Surface plasmon polaritons (SPPs) are known to preserve quantum optical properties --such as squeezing-- over distances far exceeding those of classical field amplitudes. However, the surviving squeezing typically becomes so weak that its detection requires prohibitively large numbers of measurements. Here we introduce a fundamentally new paradigm for plasmonic communication in which nonclassicality itself carries the information. We (i) encode classical data (bits or dits) directly into the {\it degree of nonclassicality} (e.g., squeezing) of SPPs, thereby enabling information transfer over distances where classical amplitude encoding fails. We further (ii) show that this information can be retrieved from long-lived correlations generated at the readout stage via a beam splitter. Crucially, we demonstrate that (iii) encoding on initially noisy states leads to a counterintuitive enhancement: the encoded information remains accessible after long propagation distances using only a few measurements, outperforming both squeezed vacuum and amplitude-based schemes by orders of magnitude. Finally, (iv) in the THz regime --relevant for graphene and carbon-nanotube platforms at room temperature-- we \textit{exploit}, rather than suppress, the intrinsic thermal background, enabling robust, high-bandwidth nanoscale communication.
Surface plasmon polaritons (SPPs) are known to preserve quantum optical properties --such as squeezing-- over distances far exceeding those of classical field amplitudes. However, the surviving squeezing typically becomes so weak that its detection requires prohibitively large numbers of measurements. Here we introduce a fundamentally new paradigm for plasmonic communication in which nonclassicality itself carries the information. We (i) encode classical data (bits or dits) directly into the {\it degree of nonclassicality} (e.g., squeezing) of SPPs, thereby enabling information transfer over distances where classical amplitude encoding fails. We further (ii) show that this information can be retrieved from long-lived correlations generated at the readout stage via a beam splitter. Crucially, we demonstrate that (iii) encoding on initially noisy states leads to a counterintuitive enhancement: the encoded information remains accessible after long propagation distances using only a few measurements, outperforming both squeezed vacuum and amplitude-based schemes by orders of magnitude. Finally, (iv) in the THz regime --relevant for graphene and carbon-nanotube platforms at room temperature-- we \textit{exploit}, rather than suppress, the intrinsic thermal background, enabling robust, high-bandwidth nanoscale communication.
We propose a scheme for achieving quantum entanglement between surface-plasmon-polariton (SPP) modes in a hybrid plasmonic-graphene waveguide system. We then demonstrate that the entanglement of two SPP modes can serve as a useful resource for quantum communication of continuous-variable light signals over a long distance, mediated by concatenated swap operations. The teleportation fidelity of an unknown coherent state has strong robustness to loss and imperfect detection at relatively strong effective optical-microwave coupling. Our study opens the way for continuous-variable quantum communication and quantum information based on hybrid plasmonic-graphene systems.
Quantum plasmonics presents a new insight into quantum photonic science and technology. The unique properties of surface plasmon polaritons (SPPs) provide immense potential for quantum control of light in ultracompact systems. The quantum behavior of SPPs should be described by corresponding SPP states, which have attracted a lot of investigative interest recently. In this work, we focus on the interaction of single quanta of SPPs. The generation of a squeezed SPP state through parametric down conversion (PDC) is investigated. Due to intrinsic loss of an SPP, this system deviates from those based on traditional bulk optics. Unlike the previous negative image, it is surprising to find that the role of loss is partly positive. As an illustration, the squeezing process of an SPP in a hybrid plasmonic waveguide system is calculated and shown. The degree of squeezing could reach above 7 dB with a propagation length of only about 12 μm in a single path. Moreover, the spectrum of squeezing has a band of 481.7 THz. It is a valuable advantage to keep a high squeezing degree in such a wide band, which is hard to realize in bulk systems. In addition, the tolerance of the system to loss is also shown to be good. The plasmonic system exhibits an attractive advantage in constructing integrated quantum circuits.
This thesis investigates properties of quantum states of light while travelling as surface plasmon polaritons in plasmonic waveguides and structures. The bosonic nature of SPPs has been shown in previous work by performing a Hong-Ou-Mandel interference experiment with a plasmonic scattering-based beam splitter. Here, we show the same interference with a higher statistical power thanks to an improved set-up. A visibility of 59 ± 1 % is obtained in the two-photon interference, clearly breaking the classical limit of 50 %. The importance of the phase-relations between the different modes in the beam splitter is experimentally probed. The output state of the interference is then further analyzed by a quantum state tomography set-up. This makes it possible to quantify the entanglement generated in the interference. As the interference happens in the plasmonic beam splitter, this shows, to our knowledge for the first time, entanglement generated in a plasmonic structure. Together with the recent results in terms of entanglement and coherence preservation of SPPs, this clearly shows the potential of quantum plasmonic devices. To move into the realm of applications, we also investigate theoretically the enhancements in sensitivity quantum states of light can deliver for plasmonic sensing. It is shown that despite the losses, quantum metrology techniques can be useful in an interferometer with plasmonic waveguides. Considering the strengths and successes of plasmonic sensing techniques in a wide range of fields, we envision that entangled and squeezed states of light will become a new route to push the limits in sensitivity.
