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This paper solves the open problem of characterizing the performance of quantum illumination (QI) with discrete variable states. By devising a novel quantum measurement approach along with meticulous analysis, our investigation demonstrates that, in the limit as $M \rightarrow \infty$, the maximally entangled $M$ mode Bell state achieves optimal performance, matching the two-mode squeezed vacuum in a high-noise regime and exceeding it in low-noise. This result challenges the dominance of continuous variable states in photonic sensing applications and extends the novelty of QI to regimes where no quantum advantage was believed to exist. A closer analysis reveals that this advantage stems from retained entanglement in the transmitted Bell state, a paradigm-shifting discovery since interaction with the environment in optical systems is believed to break entanglement. The complete mathematical analysis of this work provides granular insights into the interaction between photonic systems and environmental noise, motivating further research into discrete variable quantum sensing., Comment: Supplementary material available upon request. Please contact the authors via email

Squeezed light sources, featuring significant degrees of squeezing, flexible time-frequency attributes, and a compact salable form factor, serve as crucial building blocks in an expanding range of applications, spanning from quantum computing to quantum sensing and communications. In this study, we introduce and demonstrate a novel approach to generating squeezed light that exclusively employs standard telecommunication fiber-optic components. The technique leverages the entanglement properties of spontaneous four-wave mixing (SFWM) to generate high squeezing with flexible time-frequency properties. Notably, a record squeezing of 7.5 \(\pm\)0.1 dB is measured within an all-fiber, all-guided-wave platform. The entanglement-assisted squeezing methodology empowers the attainment of squeezing for arbitrary time-frequency modes within the SFWM phase-matching bandwidth, extending beyond coherent laser modes, for the first time. In particular, we measured 5.1 dB and 1.1 dB squeezing on partially coherent and chaotic time-frequency modes that are defined by randomly modulated laser sources and filtered amplified spontaneous emission light, respectively.

In the realm of quantum information, entanglement stands as a cornerstone phenomenon. It underpins a vast array of quantum information processes, offering significant potential for advancements in quantum computing, communication, and sensing. This paper introduces a novel multi-photon entangled state, which generalizes the maximally entangled single-photon state and exhibits remarkable resilience to signal attenuation in photonic applications. We demonstrate the novelty of the proposed state through a simplified target detection model and illustrate its superior performance over traditional single-photon protocols, attributed to its higher entanglement level and enhanced noise suppression capabilities. Our findings suggest that the proposed multi-photon state holds significant promise for enhancing the efficiency and reliability of photonic applications subject to loss. This work lays the groundwork for future exploration into the practical applications of multi-photon entangled states in quantum technologies, potentially revolutionizing our approach to quantum sensing and beyond, Comment: QuIK'24 Workshop - IEEE ISIT conference

We present a new method for target finding and ranging in Lidar applications using high-dimensional Bell states. Combined with a sequential decision rule, this scheme asymptotically achieves zero error probability with finite energy expenditure. This result stems from the synergy of high-dimensional entanglement suppressing false positives and the sequential decision rule optimizing energy usage. It effectively provides a pathway to unbounded quantum advantage over classical methods and has substantial implications for high-precision sensing in noisy environments.

Entanglement and correlation of quantum light can enhance LiDAR sensitivity in the presence of strong background noise. However, the power of such quantum sources is fundamentally limited to a stream of single photons and cannot compete with the detection range of high-power classical LiDAR transmitters. To circumvent this, we develop and demonstrate a quantum-inspired LiDAR prototype based on coherent measurement of classical time-frequency correlations. This system uses a high-power classical source and maintains the high noise rejection advantage of quantum LiDARs. In particular, we show that it can achieve over 100dB rejection (with 100ms integration time) of indistinguishable(with statistically identical properties in every degrees of freedom) in-band noise while still being sensitive to single photon signals. In addition to the LiDAR demonstration, we also discuss the potential of the proposed LiDAR receiver for quantum information applications. In particular, we propose the chaotic quantum frequency conversion technique for coherent manipulation of high dimensional quantum states of light. It is shown that this technique can provide improved performance in terms of selectivity and efficiency as compared to pulse-based quantum frequency conversion., Comment: 10 pages, 5 figures

Photon pairs generated by spontaneous parametric down-conversion are essential for optical quantum information processing, in which the quality of biphoton states is crucial for the performance. To engineer the biphoton wavefunction (BWF) on-chip, the pump envelope function and the phase matching function are commonly adjusted, while the modal field overlap has been considered as a constant in the frequency range of interest. In this work, by utilizing modal coupling in a system of coupled waveguides, we explore the modal field overlap as a new degree of freedom for biphoton engineering. We provide design examples for on-chip generations of polarization entangled photons and heralded single photons, respectively. This strategy can be applied to waveguides of different materials and structures, offering new possibilities for photonic quantum state engineering.

