Two-plasmon quantum interference

Journal name:
Nature Photonics
Year published:
Published online


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.

At a glance


  1. Schematic of the TPQI measurement.
    Figure 1: Schematic of the TPQI measurement.

    A 407 nm diode laser and a bismuth borate (BiBO) crystal aligned for SPDC generate pairs of single photons at 814 nm. The photons pass through band-pass filters (BPF) and enter polarization-maintaining (PM) fibres, where one is delayed by an adjustable amount. A second pair of collimators couples the photons back into free space and a ×40 microscope objective focuses them into separate waveguides on a photonic chip. At the outputs of the chip, multimode (MM) optical fibres collect the photons and send them to SPAD detectors.

  2. Design of the waveguides.
    Figure 2: Design of the waveguides.

    a, Silicon nitride waveguides deliver pairs of photons to a directional coupler made of DLSPPWs. The nominal coupling length shown here is the same as that which is varied systematically in Fig. 4. b, S-bends in the dielectric waveguides shift the outputs by 500 µm with respect to the inputs, ensuring that no stray light reaches the outputs. c, The dielectric and plasmonic waveguides were designed for optimal overlap of their modes. The colour plots show |E|2 and the scale bars are 300 nm. d, For wavelengths shorter than 770 nm, a second mode appears in the DLSPPWs. At 814 nm, however, the DLSPPWs support only a single mode. The red line marks the effective index of the slab surface plasmon-polariton mode, which is not guided by the PMMA.

  3. Measurements of TPQI in 50-50 directional couplers.
    Figure 3: Measurements of TPQI in 50–50 directional couplers.

    a,b, In the dielectric coupler (a) we observe TPQI with a visibility of 0.944 ± 0.003 and a temporal width of 0.12 ± 0.01 ps, while in the plasmonic case (b) we observe a visibility of 0.932 ± 0.01 and a width of 0.11 ± 0.01 ps. Each point represents the mean of a set of five measurements of ~3,000 counts each (dielectric coupler) or three measurements of ~1,600 counts each (plasmonic coupler). The red lines show fits to an inverted Gaussian function (Supplementary Section 2), from which we extracted the visibility and width of each interference dip. The error bars on individual points show ±1 s.d. of the measurements taken, while estimated errors in the visibilities, also ±1 s.d., were derived from fitting the model function to the data. The estimated error in the widths of the dips represents the precision of the adjustable delay line.

  4. TPQI in plasmonic couplers of different lengths.
    Figure 4: TPQI in plasmonic couplers of different lengths.

    As expected, the visibility of TPQI decreases as the splitting ratio of the directional coupler deviates from 50–50. Insets: images showing light diverging from the outputs of the waveguides, confirming that the coupler becomes more asymmetric as the coupling length increases.


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Author information


  1. Kavli Nanoscience Institute, California Institute of Technology, Pasadena, California 91125, USA

    • James S. Fakonas &
    • Harry A. Atwater
  2. Thomas J. Watson Laboratories of Applied Physics, California Institute of Technology, Pasadena, California 91125, USA

    • James S. Fakonas,
    • Hyunseok Lee,
    • Yousif A. Kelaita &
    • Harry A. Atwater


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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