Quantum Plasmonic Sensing Using Squeezed Light

Quantum sensing is an emerging field of quantum optics that seeks to take advantage of quantum correlations available in quantum states of light to enable sensitivities beyond the fundamental classical limits. The sensitivity of measurements and sensing apparatus when using classical states of light is limited to the shot-noise limit (SNL), which is achieved with coherent states of light. Two-mode squeezed states of light (twin beams) have quantum correlations both in time and space, leading to temporal and spatial squeezing properties. Several applications can benefit from such noise reduction to enable new approaches, such as quantum-enhanced interferometry, quantum imaging, and quantum sensing. The emergence of quantum technologies has been referred to as the second quantum revolution. For metrology and sensing applications, in particular, it has led to new state-of-the-art sensitivity limits. In this thesis, we discuss the implementation of quantum sensing based on squeezed states of light and plasmonic sensors as a platform for the demonstration of real-life quantum sensing. We present a quantum-enhanced plasmonic sensing setup that can detect changes in the refractive index of air beyond the SNL. Furthermore, we generalize such experimental apparatus to probe an array of sensors using the quantum correlations present in different spatial locations to demonstrate a parallel quantum-enhanced plasmonic sensing scheme that can simultaneously detect changes in the refractive index of air in multiple locations with a single probing beam. These results prove the applicability of twin beams for real-life applications based on plasmonic sensors. The spatially resolved sensing scheme can be extended to pixel-size sensing of multiple sensors for multi-parameter estimation and detection applications to reach more complex sensing architectures

Fundamental Sensitivity Bounds for Quantum Enhanced Optical Resonance Sensors Based on Transmission and Phase Estimation

Quantum states of light can enable sensing configurations with sensitivities beyond the shot-noise limit (SNL). In order to better take advantage of available quantum resources and obtain the maximum possible sensitivity, it is necessary to determine fundamental sensitivity limits for different possible configurations for a given sensing system. Here, due to their wide applicability, we focus on optical resonance sensors, which detect a change in a parameter of interest through a resonance shift. We compare their fundamental sensitivity limits set by the quantum Cram\'er-Rao bound (QCRB) based on the estimation of changes in transmission or phase of a probing bright two-mode squeezed state (bTMSS) of light. We show that the fundamental sensitivity results from an interplay between the QCRB and the transfer function of the system. As a result, for a resonance sensor with a Lorentzian lineshape a phase-based scheme outperforms a transmission-based one for most of the parameter space; however, this is not the case for lineshapes with steeper slopes, such as higher order Butterworth lineshapes. Furthermore, such an interplay results in conditions under which the phase-based scheme provides a higher sensitivity than the transmission-based one but a smaller degree of quantum enhancement. We also study the effect of losses external to the sensor on the degree of quantum enhancement and show that for certain conditions probing with a classical state can provide a higher sensitivity than probing with a bTMSS. Finally, we discuss detection schemes, namely optimized intensity-difference and optimized homodyne detection, that can achieve the fundamental sensitivity limits even in the presence of external losses

An efficient and low-cost method to create high-density nitrogen-vacancy centers in CVD diamond for sensing applications

The negatively charged Nitrogen-Vacancy (NV-) center in diamond is one of the most versatile and robust quantum sensors suitable for quantum technologies, including magnetic field and temperature sensors. For precision sensing applications, densely packed NV- centers within a small volume are preferable due to benefiting from 1/N^1/2 sensitivity enhancement (N is the number of sensing NV centers) and efficient excitation of NV centers. However, methods for quickly and efficiently forming high concentrations of NV- centers are in development stage. We report an efficient, low-cost method for creating high-density NV- centers production from a relatively low nitrogen concentration based on high-energy photons from Ar+ plasma. This study was done on type-IIa, single crystal, CVD-grown diamond substrates with an as-grown nitrogen concentration of 1 ppm. We estimate an NV- density of ~ 0.57 ppm (57%) distributed homogeneously over 200 um deep from the diamond surface facing the plasma source based on optically detected magnetic resonance and fluorescence confocal microscopy measurements. The created NV-s have a spin-lattice relaxation time (T1) of 5 ms and a spin-spin coherence time (T2) of 4 us. We measure a DC magnetic field sensitivity of ~ 104 nT Hz^-1/2, an AC magnetic field sensitivity of ~ 0.12 pT Hz^-1/2, and demonstrate real-time magnetic field sensing at a rate over 10 mT s-1 using an active sample volume of 0.2 um3

Plasmon Enhanced Quantum Properties of Single Photon Emitters with Hybrid Hexagonal Boron Nitride Silver Nanocube Systems

