent photon-plasmon-photon conversion on a metallic subwavelength hole array, complete Bell-state measurements and an active feed-forward technique. The results of both quantum state and quantum process tomography confirm the quantum nature of the SPP mediated teleportation. An average state fidelity of [Formula: see text] and a process fidelity of [Formula: see text], which are well above the classical limit, are achieved. Our work shows that SPPs may be useful for realizing complex quantum protocols in a photonic-plasmonic hybrid quantum network.
We experimentally demonstrated preservation of indistinguishability between two photons via mode conversions, namely, photon-to-plasmon and plasmon-to-photon conversions. A two-photon interference experiment was carried out using a broadband photon pair generated through a spontaneous parametric downconversion process. We observed the so-called Hong-Ou-Mandel dip with an interferometer including a 1-mm-long surface-plasmon-polariton (SPP) waveguide. The photon indistinguishability of 92.4% was retained after propagation in the SPP waveguide.
Enabling the confinement of light to a scale far below the one of conventional optics, surface plasmon polaritons (SPPs) induced by an electromagnetic field in a metal-dielectric interface supply an ideal system to explore strong quantized light-matter coupling. The fast matter-SPP population exchange reported in previous works makes it a candidate for spin manipulation, but such reversible dynamics asymptotically vanishes accompanying the quantum matter relaxing completely to its ground state. Here, we study the exact dissipative dynamics of a quantum emitter (QE) coupled to SPPs. It is interesting to find that, qualitatively different from conventional findings, the QE can be partially stabilized in its excited state even in the presence of the lossy metal. Our analysis reveals that it is the formation of a QE-SPP bound state which results in such suppressed dissipation. Enriching the decoherence dynamics of the QE in the lossy medium, our result is helpful to understand QE-SPP interactions and apply plasmonic nanostructures in quantum devices.
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.
We develop a microscopic quantum theory of surface plasmon polaritons valid for arbitrary metal-dielectric geometries. Our framework is based on the Power-Zienau-Woolley representation of quantum electrodynamics, which provides an optimal separation between electronic and photonic degrees of freedom and is therefore particularly well suited for constructing quantum descriptions of polaritonic excitations in strongly dispersive media. Within this formulation, the fundamental electronic oscillator is identified as the bulk plasmon mode, which is nonperturbatively coupled to the radiative continuum of free photon modes. This coupling induces a geometry-dependent renormalization of the bulk plasma frequency, giving rise to confined plasmonic resonances. As specific applications, we recover the localized surface plasmon modes of metallic nanoparticles, including radiative frequency shifts and decay, as well as the exact dispersion relation of propagating surface plasmon polaritons at planar interfaces. Our quantum treatment further reveals that light-matter interactions at metal-dielectric interfaces are inherently in the ultrastrong coupling regime. As a result, in the quasistatic limit, the system exhibits unconventional ground-state quantum fluctuations that can be controlled through the refractive index. These results open new intriguing perspectives in the field of quantum plasmonics.