Direct observation of bosonic quantum interference of surface plasmon polaritons using photon-number-resolving detectors

Quantum plasmonics is a field of research combining plasmonics with quantum optics and investigates interactions between photons and metallic nanostructures. So far, it has been proven that quantum properties of single photons to excite single surface plasmon polaritons (SPPs) are preserved in the process of photon-SPP-photon mode conversion in plasmonic nanostructures, which suggests the potential application of SPPs to the quantum information processing (QIP). Recently the Hong-Ou-Mandel (HOM) interference of single SPPs was observed in a plasmonic circuitry. However, the visibility was below the classical limit (50%) due to the simultaneous excitation of distinguishable SPP modes. We employed a directional coupler based on long-range surface-plasmon-polariton waveguides (LRSPP-DC) and superconducting photon-number-resolving detectors to directly observe the bosonic quantum interference of single SPPs beyond the classical limit. In addition, we demonstrated the indistinguishability of photons that excite single SPPs is well preserved in the process of photon-SPP mode conversion.

Figure 1

(a) Schematic of the LRSPP-DC. (b) Microscope image of the fabricated device. (c) The far field image of the LRSPP-DC outputs.

Figure 2

(a) Cross-sectional image of our TES. The layers highlighted by the yellow, red, and blue boxes are the dielectric mirror, Ti layer, and antireflection coating, respectively. (b) Counts of $C_{20}$ (blue circle) and $C_{11}$ (red triangle) in the two-photon interference using a 50/50 optical splitter as a function of the optical path difference. The best-fit curves of $C_{20}$ (blue line) and $C_{11}$ (red line) are derived from Eq. (2).

Figure 3

Experimental setup. BPF, band-pass filter; LWPF, long-wavelength pass filter; PBS, polarizing beam splitter; HWP, half-wave plate; PMF, polarization maintaining fiber; SMF, single mode fiber; NBPF, narrow band-pass filter; FBPF, fiber based band-pass filter; TESs, superconducting transition edge sensors; DISC, discriminator; TIA, time interval analyzer.

Figure 4

Anticoincidence counts between two-photon detection and one-photon detection as a function of the optical path difference. The best-fit curve (solid line) is derived from Eq. (2). The estimated total accidental coincidence count (dashed line) is also shown.