Hexagonal boron nitride (hBN) has emerged as a promising ultrathin host of single photon emitters (SPEs) with favorable quantum properties at room temperature, making it a highly desirable element for integrated quantum photonic networks. One major challenge of using these SPEs in such applications is their low quantum efficiency. Recent studies have reported an improvement in quantum efficiency by up to two orders of magnitude when integrating an ensemble of emitters such as boron vacancy defects in multilayered hBN flakes embedded within metallic nanocavities. However, these experiments have not been extended to SPEs and are mainly focused on multiphoton effects. Here, we study the quantum single photon properties of hybrid nanophotonic structures composed of SPEs created in ultrathin hBN flakes coupled with plasmonic silver nanocubes. We demonstrate > 200% plasmonic enhancement of the SPE properties, manifested by a strong increase in the SPE fluorescence. Such enhancement is explained by rigorous numerical simulations where the hBN flake is in direct contact with the Ag nanocubes that cause the plasmonic effects. The presented strong and fast single photon emission obtained at room-temperature with a compact hybrid nanophotonic platform can be very useful to various emerging applications in quantum optical communications and computing

Parallel Quantum-Enhanced Sensing

Quantum metrology takes advantage of quantum correlations to enhance the sensitivity of sensors and measurement techniques beyond their fundamental classical limit given by the shot noise limit. The use of both temporal and spatial correlations present in quantum states of light can extend quantum-enhanced sensing to a parallel configuration that can simultaneously probe an array of sensors or independently measure multiple parameters. To this end, we use multi-spatial mode twin beams of light, which are characterized by independent quantum-correlated spatial subregions in addition to quantum temporal correlations, to probe a four-sensor quadrant plasmonic array. We show that it is possible to independently and simultaneously measure local changes in refractive index for all four sensors with a quantum enhancement in sensitivity in the range of $22\%$ to $24\%$ over the corresponding classical configuration. These results provide a first step towards highly parallel spatially resolved quantum-enhanced sensing techniques and pave the way toward more complex quantum sensing and quantum imaging platforms

Lattice resonances of nanohole arrays for quantum enhanced sensing

10 pags., 7 figs.Periodic arrays of nanoholes perforated in metallic thin films interact strongly with light and produce large electromagnetic near-field enhancements in their vicinity. As a result, the optical response of these systems is very sensitive to changes in their dielectric environment, thus making them an exceptional platform for the development of compact optical sensors. Given that these systems already operate at the shot-noise limit when used as optical sensors, their sensing capabilities can be enhanced beyond this limit by probing them with quantum light, such as squeezed or entangled states. Motivated by this goal, here, we present a comparative theoretical analysis of the quantum enhanced sensing capabilities of metallic nanohole arrays with one and two holes per unit cell. Through a detailed investigation of their optical response, we find that the two-hole array supports resonances that are narrower and stronger than its one- hole counterpart, and therefore have a higher fundamental sensitivity limit as defined by the quantum Cramér-Rao bound. We validate the optical response of the analyzed arrays with experimental measure- ments of the reflectance of representative samples. The results of this work advance our understanding of the optical response of these systems and pave the way for developing sensing platforms capable of taking full advantage of the resources offered by quantum states of light.This work was sponsored by Grant No. TEM-FLU PID2019-109502GA-I00 funded by MCIN/AEI/10.13039/ 501100011033 and the U.S. National Science Foundation (Grant No. DMR-1941680). This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. DOE’s National Nuclear Security Administration under contract DE-NA0003525

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Details on calibration, data taking and analysis, expected quantum sensitivity enhancement, and sensitivity order of magnitude estimation

Plasmon Enhanced Quantum Properties of Single Photon Emitters with Hybrid Hexagonal Boron Nitride Silver Nanocube Systems

Hexagonal boron nitride (hBN) has emerged as a promising ultrathin host of single photon emitters (SPEs) with favorable quantum properties at room temperature, making it a highly desirable element for integrated quantum photonic networks. One major challenge of using these SPEs in such applications is their low quantum efficiency. Recent studies have reported an improvement in quantum efficiency by up to two orders of magnitude when integrating an ensemble of emitters such as boron vacancy defects in multilayered hBN flakes embedded within metallic nanocavities. However, these experiments have not been extended to SPEs and are mainly focused on multiphoton effects. Here, the quantum single-photon properties of hybrid nanophotonic structures composed of SPEs created in ultrathin hBN flakes coupled with plasmonic silver nanocubes (SNCs) are studied. The authors demonstrate 200% plasmonic enhancement of the SPE properties, manifested by a strong increase in the SPE fluorescence. Such enhancement is explained by rigorous numerical simulations where the hBN flake is in direct contact with the SNCs that cause the plasmonic effects. The presented strong and fast single photon emission obtained at room temperature with a compact hybrid nanophotonic platform can be very useful to various emerging applications in quantum optical communications and computing