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arXiv:1810.00114 · Physical Review Applied

Quantum Coherence Preservation in Extremely Dispersive Plasmonic Media

Yury S. Tokpanov, James S. Fakonas, Benjamin Vest, Harry A. Atwater

Published Oct 16, 2019• 5 citations
arXivHTMLPDF

Abstract

Decoherence is one of the limiting factors in quantum technology. To use plasmonic components here, knowing whether the quantum properties of individual surface plasmons can be protected over long distances is crucial, yet has not been fully addressed experimentally. The authors investigate the quantum decoherence of single surface plasmons in the high-confinement regime, where the excitation's significant matter component is expected to cause prompt decoherence. Surprisingly, the coherence properties of plasmons are well preserved even in this regime, highlighting that, despite intrinsic losses, plasmonic devices can have a remarkable range of utility for quantum applications.

DOI
10.1103/physrevapplied.12.044037

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Decoherence is one of the limiting factors in quantum technology. To use plasmonic components here, knowing whether the quantum properties of individual surface plasmons can be protected over long distances is crucial, yet has not been fully addressed experimentally. The authors investigate the quantum decoherence of single surface plasmons in the high-confinement regime, where the excitation's significant matter component is expected to cause prompt decoherence. Surprisingly, the coherence properties of plasmons are well preserved even in this regime, highlighting that, despite intrinsic losses, plasmonic devices can have a remarkable range of utility for quantum applications.
Previous experiments have shown that surface plasmon polaritons (SPPs) preserve their entangled state and do not cause measurable decoherence. However, essentially all of them were done using SPPs whose dispersion was in the linear “photon-like” regime. We report in this presentation on experiments showing how transition to “true-plasmon” non-linear dispersion regime, which occurs near SPP resonance frequency, will affect quantum coherent properties of light. To generate a polarization-entangled state we utilize type-I parametric down-conversion, occurring in a pair of non-linear crystals (BiBO), glued together and rotated by 90 degrees with respect to each other. For state projection measurements, we use a pair of polarizers and single-photon avalanche diode coincidence count detectors. We interpose a plasmonic hole array in the path of down-converted light before the polarizer. Without the hole array, we measure visibility V=99-100% and Bell’s number S=2.81±0.03. To study geometrical effects we fabricated plasmonic hole arrays (gold on optically polished glass) with elliptical holes (axes are 190nm and 240nm) using focused ion beam. When we put this sample in our system we measured the reduction of visibility V=86±5% using entangled light. However, measurement using classical light gave exactly the same visibility; hence, this reduction is caused only by the difference in transmission coefficients of different polarizations. As samples with non-linear dispersion we fabricated two-layer (a-Si - Au) and three-layer (a-Si – Au – a-Si) structures on optically polished glass with different pitches and circular holes. The results of measurements with these samples will be discussed along with the theoretical investigations.
$Q\phantom{\rule{0}{0ex}}u\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}u\phantom{\rule{0}{0ex}}m$ $p\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}s\phantom{\rule{0}{0ex}}m\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}s$ is an emerging field with a wide range of applications in quantum information science. Despite significant progress so far, it is not known how decoherence affects quantum plasmonic systems. This experimental study shows that damping of either amplitude or phase can lead to decoherence in these systems, and provides important information for designing plasmonic waveguide systems for loss-tolerant and phase-sensitive applications, such as quantum sensing and imaging. The techniques developed here may be useful for studying decoherence in other plasmonic structures, too, including nanoantennas, unit cells in metamaterials, and nanotraps for cold atoms.
Previous experiments have shown that surface plasmon polaritons (SPPs) preserve their entangled state and do not cause measurable decoherence. However, essentially all of them were done using SPPs whose dispersion was in the linear “photon-like” regime. We report in this presentation on experiments showing how transition to “true-plasmon” non-linear dispersion regime, which occurs near SPP resonance frequency, will affect quantum coherent properties of light. To generate a polarization-entangled state we utilize type-I parametric down-conversion, occurring in a pair of non-linear crystals (BiBO), glued together and rotated by 90 degrees with respect to each other. For state projection measurements, we use a pair of polarizers and single-photon avalanche diode coincidence count detectors. We interpose a plasmonic hole array in the path of down-converted light before the polarizer. Without the hole array, we measure visibility V=99-100% and Bell’s number S=2.81±0.03. To study geometrical effects we fabricated plasmonic hole arrays (gold on optically polished glass) with elliptical holes (axes are 190nm and 240nm) using focused ion beam. When we put this sample in our system we measured the reduction of visibility V=86±5% using entangled light. However, measurement using classical light gave exactly the same visibility; hence, this reduction is caused only by the difference in transmission coefficients of different polarizations. As samples with non-linear dispersion we fabricated two-layer (a-Si - Au) and three-layer (a-Si – Au – a-Si) structures on optically polished glass with different pitches and circular holes. The results of measurements with these samples will be discussed along with the theoretical investigations.
Plasmonics offers sub-diffraction confinement of light, which affords enhanced coupling to emitter systems (e.g.quantum dots).This feature makes possible a range of on-chip quantum photonic components -most notably single-photon sources and switches.This potential use of plasmonics, along with the nonlinearity provided by emitter systems, opens up quantum plasmonics as a viable route to realising quantum information processing.In this setting, the excitation of single surface plasmon polaritons (SPPs) on waveguides via single photons and the confirmation of single-photon states upon output is an important goal.In the work reported here, plasmonic waveguides were experimentally probed with single photons.A measurement of the secondorder quantum coherence function yielded a value of g (2) (0) = 0.160 ± 0.002.A value less than 0.5 is indicative of single-excitation states.Furthermore, to confirm successful SPP excitation, the transverse-magnetic mode restriction and exponential decay of SPPs were verified.Having firmly established the ability to probe plasmonic waveguides in the classical and quantum regimes, quantum random number generation was implemented using a plasmonic beam splitter.The random bit sequences produced passed the NIST Statisitical Test Suite once post-processed to correct for a slightly asymmetric beamsplitter.
The quantum control of emitters is a key issue for quantum information processing at the nanoscale. This generally necessitates the strong coupling of emitters to a high Q-cavity for efficient manipulation of the atoms and field dynamics (cavity quantum electrodynamics or cQED). Since almost a decade, strong efforts are put to transpose cQED concepts to plasmonics in order to profit of the strong mode confinement of surface plasmons polaritons. Despite the intrinsic presence of lossy channels leading to strong decoherence in plasmonics systems, it has been experimentally proven that it is possible to reach the strong coupling regim [1].
Plasmonics is a rapidly emerging platform for quantum state engineering with the potential for building ultra-compact and hybrid optoelectronic devices. Recent experiments have shown that despite the presence of decoherence and loss, photon statistics and entanglement can be preserved in single plasmonic systems. This preserving ability should carry over to plasmonic metamaterials, whose properties are the result of many individual plasmonic systems acting collectively, and can be used to engineer optical states of light. Here, we report an experimental demonstration of quantum state filtering, also known as entanglement distillation, using a metamaterial. We show that the metamaterial can be used to distill highly entangled states from less entangled states. As the metamaterial can be integrated with other optical components this work opens up the intriguing possibility of incorporating plasmonic metamaterials in on-chip quantum state engineering tasks.