Non-local effects have the potential to radically move forward quantum enhanced LiDAR to provide an advantage over classical LiDAR not only in laboratory environments but practical implementation. In this work, we demonstrate a 43dB lower signal-to-noise ratio using a quantum enhanced LiDAR based on time-frequency entanglement compared with a classical phase-insensitive LiDAR system. Our system can tolerate more than 3 orders of magnitude higher noise than classical singlephoton counting LiDAR systems before detector saturation. To achieve these advantages, we use non-local cancellation of dispersion to take advantage of the strong temporal correlations in photon pairs in spite of the orders of magnitude larger detector temporal uncertainty. We go on to incorporate this scheme with purpose-built scanning collection optics to image non-reflecting targets in an environment with noise., Comment: 6 pages, 4 figures

We report parametric gain by utilizing $ \chi^{(2)} $ non-linearities in a semiconductor Bragg Reflection Waveguide (BRW) waveguide chip. Under the two-mode degenerate type II phase matching, it can be shown that more than 18 dBs of parametric gain for both TE and TM modes is tenable in 100s of micrometers of device length. Polarization insensitive parametric gain can be attained within the 1550 nm region of the spectrum. These AlGaAs BRW waveguides exhibit sub-photon per pulse sensitivity. This is in sharp contrast to other types of parametric gain devices which utilize $ \chi^{(3)} $, where the pump wavelength is in the vicinity of the signal wavelength. This sensitivity, which reached 0.1~photon/pulse, can usher a new era for on-chip quantum information processing using compact, micrometer-scale devices., Comment: 9 pages, 9 figures

An approach to significantly enhance spontaneous Raman sensitivity through the formation of a thin film via thermophoresis along with evaporation at the facet of a Hollow-Core Photonic Crystal Fiber is reported for the first time. Sensitivity of detection is increased by more than 6 orders of magnitude for both organic and inorganic nanoparticles, facilitating the search for trace analytes in solution. Detection of two nanoparticles, Alumina and Polystyrene, is demonstrated down to 392 nM without the use of Surface-Enhanced Raman Spectroscopy or other chemical-based procedures. This new thin-film deposition approach simplifies the simultaneous detection and analysis of small trace compounds, a previously arduous task using conventional spontaneous Raman., Comment: In press

In this work, we develop a design methodology to generate spectrally pure photon pairs in asymmetric heterogeneously coupled waveguides by spontaneous parametric down-conversion. Mode coupling in a system of waveguides is used to directly tailor the group velocity of a supermode to achieve group velocity matching (GVM) that is otherwise not allowed by material dispersion. Design examples based on thin film lithium niobate waveguides are provided, demonstrating high spectral purity and temperature tunability. This approach is a versatile strategy applicable to waveguides of different materials and structures, allowing more versatility in single-photon source designs., Comment: 4 pages, 5 figures

Modal phase matching (MPM) is a widely used phase matching technique in Al$_x$Ga$_{1-x}$As and other $\chi^{(2)}$ nonlinear waveguides for efficient wavelength conversions. The use of a non-fundamental spatial mode compensates the material dispersion but also reduces the spatial overlap of the three interacting waves and therefore limits the conversion efficiency. In this work, we develop a technique to increase the nonlinear overlap by modifying the material nonlinearity, instead of the traditional method of optimizing the modal field profiles. This could eliminate the limiting factor of low spatial overlap inherent to MPM and significantly enhance the conversion efficiency. Among the design examples provided, this technique could increase the conversion efficiency by a factor of up to $\sim$290 in an Al$_x$Ga$_{1-x}$As waveguide. We further show that this technique is applicable to all $\chi^{(2)}$ material systems that utilize MPM for wavelength conversion., Comment: 10 pages, 5 figures

Phase sensitive amplifiers (PSAs) have the potential to empower substantial advances in emerging generations of optical communication systems as well as classical and quantum on-chip signal processing. The core building block of a PSA is a nonlinear medium. While the second-order nonlinearity ($\chi^{(2)}$) is stronger than the third-order nonlinearity ($\chi^{(3)}$), it is used less often in semiconductors for parametric amplification owing to the challenges of effectively phase matching the interacting waves as well as two-photon absorption of the pump. In this work, we demonstrate the successful design, fabrication, and characterization of the first $\chi^{(2)}$-based semiconductor PSA using an efficient phase matching approach and a pulsed pump, based on an aluminium gallium arsenide (AlGaAs) waveguide platform. Non-centrosymmetric semiconductors such as AlGaAs offer appreciable $\chi^{(2)}$. Such waveguides also achieve more than one order of magnitude greater pump field confinement when compared to other materials with large $\chi^{(2)}$ such as Periodically Poled Lithium Niobate (PPLN). Our AlGaAs PSA achieves an on-chip in-phase gain, a sensitivity of 0.005 photons per pulse, and approaches theoretical minimal noise figure (NF) of 0~dB. With the capability of operating on signal states with sub-single photons per pulse, our PSA could usher in a new era of on-chip quantum circuits.

