This PDF file contains the front matter associated with SPIE Proceedings Volume 12016, including the Title Page, Copyright information, Table of Contents and Conference Committee lists.

Matter-wave interferometers operating as clocks and gravimeters allow for precision measurements of time and gravity at unprecedented levels. In all these sensors, the exquisite control of both the internal (electronic) and external (center-of-mass motion) degrees of freedom of ultracold atomic samples enable interactions at their most basic, quantum level to be studied, paving the way for new tests of fundamental physics. For all these applications, novel interferometric schemes based on the narrow intercombination transitions of alkali-earth (and alkali-earth like) atoms have recently gained considerable attention. Here we present the work towards the realization of a simultaneous atom interferometer with cadmium and strontium atoms and the potential physics such a system could explore, along the idea of the ERC - “TICTOCGRAV” EU research project. In particular, we present the design and implementation of the high-power UV laser sources necessary for performing atom interferometry with cadmium and the prospects for developing a high-flux, cold source of cadmium. Potential applications of a simultaneous cadmium and strontium atom interferometer, including to weak equivalence principle and quantum time dilation tests, are discussed.

Through the use of a high-flux rubidium beam source with sub-Doppler temperatures in three dimensions, we have demonstrated an inertially sensitive atom interferometer featuring high contrast, low noise, and continuous measurement with high bandwidth. We describe the cold-atom source and the optical design that optimizes interferometer contrast. Finally, we demonstrate useful features enabled by this architecture, such as continuous phase shear readout and rapid reversal of inertial sensitivity. This demonstration may enable future cold-atom sensors that measure with both high sensitivity and high bandwidth.

Low noise microwave signals, generated by coherent division from optical atomic clock signals with an optical frequency comb, are amongst the lowest noise and highest stability of both room temperature and cryogenic electronic sources. The latter conversion is necessary for the realization of timescales, and redefinition of the SI second, from next generation atomic clocks. To this end, I will describe a highly robust technique that uses feedforward to suppress noise in optical-to-microwave division. This technique also supports the simultaneous conversion of multiple independent optical signals to microwave signals with a single comb, permitting the independent synthesis of microwave signals from multiple atomic clocks with accuracy near 1 part in 10-18.

Chip-scale atomic clocks (CSACs) based on Coherent Population Trapping (CPT) are at the forefront of next-generation timekeeping for diverse applications, including global navigation satellite systems (GNSS), satellite communications, cell-phone networks, and hand-held GNSS receivers. Notwithstanding the potential ubiquity of this atomic device, a performance-limiting aspect of CSACs is the vapor-phase signal-to-noise ratio (SNR) of their ground-state (mF = 0 to mF = 0) atomic hyperfine resonance. Specifically, in commercially available devices angular-momentum optical pumping “pushes” atomic population towards high |mF| Zeeman sublevels at the expense of population in the 0-0 clock transition. Though mitigation strategies for this SNR limiting process have been proposed and demonstrated there has, to date, been little direct measurement of the population distribution among Zeeman sub-states for atoms undergoing CPT, and how that population distribution is altered by SNR improving mitigation strategies. Here, we describe our initial studies examining this question.

Axion-like particles which couple to photons can induce small nonlinearities in electromagnetism, leading to light-by-light scattering. We will describe a proposed experiment at the SQMS National Quantum Center at Fermilab to detect this process, using a high-Q superconducting radiofrequency (SRF) cavity. When the cavity is pumped with two resonant modes ω₁ and ω₂, the axion-induced nonlinearity will source a small signal at ωs = 2ω₁ − ω₂, which may be detected if ω_s is also a resonant frequency of the cavity. An experiment sensitive enough to set world-leading limits on axions may also be sensitive to the nonlinearity already present in quantum electrodynamics (QED) as encapsulated by the Euler-Heisenberg Lagrangian, which has never been probed at energies below the electron mass.

Quantum-enhanced measurements using squeezed light provide sensitivities beyond the shot-noise limit and have attracted much attention in many fields such as spectroscopy, gravitational wave detection, and biological imaging. To achieve high squeezing levels, the precise matching of the amplitude and phase between squeezed vacuum and local oscillator (LO) is essential. Still, it is not a simple task, especially when optical pulses are used. For continuous-wave beams, a mode-cleaning cavity or a single-mode waveguide is commonly used to control the spatial profile of the two beams. However, the cavity must be large enough to couple the optical pulses, and the waveguide causes unwanted nonlinearities for intense optical pulses and has a limited variety of materials. This study implemented precise amplitude and phase matching by integrating phase-shift digital holography and spatial light modulator (SLM). Specifically, we used a periodically-poled stoichiometric lithium tantalate (PPSLT) waveguide for optical parametric amplification (OPA). We measured the spatially resolved phase difference and intensity profile of idler light and LO by analyzing their interference movie with phase- shifting digital holography. We performed spatial mode matching by controlling the amplitude and phase of LO using SLM. As a result, we achieved high visibility of 92.9% and observed a squeezing level of -3.61 dB.