We investigate the excitation of quantum plasmonic states of light in graphene using end-fire and prism coupling. In order to model the excitation process quantum mechanically, we quantize the transverse-electric and transverse-magnetic surface plasmon polariton (SPP) modes in graphene. A selection of regimes are then studied that enable the excitation of SPPs by photons and we show that efficient coupling of photons to graphene SPPs is possible at the quantum level. Furthermore, we study the excitation of quantum states and their propagation under the effects of loss induced from the electronic degrees of freedom in the graphene. Here we investigate whether it is possible to protect quantum information using quantum-error correction techniques. We find that these techniques provide a robust-to-loss method for transferring quantum states of light in graphene over large distances.
Quantum metrology leverages quantum resources to surpass classical limits in measurement sensitivity, enabling new capabilities across fields ranging from fundamental physics to sensing, imaging, and navigation. As these techniques mature, an increasing emphasis is being placed on translating quantum-enhanced performance into practical, robust systems that can operate outside controlled laboratory environments. The overarching objective of the work presented in this dissertation is to develop sensing platforms that achieve improved performance by leveraging quantum resources in realistic, noisy environments. In particular, this work focuses on the use of squeezed states of light, and investigates how two-mode squeezed states generated through four-wave mixing can be developed into compact sources and integrated with practical fiber-optic and plasmonic sensing platforms. Squeezed states of light, and in particular two-mode squeezed states (twin beams), constitute a powerful resource in quantum metrology. Their reduced quantum noise enables measurements below the noise level that can be achieved with classical resources, know as the shot-noise limit, offering genuine quantum advantages in precision sensing and measurement. Beyond their fundamental interest, such states provide a pathway toward practical quantum technologies capable of outperforming classical approaches in real-world applications. For many practical implementations, it is therefore highly advantageous to demonstrate squeezed light sources that are compact, stable, and compatible with existing photonic infrastructures. This is needed to lower the barrier to incorporate quantum resources into experiments and sensing platforms, particularly in environments where bulky or delicate optical setups are impractical. To demonstrate a pathway toward practical quantum sensing with squeezed light, this dissertation presents experimental studies spanning both the development of deployable squeezed-light sources and their application in practical or real-world sensing platforms. We present the development of a compact, narrowband, fiber-coupled source of squeezed light based on four-wave mixing in hot Rb 85 vapor. The system achieves more than 4 dB of intensity-difference squeezing after propagation through polarization-maintaining fibers, demonstrating that strong quantum correlations can be preserved even when fiber coupled. This platform serves as a standalone proof-of-principle demonstration of a deployable, low–size, weight, and power (low-SWaP) squeezed-light source that is well suited for applications where fiber delivery of quantum-correlated light can be readily integrated into existing experimental and sensing platforms. In addition to this source development, the dissertation investigates two distinct applications of twin-beam for quantum-enhanced sensing. The first one focuses on fiber Bragg grating (FBG) sensors, which are widely used to monitor strain, vibration, and temperature in applications ranging from civil infrastructure and aerospace systems to oil and gas pipeline monitoring. By coupling the squeezed light into separate FBGs and employing differential detection, common-mode noise is suppressed while localized perturbations are retained. This work provides a feasibility study for using twin-beams to enhance the sensitivity of FBG-based sensors beyond classical limits while providing robustness against environmental noise. The second application examines plasmonic sensors based on surface plasmon resonances in metallic nanostructures, which enable highly sensitive detection of small changes in refractive index. Such plasmonic sensors are particularly useful for applications in chemical and biological sensing, environmental monitoring, and lab-on-chip platforms, where compactness and high sensitivity to refractive index changes are critical. A dual-probing scheme is introduced in which both beams of a two-mode squeezed state interact with a polarization-dependent plasmonic nanostructure. By exploiting opposite spectral response for orthogonal polarizations, this approach improves measurement sensitivity relative to our previous technique of using one beam to probe the plasmonic sensor and the other to serve as a reference. This new technique takes advantage of transducing information into both the beams while preserving the benefits of differential detection for noise cancellation. Taken together, these studies illustrate complementary approaches for leveraging quantum correlations for practical sensing problems. Through the development of a compact, fiber-compatible squeezed-light source and independent demonstrations of enhanced performance in both fiber-optic and plasmonic sensors, this dissertation advances pathways for translating quantum sensing approaches based on quantum states of light from laboratory demonstrations into deployable metrology tools.
Surface plasmon polaritons (SPPs) are collective excitations of free electrons propagating along a metal-dielectric interface. Although some basic quantum properties of SPPs, such as the preservation of entanglement, the wave-particle duality of a single plasmon, the quantum interference of two plasmons, and the verification of entanglement generation, have been shown, more advanced quantum information protocols have yet to be demonstrated with SPPs. Here, we experimentally realize quantum state teleportation between single photons and SPPs. To achieve this, we use polarization-entangled photon pairs, coherent photon-plasmon-photon conversion on a metallic subwavelength hole array, complete Bell-state measurements and an active feed-forward technique. The results of both quantum state and quantum process tomography confirm the quantum nature of the SPP mediated teleportation. An average state fidelity of [Formula: see text] and a process fidelity of [Formula: see text], which are well above the classical limit, are achieved. Our work shows that SPPs may be useful for realizing complex quantum protocols in a photonic-plasmonic hybrid quantum network.