Quantum Interference in Plasmonic Circuits (1309.6942v1)

Abstract: Surface plasmon polaritons (plasmons) are a combination of light and a collective oscillation of the free electron plasma at metal-dielectric interfaces. This interaction allows sub-wavelength confinement of light, beyond the diffraction limit inherent to dielectric structures. The resulting electromagnetic fields are more intense and the strength of optical interactions between metallic structures and light-sources or detectors can be increased. Plasmons maintain non-classical photon statistics and preserve entanglement on plasmon-assisted transmission through thin, patterned metallic films or weakly confining waveguides. For quantum applications it is essential that plasmons behave as indistinguishable quantum particles. Here we report on a quantum interference experiment in a nanoscale plasmonic circuit consisting of an on-chip plasmon beam splitter with integrated superconducting single-photon detectors to allow efficient single plasmon detection. We demonstrate quantum mechanical interaction between pairs of indistinguishable plasmons by observing Hong-Ou-Mandel interference, a hallmark non-classical effect which is the basis of linear optics-based quantum computation. Our work shows that it is feasible to shrink quantum optical experiments to the nanoscale and demonstrates a promising route for sub-wavelength quantum optical networks.

Analysis of Plasmonic Circuits in Quantum Optics

The paper provides a comprehensive exploration into the integration of superconducting nanowire single-photon detectors (SSPDs) within plasmonic circuits, specifically focusing on quantum interference experiments. Through a series of intricate experiments, the authors have illustrated various aspects of plasmon excitation, waveguide modes, and photon number entanglement within a plasmonic framework.

Experimental Framework

The research employs a spontaneous parametric downconversion (SPDC) photon pair source to facilitate quantum experiments. Utilizing a 2 cm long KTP crystal, the authors achieved a pair production rate of 288 MHz, leveraging a 532 nm laser to pump parametric downconversion from 532 nm to 1064 nm. The photon count rates suggest a robust setup for high-fidelity quantum experiments, evidenced by pair rates of 8050 Hz and individual rates exceeding 1 MHz. The application of long pass filters and beam splitters assures optimal phase matching, with spectral data reflecting a coherence time of approximately 4.8 ps.

Waveguide Fabrication and Measurement

The fabrication of waveguides on sapphire substrates involves meticulous processes including e-beam lithography and reactive ion etching. The implementation of e-beam writing in combination with atomic layer deposition on materials like NbN and Al2O3 ensures high precision in device structure, critical for plasmonic applications. Subsequent measurements utilize amplified SSPD signals and time-correlated photon counting, showcasing high sensitivity with sub-nanosecond accuracy.

Investigation of Polarization Dependency

The polarization dependence measurements reveal significant variations in excitation efficiency across different modes, primarily due to differences in input orientations and modal overlap. The coupled waveguide analysis underscores the complexity of achieving efficient mode overlap, with a cross-coupling factor illustrating directional coupler functionality. The coupling factor indicates substantial variability, attributed to fabrication inconsistencies which affect coupling length and uniformity in waveguide structures.

Classical and Quantum Interference Studies

Classical interference experiments conducted with phase-modulated laser inputs suggest notable differences in detector signal visibilities, a key indicator of mode coupling efficacy. With visibilities ranging around 0.6-0.75, these experiments set the stage for understanding quantum interference characteristics. The paper discusses a calculated entanglement visibility of around 0.45, correlating well with observations, while acknowledging potential contributions from LRSP modes which introduce deviations from ideal quantum interference predictions.

Simulation and Mode Analysis

Simulations focus on the eigenmode analysis of single and coupled waveguides, identifying SRSP modes and their impact on propagation length and effective index. Calculations of effective mode areas highlight the comparative confinement achievable within plasmonic structures against diffraction-limited dielectric waveguides. The design and optimization of mode confinement underscores the potential to tailor waveguide dimensions for enhanced performance, albeit with increased propagation loss as a trade-off.

Practical Implications and Future Directions

The findings have significant implications for the utilization of plasmonic circuits in quantum optics, providing pathways to minimize losses and enhance single-photon detection capabilities. The robust experimental techniques and insightful theoretical analyses contribute meaningfully to the advancement of integrated optics and photonics. Future work may explore resolving fabrication challenges and improving coherence times in plasmonic circuits, with potential applications in quantum computing and secure communication technologies.

Conclusion

This paper offers a detailed paper into the interplay between plasmonic circuits and quantum interference, leveraging advanced techniques and simulations to unravel the complexities in mode dynamics and photon statistics. The methodology and insights serve as a foundation for future exploration in enhancing quantum photonic devices and improving the integration capacity of plasmonic elements in quantum networks.