Optical devices capable of suppressing diffraction nature of light are of great technological importance to many nanophotonic applications. One important technique to achieve diffractionless optics is to exploit field canalization effect. However, current technological platforms based on metamaterial structures typically suffer from strict loss-confinement tradeoff, or lack dynamic reconfigurability over device operations. Here we report an integrated canalization platform that can alleviate this performance tradeoff. It is found that by leveraging material absorption of anisotropic 2D materials, the dispersion of this class of materials can flatten without increasing propagation losses and compromising confinement. The realization of such plasmon canalization can be considered using black phosphorus (BP), where topological transition from elliptic to hyperbolic curves can be induced by dynamically leveraging material absorption of BP. At the transition point, BP film can support long range, deeply subwavelength, near-diffractionless field propagation, exhibiting diffraction angle of 5.5o, propagation distance of 10{\lambda}spp, and {\lambda}spp < {\lambda}0/300.

Plasmonic waveguides based on 2D materials, which enable the formations of guided modes confined around few-layered material, are promising plasmonic platforms for the miniaturization of photonic devices. Nonetheless, such waveguides support modes that are evanescent in the waveguide core with the majority of the fields concentrated around waveguide edges, which are different from those supported by 3D dielectric waveguides where the modal fields are of oscillatory nature and peak at the center. As a result, many photonic devices and functionalities that can be achieved within 3D dielectric waveguides based on total-internal-reflation modes cannot be realized using 2D material-based plasmonic structures. In this work, we propose and demonstrate how to leverage anisotropy in 2D materials to tailor of modal fields supported by 2D material waveguide for the first time. By regulating material absorption of the constituent 2D materials, the modal fields of these 2D modes can be tailored to localize around the waveguide center, which in turn can improve the efficiencies of coupling-based photonic functions using 2D materials, from in-plane multimode-interference couplers to out-of-plane optical radiation. Using natural anisotropic 2D materials such as black phosphorus, these pivotal functions can expand existing device capabilities that are typically achieved in 3D dielectrics but using 2D materials, thus allowing for the implementation of 2D plasmonic circuits with no need to relying on 3D layers.

We demonstrate novel instrumentation for spontaneous Raman spectroscopy in biofluids, enabling development of a portable, automated, reliable diagnostics technique requiring minimal operator expertise to quantify disease markers. Label-free Raman analysis of biofluids at physiologically-relevant sensitivities is achieved using a microfluidic-embedded liquid-core-waveguide augmented with a unique circulation approach: thermal damage and spectrum variance is minimized, eliminating conventional limits on integration time for excellent signal-to-noise ratio and temporal stability. Machine-learning then optimizes spectrum processing, yielding quantitative results independent of end-user proficiency. Sub-mM accuracy is achieved in solutions of both high and low turbidity, surpassing the sensitivity of previous techniques for analytes with a small scattering cross-section, such as glucose. We attain a new record for label-free glucose measurements in an artificial whole-blood, achieving an accuracy up to 0.14 mM, well-exceeding the 0.78 mM accuracy required for diabetic monitoring, establishing our technique's potential to significantly facilitate portable Raman for complex biofluid analysis., Comment: May 2021 Accepted for publication in Journal of Lightwave Technology

Laser cooling of matter through anti-Stokes photoluminescence, where the emitted frequency of light exceeds that of the impinging laser by virtue of absorption of thermal vibrational energy, has been successfully realized in condensed media, and in particular with rare earth doped systems achieving sub-100K solid state optical refrigeration. Studies suggest that laser cooling in semiconductors has the potential of achieving temperatures down to ~10K and that its direct integration can usher unique high-performance nanostructured semiconductor devices. While laser cooling of nanostructured II-VI semiconductors has been reported recently, laser cooling of indirect bandgap semiconductors such as group IV silicon and germanium remains a major challenge. Here we report on the anomalous observation of dominant anti-Stokes photoluminescence in germanium nanocrystals. We attribute this result to the confluence of ultra-high purity nanocrystal germanium, generation of high density of electron-hole plasma, the inherent degeneracy of longitudinal and transverse optical phonons in non-polar indirect bandgap semiconductors, and commensurate spatial confinement effects. At high laser intensities, laser cooling with lattice temperature as low as ~50K is inferred., Comment: 17 pages, 5 Figures

