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High visibility on-chip quantum interference of single surface plasmons

Yong-Jing Cai, Ming Li, Xi‐Feng Ren, Chang‐Ling Zou, Xiao Xiong, Hua-Lin Lei, Bi‐Heng Liu, Guo‐Ping Guo, Guang‐Can Guo

Published Jan 1, 2015• 2 citations
Publisher

Abstract

In this work, the on-chip quantum interference of two single surface plasmons was achieved and the high visibility (greater than 90%) proves the bosonic nature of single plasmons. The effect of intrinsic losses in plasmonic waveguides is also discussed.
DOI
10.1364/cleo_at.2015.jtu5a.71

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•Jan 8, 2017•0 citations
Surface plasma waves arise from the collective oscillations of billions of electrons at the surface of a metal in unison. The simplest way to quantize these waves is by direct analogy to electromagnetic fields in free space, with the surface plasmon, the quantum of the surface plasma wave, playing the same role as the photon. It follows that surface plasmons should exhibit all of the same quantum phenomena that photons do, including quantum interference and entanglement. Unlike photons, however, surface plasmons suffer strong losses that arise from the scattering of free electrons from other electrons, phonons, and surfaces. Under some circumstances, these interactions might also cause “pure dephasing,” which entails a loss of coherence without absorption. Quantum descriptions of plasmons usually do not account for these effects explicitly, and sometimes ignore them altogether. In light of this extra microscopic complexity, it is necessary for experiments to test quantum models of surface plasmons. In this thesis, I describe two such tests that my collaborators and I performed. The first was a plasmonic version of the Hong-Ou-Mandel experiment, in which we observed two-particle quantum interference between plasmons with a visibility of 93 ± 1%. This measurement confirms that surface plasmons faithfully reproduce this effect with the same visibility and mutual coherence time, to within measurement error, as in the photonic case. The second experiment demonstrated path entanglement between surface plasmons with a visibility of 95 ± 2%, confirming that a path-entangled state can indeed survive without measurable decoherence. This measurement suggests that elastic scattering mechanisms of the type that might cause pure dephasing must have been weak enough not to significantly perturb the state of the metal under the experimental conditions we investigated. These two experiments add quantum interference and path entanglement to a growing list of quantum phenomena that surface plasmons appear to exhibit just as clearly as photons, confirming the predictions of the simplest quantum models.
This thesis investigates properties of quantum states of light while travelling as surface plasmon polaritons in plasmonic waveguides and structures. The bosonic nature of SPPs has been shown in previous work by performing a Hong-Ou-Mandel interference experiment with a plasmonic scattering-based beam splitter. Here, we show the same interference with a higher statistical power thanks to an improved set-up. A visibility of 59 ± 1 % is obtained in the two-photon interference, clearly breaking the classical limit of 50 %. The importance of the phase-relations between the different modes in the beam splitter is experimentally probed. The output state of the interference is then further analyzed by a quantum state tomography set-up. This makes it possible to quantify the entanglement generated in the interference. As the interference happens in the plasmonic beam splitter, this shows, to our knowledge for the first time, entanglement generated in a plasmonic structure. Together with the recent results in terms of entanglement and coherence preservation of SPPs, this clearly shows the potential of quantum plasmonic devices. To move into the realm of applications, we also investigate theoretically the enhancements in sensitivity quantum states of light can deliver for plasmonic sensing. It is shown that despite the losses, quantum metrology techniques can be useful in an interferometer with plasmonic waveguides. Considering the strengths and successes of plasmonic sensing techniques in a wide range of fields, we envision that entangled and squeezed states of light will become a new route to push the limits in sensitivity.
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.