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Two-plasmon quantum interference


Surface plasma waves on metals arise from the collective oscillation of many free electrons in unison. These waves are usually quantized by direct analogy to electromagnetic fields in free space1,2,3, 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 the same quantum phenomena that photons do. Here, we report a plasmonic version of the Hong–Ou–Mandel experiment4, in which we observe unambiguous two-photon quantum interference between plasmons, confirming that surface plasmons faithfully reproduce this effect with the same visibility and mutual coherence time, to within measurement error, as in the photonic case. These properties are important if plasmonic devices are to be employed in quantum information applications5, which typically require indistinguishable particles.

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Figure 1: Schematic of the TPQI measurement.
Figure 2: Design of the waveguides.
Figure 3: Measurements of TPQI in 50–50 directional couplers.
Figure 4: TPQI in plasmonic couplers of different lengths.


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This work was supported by the Air Force Office of Scientific Research under MURI awards FA9550-12-1-0488 (J.S.F.), FA9550-12-1-0024 (H.L.) and FA9550-04-1-0434 (Y.A.K.). The authors thank the Kavli Nanoscience Institute at Caltech for access to and maintenance of fabrication equipment. Additionally, J.S.F. would like to thank R.M. Briggs for fabrication training and advice.

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J.S.F. and H.A.A. designed the experiment. J.S.F. and Y.A.K. built and tested the SPDC source. J.S.F. and H.L. built and tested the waveguide-coupling set-up. J.S.F. fabricated the waveguides. H.L. performed the measurements of quantum interference. All authors contributed to writing the manuscript.

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Correspondence to Harry A. Atwater.

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The authors declare no competing financial interests.

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Fakonas, J., Lee, H., Kelaita, Y. et al. Two-plasmon quantum interference. Nature Photon 8, 317–320 (2014).

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