Guided-wave plasmonic circuits are promising platforms for sensing, interconnection, and quantum applications in the sub-diffraction regime. Nonetheless, the loss-confinement trade-off remains a collective bottleneck for plasmonic-enhanced optical processes. Here, we report a unique plasmonic waveguide that can alleviate such trade-off and improve the efficiencies of plasmonic-based emission, light-matter-interaction, and detection simultaneously. Through different bias configurations, record experimental attributes such as normalized Purcell factor approaching 10^4, 10-dB amplitude modulation with <1 dB insertion loss and fJ-level switching energy, and photodetection sensitivity and internal quantum efficiency of -54 dBm and 6.4 % respectively can be realized within the same amorphous-based plasmonic structure. The ability to support multiple optoelectronic phenomena while providing performance gains over existing plasmonic and dielectric counterparts offers a clear path towards reconfigurable, monolithic plasmonic circuits., Comment: 17 pages, 4 figures

Schottky photodetectors are attractive for CMOS-compatible photonic integrated circuits, but the trade-off between photon absorption and hot carrier emission often compromises the detection fidelity and sensitivity. Here, we report a hybridization-based waveguiding effect that can improve the sensitivity of guided-wave Schottky detection by >200x. By hybridizing the supermodes guided by asymmetrical coupled plasmonic nanostructures, light-matter-interaction can be significantly enhanced even if the thickness of the light absorbing region is only a few nanometers, thus allowing the absorption and emission efficiencies to be simultaneously optimized for the first time. Using amorphous-based active junction with absorbing volume of only 0.031 {\mu}m3, our detectors demonstrate record experimental sensitivity of -55 dBm, approaching that of crystalline-based Ge counterparts with >36x larger absorption region. The ability to maximize field overlap within small device volume and improve photodetection fidelity without using crystalline materials pave the way towards improved back-end-of-line interconnection, single photon detection, and 2D optoelectronics., Comment: 26 pages, 5 figures

The detection of objects in the presence of significant background noise is a problem of fundamental interest in sensing. In this work, we theoretically analyze a prototype target detection protocol, the quantum temporal correlation (QTC) detection protocol, which is implemented in this work utilizing spontaneous parametric down-converted photon-pair sources. The QTC detection protocol only requires time-resolved photon-counting detection, which is phase-insensitive and therefore suitable for optical target detection. As a comparison to the QTC detection protocol, we also consider a classical phase-insensitive target detection protocol based on intensity detection that is practical in the optical regime. We formulated the target detection problem as a total probe photon transmission estimation problem and obtain an analytical expression of the receiver operating characteristic (ROC) curves. We carry out experiments using a semiconductor waveguide source, which we developed and previously reported. The experimental results agree very well with the theoretical prediction. In particular, we find that in a high-level environment noise and loss, the QTC detection protocol can achieve performance comparable to that of the classical protocol (that is practical in the optical regime) but with \(\simeq 57\) times lower detection time in terms of ROC curve metric. The performance of the QTC detection protocol experiment setup could be further improved with a higher transmission of the reference photon and better detector time uncertainty. Furthermore, the probe photons in the QTC detection protocol are completely indistinguishable from the background noise and therefore useful for covert ranging applications. Finally, our technological platform is highly scalable as well as tunable and thus amenable to large scale integration, which is necessary for practical applications.

We demonstrate and analyze how deploying non-classical intensity correlations obtained from a monolithic semiconductor quantum photon source can enhance classical target detection systems. This is demonstrated by examining the advantages offered by the utilization of the non-classical correlations in a correlation based target detection protocol. We experimentally demonstrate that under the same condition, the target contrast obtained from the protocol when non-classical correlations are utilized exhibits an improvement of up to 17.79dB over the best classical intensity correlation-based target detection protocol, under 29.69dB channel loss and excess noise 13.40dB stronger than the probe signal. We also assessed how the strong frequency correlations within the non-classical photon pairs can be used to further enhance this protocol.

In this article, we demonstrate theoretically and experimentally how one can exploit correlations generated in monolithic semiconductor quantum light sources to enhance the performance of optical target detection. A prototype target detection protocol, the quantum time-correlation (QTC) detection protocol, with spontaneous parametric down-converted photon-pair sources, is discussed. The QTC protocol only requires time-resolved photon-counting detection, which is phase-insensitive and therefore suitable for optical target detection. As a comparison to the QTC detection protocol, we also consider a classical phase-insensitive target detection protocol based on intensity detection. We formulated the target detection problem as a probe light transmission estimation problem, and we quantify the target detection performance with the Fisher information criterion and the receiver operation characteristic analysis. Unlike classical target detection and ranging protocols, the probe photons in our QTC detection protocol are completely indistinguishable from the background noise and therefore useful for covert ranging applications. Finally, our technological platform is highly scalable and tunable and thus amenable to large scale integration necessary for practical applications.

