The control of quantum states of light at the nanoscale has become possible in recent years with the use of plasmonics. Here, many types of nanophotonic devices and applications have been suggested that take advantage of quantum optical effects, despite the inherent presence of loss. A key example is quantum plasmonic sensing, which provides sensitivity beyond the classical limit using entangled N00N states and their generalizations in a compact system operating below the diffraction limit. In this work, we experimentally demonstrate the excitation and propagation of a two-plasmon entangled N00N state ($N=2$) in a silver nanowire and assess the performance of our system for carrying out quantum sensing. Polarization entangled photon pairs are converted into plasmons in the silver nanowire, which propagate over a distance of 5 μm and reconvert back into photons. A full analysis of the plasmonic system finds that high-quality entanglement is preserved throughout. We measure the characteristic super-resolution phase oscillations of the entangled state via coincidence measurements. We also identify various sources of loss in our setup and show how they can be mitigated, in principle, in order to reach super-sensitivity that goes beyond the classical sensing limit. Our results show that polarization entanglement can be preserved in a plasmonic nanowire and that sensing with a quantum advantage would be possible with only moderate loss present.

中文翻译：

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银纳米线中的量子等离子体N00N态及其在量子传感中的应用

近年来，通过使用等离激元技术，可以控制纳米级光的量子态。在此，尽管存在固有的损耗，但已经提出了利用量子光学效应的许多类型的纳米光子器件和应用。一个关键的例子是量子等离激元传感，它使用纠缠的N00N态及其在衍射极限以下工作的紧凑系统中的泛化，提供了超出经典极限的灵敏度。在这项工作中，我们通过实验证明了两等离激元纠缠的N00N态的激发和传播（$ñ=\mathrm{2个}$），并评估我们用于执行量子传感的系统的性能。偏振纠缠的光子对在银纳米线中转换为等离激元，并在5μm的距离内传播并重新转换回光子。对等离子体系统的全面分析发现，始终保留了高质量的纠缠。我们通过巧合测量来测量纠缠态的特征超分辨率相位振荡。我们还将确定设置中的各种损耗源，并说明如何从原则上减轻损耗，以达到超越传统传感极限的超灵敏性。我们的结果表明，极化纠缠可以保留在等离激元纳米线中，并且在仅存在中等损耗的情况下，具有量子优势的传感是可能的。