In order to enhance LIDAR performance metrics such as target detection sensitivity, noise resilience and ranging accuracy, we exploit the strong temporal correlation within the photon pairs generated in continuous-wave pumped semiconductor waveguides. The enhancement attained through the use of such non-classical sources is measured and compared to a corresponding target detection scheme based on simple photon-counting detection. The performances of both schemes are quantified by the estimation uncertainty and Fisher information of the probe photon transmission, which is a widely adopted sensing figure of merit. The target detection experiments are conducted with high probe channel loss (\(\simeq 1-5\times10^{-5}\)) and formidable environment noise up to 36 dB stronger than the detected probe power of \(1.64\times 10^{-5}\) pW. The experimental result shows significant advantages offered by the enhanced scheme with up to 26.3 dB higher performance in terms of estimation uncertainty, which is equivalent to a reduction of target detection time by a factor of 430 or 146 (21.6 dB) times more resilience to noise. We also experimentally demonstrated ranging with these non-classical photon pairs generated with continuous-wave pump in the presence of strong noise and loss, achieving \(\approx\)5 cm distance resolution that is limited by the temporal resolution of the detectors.

We propose, analyze, and evaluate a technique for the joint measurement of time-frequency entanglement between two photons. In particular, we show that the frequency sum and time difference of two photons could be simultaneously measured through the sum-frequency generation process, without measuring the time or frequency of each individual photon. We demonstrate the usefulness of this technique by using it to design a time-frequency entanglement based continuous variable superdense coding and a quantum illumination protocol. Performance analysis of these two protocols suggests that the joint measurement of strong time-frequency entanglement of non-classical photon pairs can significantly enhance the performance of joint-measurement based quantum communication and metrology protocols.

For integrated optical devices, realistic utilization of the superior wave-matter interaction offered by plasmonics is typically impeded by optical losses, which increase rapidly with mode volume reduction. Although coupled-mode plasmonic structures has demonstrated effective alleviation of the loss-confinement trade-off, stringent symmetry requirements must be enforced for such reduction to prevail. In this work, we report an asymmetric plasmonic waveguide that is not only capable of guiding subwavelength optical mode with long-range propagation, but is also unrestricted by structural, material, or modal symmetry. In these composite hybrid plasmonic waveguides (CHPWs), the versatility afforded by coupling dissimilar plasmonic structures, within the same waveguide, allow better fabrication tolerance and provide more degrees of design freedom to simultaneously optimize various device attributes. The CHPWs used to demonstrated the concept in this work, exhibit propagation loss and mode area of only 0.03 dB/{\mu}m and 0.002 {\mu}m^2 respectively, corresponding to the smallest combination amongst experimentally demonstrated long-range plasmonic structures to-date. Moreover, CHPW micro-rings were realized with record in/out coupling excitation efficiency (71%), extinction ratio (29 dB), as well as Purcell factor (1.5\times10^4).

Raman spectroscopy's capability to provide meaningful composition predictions is heavily reliant on a pre-processing step to remove insignificant spectral variation. This is crucial in biofluid analysis. Widespread adoption of diagnostics using Raman requires a robust model which can withstand routine spectra discrepancies due to unavoidable variations such as age, diet, and medical background. A wealth of pre-processing methods are available, and it is often up to trial-and-error or user experience to select the method which gives the best results. This process can be incredibly time consuming and inconsistent for multiple operators. In this study we detail a method to analyze the statistical variability within a set of training spectra and determine suitability to form a robust model. This allows us to selectively qualify or exclude a pre-processing method, predetermine robustness, and simultaneously identify the number of components which will form the best predictive model. We demonstrate the ability of this technique to improve predictive models of two artificial biological fluids. Raman spectroscopy is ideal for noninvasive, nondestructive analysis. Routine health monitoring which maximizes comfort is increasingly crucial, particularly in epidemic-level diabetes diagnoses. High variability in spectra of biological samples can hinder Raman's adoption for these methods. Our technique allows the decision of optimal pre-treatment method to be determined for the operator; model performance is no longer a function of user experience. We foresee this statistical technique being an instrumental element to widening the adoption of Raman as a monitoring tool in a field of biofluid analysis.

We theoretically investigate several types of plasmonic slot waveguides for enhancing the measured signal in Raman spectroscopy, which is a consequence of electric field and Purcell factor enhancements, as well as an increase in light-matter interaction volume and the Raman signal collection efficiency. An intuitive methodology is presented for calculating the accumulated Raman enhancement factor of an ensemble of molecules in waveguide sensing, which exploits an analytical photon Green function expansion in terms of the waveguide normal modes, and we combine this with a quantum optics formalism of the molecule-waveguide interaction to model Raman scattering. We subsequently show how integrated plasmonic slot waveguides can attain significantly higher Raman enhancement factors: $\sim$5.3$\times$ compared to optofluidic fibers and $\sim$3.7$\times$ compared to planar integrated dielectric waveguides, with a device size and thus analyte volume of at least three-orders of magnitude less. We also provide a comprehensive comparison between the different types of plasmonic slot waveguides based on the important figures-of-merit, and determine the optimal approaches to maximize Raman enhancement., Comment: 24 pages, 21 figures

Vanadium dioxide (${\rm VO}_2$) is a phase change material (PCM) that exhibits a large change in complex refractive index on the order of unity upon switching from its dielectric to its metallic phase. Although this property is key for the design of ultracompact optical modulators of only a few microns in footprint, the high absorption of ${\rm VO}_2$ leads to appreciable insertion loss (IL) that limits the modulator performance. In this paper, through theory and numerical modeling, we report on a new paradigm, which demonstrates how the use of a hybrid plasmonic waveguide to construct a ${\rm VO}_2$ based modulator can improve the performance by minimizing its IL while achieving high extinction ratio (ER) in comparison to a purely dielectric waveguide. The hybrid plasmonic waveguide that contains an additional metal layer with even higher loss than ${\rm VO}_2$ enables unique approaches to engineer the electric field (E-field) intensity distribution within the cross section of the modulator. The resulting figure-of-merit, FoM = ER/IL, is much higher than what is possible by simply incorporating ${\rm VO}_2$ into a silicon wire waveguide. A practical modulator design using this new approach, which also includes input and output couplers yields ER = 3.8 dB/{\mu}m and IL = 1.4 dB/{\mu}m (FoM = 2.7), with a 3-dB optical bandwidth >500 nm, in a device length = 2 {\mu}m, and cross-sectional dimensions = 200 nm $\times$ 450 nm. To the best of our knowledge, this is one of the smallest modulator designs proposed to date that also exhibits amongst the highest ER, FoM, and optical bandwidth, in comparison to existing designs. In addition to ${\rm VO}_2$, we investigate two other PCMs incorporated within the waveguide structure. The improvements obtained for ${\rm VO}_2$ modulators do not extend to other PCMs., Comment: 13 pages, 17 figures, 3 tables

The faithful distribution of entanglement in continuous variable systems is essential to many quantum information protocols. As such, entanglement distillation and enhancement schemes are a cornerstone of many applications. The photon subtraction scheme offers enhancement with a relatively simple setup and has been studied in various scenarios. Motivated by recent advances in integrated optics, particularly the ability to build stable multimode interferometers with squeezed input states, a multimodal extension to the enhancement via photon subtraction protocol is studied. States generated with multiple squeezed input states, rather than a single input source, are shown to be more sensitive to the enhancement protocol, leading to increased entanglement at the output. Numerical results show the gain in entanglement is not monotonic with the number of modes or the degree of squeezing in the additional modes. Consequently, the advantage due to having multiple squeezed inputs states can be maximized when the number of modes is still relatively small (e.g., $4$). The requirement for additional squeezing is within the current realm of implementation, making this scheme achievable with present technologies., Comment: 13 pages, 10 figures, comments welcomed

We present a modular design for integrated programmable multimode sources of arbitrary Gaussian states of light. The technique is based on current technologies, in particular recent demonstrations of on-chip photon manipulation and generation of highly squeezed vacuum states in semiconductors. While the design is generic and independent of the choice of integrated platform, we adopt recent experimental results on compound semiconductors as a demonstrative example. Such a device would be valuable as a source for many quantum protocols that range from imaging to communication and information processing., Comment: 14 pages, 5 figures

We theoretically investigate the enhancement of surface enhanced Raman spectroscopy (SERS) using hyperbolic stratified nanostructures and compare to metal nanoresonators. The photon Green function of each nanostructure within its environment is first obtained from a semi-analytical modal theory, which is used in a quantum optics formalism of the molecule-nanostructure interaction to model the SERS spectrum. An intuitive methodology is presented for calculating the single molecule enhancement factor (SMEF), which is also able to predict known experimental SERS enhancement factors of an example gold nano-dimer. We elucidate the important figures-of-merit of the enhancement and explore these for different designs. We find that the use of hyperbolic stratified materials can enhance the photonic local density of states (LDOS) by close to 2 times in comparison to pure metal nanostructures, when both designed to work at the same operating wavelengths. However, the increased LDOS is accompanied by higher electric field concentration within the lossy hyperbolic material, which leads to increased quenching that serves to reduce the overall detected SERS enhancement in the far field. For nanoresonators with resonant localized surface plasmon wavelengths in the near-infrared, the SMEF for the hyperbolic stratified nanostructure is approximately an order of magnitude lower than the pure metal counterpart. Conversely, we show that by detecting the Raman signal using a near-field probe, hyperbolic materials can provide an improvement in SERS enhancement compared to using pure metal nanostructures when the probe is sufficiently close (<50 nm) to the Raman active molecule at the plasmonic hotspot., Comment: 18 pages, 9 figures

Hybrid integration of plasmonics and Si photonics is a promising architecture for global microprocessor interconnects. To this end, practical plasmonic devices not only should provide athermal, broadband operation over wavelength-scale footprint, but also support non-intrusive integration with low-loss Si waveguides as well as CMOS back-end-of-line processes. Here, we demonstrate a hybrid plasmonic photodetector with a single active junction fabricated via back-end deposited amorphous materials coupled to Si nanowires with only 1.5dB loss. Utilizing internal photoemission, our detectors measured sensitivity of -35dBm in a 620nm by 5{\mu}m footprint at 7V bias. Moreover, responsivity up to 0.4mA/W and dark current down to 0.2nA were obtained. The high process tolerance is demonstrated between {\lambda}=1.2-1.8{\mu}m and up to 100{\deg}C. The results suggest the potential towards plasmonic-photonic optoelectronic integration on top of Si chips without costly process modifications., Comment: 4 pages, 4 figures

Graphene photonics has emerged as a promising platform for providing desirable optical functionality. However, graphene's monolayer-scale thickness fundamentally restricts the available light matter interaction, posing a critical design challenge for integrated devices, particularly in wavelength regimes where graphene plasmonics is untenable. While several plasmonic designs have been proposed to enhance graphene light interaction in these regimes, they suffer from substantial insertion loss due to metal absorption. Here we report a non-resonant metamaterial-based waveguide platform to overcome the design bottleneck associated with graphene device. Such metamaterial structure enables low insertion loss even though metal is being utilized. By examining waveguide dispersion characteristics via closed-form analysis, it is demonstrated that the metamaterial approach can provide optimized optical field that overlaps with the graphene monolayer. This enables graphene-based integrated components with superior optical performance. Specifically, the metamaterial-assisted graphene modulator can provide 5-fold improvement in extinction ratio compared to Si nanowire, while reducing insertion loss by one order magnitude compared to plasmonic structures. Such a waveguide configuration thus allows one to maximize the optical potential that graphene holds in the telecom and visible regimes., Comment: 4 figures, 5 pages

We study the optical properties of quantum dipole emitters coupled to hyperbolic metamaterial nano-resonators using a semi-analytical quasinormal mode approach. We show that coupling to metamaterial nano-resonators can lead to significant Purcell enhancements that are nearly an order of magnitude larger than those of plasmonic resonators with comparable geometry. However, the associated single photon output $\beta$-factors are extremely low (around 10$\%$), far smaller than those of comparable sized metallic resonators (70$\%$). Using a quasinormal mode expansion of the photon Green function, we describe how the low $\beta$-factors are due to increased Ohmic quenching arising from redshifted resonances, larger quality factors, and stronger confinement of light within the metal. In contrast to current wisdom, these results suggest that hyperbolic metamaterial nano-structures make poor choices for single photon sources.

Integrated optics has brought unprecedented levels of stability and performance to quantum photonic circuits. However, integrated devices are not merely micron-scale equivalents of their bulk-optics counterparts. By exploiting the inherently dispersive characteristics of the integrated setting, such devices can play a remarkably more versatile role in quantum circuit architectures. We show this by examining the implications of linear dispersion in an ordinary directional coupler. Dispersion unlocks several novel capabilities for this device, including in-situ control over photon spectral and polarization entanglement, tunable photon time-ordering, and entanglement-sensitive two-photon coincidence generation. Also revealed is an ability to maintain perfect two-photon anti-coalescence while tuning the interference visibility, which has no equivalent in bulk-optics. The outcome of this work adds to a suite of state engineering and characterization tools that benefit from the advantages of integration. It also paves the way for re-evaluating the possibilities offered by dispersion in other on-chip devices., Comment: To be published in Optica (OSA); 10 pages, 8 figures

A device that achieves controllable rotation of the state of polarization by rotating the orientation of the eigenmodes of a waveguide by 45$^{\circ}$ is introduced and analyzed. The device can be implemented using lossless materials on a nanoscale and helps circumvent the inherent polarization dependence of photonic devices realized within the silicon on insulator platform. We propose and evaluate two novel polarization rotator-based schemes to achieve polarization engineering functions: (1) A multi-purpose device, with dimensions on the order of a few wavelengths which can function as a polarization splitter or an arbitrary linear polarization state generator. (2) An energy efficient optical modulator that utilizes eigenmode rotation and epsilon near zero (ENZ) effects to achieve high extinction ratio, polarization insensitive amplitude modulation without the need to sweep the device geometry to match the TE and TM mode attributes. By using indium tin oxide (ITO) as an example for a tunable material, the proposed modulator provides polarization insensitive operation and can be realized with a modulation bandwidth of 112 GHz, a length of 1800 nm an energy per bit of 7.5 $fJ$ and an optical bandwidth of 210 nm.

Entangled photon pairs generated within integrated devices must often be spatially separated for their subsequent manipulation in quantum circuits. Separation that is both deterministic and universal can in principle be achieved through anti-coalescent two-photon quantum interference. However, such interference-facilitated pair separation (IFPS) has not been extensively studied in the integrated setting, where the strong polarization and wavelength dependencies of integrated couplers -- as opposed to bulk-optics beamsplitters -- can have important implications for performance beyond the identical-photon regime. This paper provides a detailed review of IFPS and examines how these dependencies impact separation fidelity and interference visibility. Focus is given to IFPS mediated by an integrated directional coupler. The analysis applies equally to both on-chip and in-fiber implementations, and can be expanded to other coupler architectures such as multimode interferometers. When coupler dispersion is present, the separation performance can depend on photon bandwidth, spectral entanglement, and the linearity of the dispersion. Under appropriate conditions, reduction in the separation fidelity due to loss of non-classical interference can be perfectly compensated for by classical wavelength demultiplexing effects. This work informs the design as well as the performance assessment of circuits for achieving universal photon pair separation for states with tunable arbitrary properties., Comment: 11 pages, 14 figures; to be published in Laser & Photonics Reviews

We investigate the electro-optic properties of black phosphorus (BP) thin films for optical modulation in the mid-infrared frequencies. Our calculation indicates that an applied out-of-plane electric field may lead to red-, blue-, or bidirectional shift in BP's absorption edge. This is due to the interplay between the field-induced quantum-confined Franz-Keldysh effect and the Pauli-blocked Burstein-Moss shift. The relative contribution of the two electro-absorption mechanisms depends on doping range, operating wavelength, and BP film thickness. For proof-of concept, simple modulator configuration with BP overlaid over a silicon nanowire is studied. Simulation result shows that operating BP in the quantum-confined Franz-Keldysh regime can enable improved maximal attainable absorption as well as power efficiency compared to its graphene counterpart., Comment: 7 pages, 5 figures

Generating nonclassical states of photons such as polarization entangled states on a monolithic chip is a crucial step towards practical applications of optical quantum information processing such as quantum computing and quantum key distribution. Here we demonstrate two polarization entangled photon sources in a single monolithic semiconductor waveguide. The first source is achieved through the concurrent utilization of two spontaneous parametric down-conversion (SPDC) processes, namely type-0 and type-I SPDC processes. The chip can also generate polarization entangled photons via the type-II SPDC process, enabling the generation of both co-polarized and cross-polarized entangled photons in the same device. In both cases, polarization entanglement is generated directly on the chip without the use of any off-chip compensation, interferometry or bandpass filtering. This enables direct, chip-based generation of both Bell states $(|H,H\rangle+|V,V\rangle)/\sqrt{2}$ and $(|H,V\rangle+|V,H\rangle)/\sqrt{2}$ simultaneously utilizing the same pump source. In addition, based on compound semiconductors, this chip can be directly integrated with it own pump laser. This technique ushers an era of self-contained, integrated, electrically pumped, room-temperature polarization entangled photon sources.

Generating entangled photons from a monolithic chip is a major milestone towards real-life applications of optical quantum information processing including quantum key distribution and quantum computing. Ultrabroadband entangled photons are of particular interest to various applications such as quantum metrology and multi-party entanglement distribution. In this work, we demonstrate the direct generation of broadband wavelength multiplexed polarization entangled photons from a semiconductor chip for the first time. Without the use of any on-chip compensation, interferometry, entangled photons with a signal-idler separation as large as 95 nm in the telecom band were observed. The highest concurrence of 0.98$\pm$0.01 achieved in this work is also the highest, to the best of our knowledge, comparing to all previously demonstrated semiconductor waveguide sources. This work paves the way for fully integrated, ultrabroadband sources of polarization entangled photons.