Single-pass optical parametric amplification (OPA) is attractive in the generation of broadband squeezed vacuum. To achieve a higher squeezing level, precise control of the local oscillator (LO) phase is needed. However, the conventional control scheme was designed for squeezing by optical parametric oscillation (OPO). In this study, we propose a new scheme, which is applicable to single-pass OPA. In this scheme, we measure the modulation in the residual pump light after the OPA crystal to realize precise phase-locking of the LO. We anticipate that the proposed scheme will be indispensable for practical applications of squeezing with single-pass OPA.

Waveguide-based optical parametric amplification (OPA) is advantageous in generating broadband or pulsed squeezed vacuum. To enhance the squeezing level, it is crucial to clarify its limiting factors. In this study, we develop a numerical method which allows us to derive the squeezing level in waveguide OPA. Based on the developed method, we investigate the effect of the higher-order mode of the pump light, which generates noise due to the OPA between the fundamental mode and the higher-order mode. We also investigate the effect of optical loss in the waveguide. Specifically, we formulated coupled equations that take into account the degenerate OPA between the pump and signal both in the fundamental mode, and the non-degenerate OPA between the pump in the higher-order mode, the signal in the fundamental mode, and the idler in the higher-order mode. By solving the coupled equations, we can express the quantum state as a linear combination of annihilation and creation operators, and the squeezing level can be derived from its coefficients. The analysis allowed us to quantify the effect of OPA on the fundamental and higher-order modes, and the requirement for mode matching of pump light with the fundamental mode. We also analyzed the effect of loss in the waveguide based on the same procedure. The results show the loss in the waveguide to have less effect than the same loss at the output of the waveguide. These results provide important design criteria of waveguide OPA for squeezing.

Kilometer-scale ground-based gravitational-wave interferometers have generated a new field of astronomy by viewing the universe in gravitational wave (GW) radiation, operating at a peak sensitivity of frequencies ranging from 10s of Hz to a few kHz.1 A great many discoveries have resulted from these detectors, such as the existence of binary black hole and neutron star systems.² It is of great scientific value to extend the GW search to other frequencies, just as has been the case for the exploration of the EM spectrum. The levitated sensor detector (LSD) is a 1 m tabletop scale high frequency (< 10 kHz) gravitational wave detection experiment currently under construction at Northwestern University.³ It will serve as a 1 m prototype for future generations of levitated mass based instruments. The LSD sensitivity has more favorable frequency scaling at these frequencies compared to laser interferometer detectors such as LIGO and VIRGO due to different limiting noise factors, the LIGO free spectral range, and the fact that the LSD is a resonant sensor. The LSD is sensitive to GWs from binary coalescence of sub-solar-mass primordial black holes and as-yet unexplored new physics in the high-frequency GW window, such as the annihiation of gravitationally bound states of the QCD axion by black hole superradiance. Many promising experiments and techniques exist for probing the GW spectrum below the LIGO frequency band; they include pulsar timing arrays,⁴, ⁵ atomic clocks and other interferometers,^{6, 7} LISA,^{8, 9} and DECIGO.¹⁰ There are also a number of proposals, experiments and initial bounds set above the LIGO frequency band, largely over 100 MHz.^11–18 Fewer established methods to systematically probe the kHz-MHz part of the GW spectrum, where a variety of interesting sources could exist. At Northwestern, we are constructing a compact Michelson interferometer configuration with Fabry-P´erot arms as shown in Figure 3, designed to work in the 10-100 kHz band. In the medium and long term, a multi index dielectric stack will be suspended at an anti-node of the standing wave inside each Fabry-P´erot arm. In the short term, this is likely to be a disc or disc like object (discussed in sections 2 and 2.2) — with which a degree of experimental success has already been had. A second laser is used to read out the position of the object as well as cool it along the cavity axes.¹⁹

A hybrid structure sensor is proposed for curvature and strain measurement. The sensor is fabricated by cascading the Michelson interferometer (MI) based on the up-taper and the Fabry-Perot interferometer (FPI) based on the air cavity. The MI is sensitive to curvature, and the FPI is sensitive to strain. Therefore, the cascade of sensing structures can realize the simultaneous measurement of curvature and strain. The curvature and strain sensitivity of the sensing structure are 2.59 nm/m-1 and 1.63 pm/με, respectively. The cascade of the FPI and the MI enables the end face of the MI to be effectively encapsulated, avoiding the crosstalk of external factors. The hybrid structure sensor has the advantages of simple structure, low cost, and easy preparation. The structure has the potential to be applied to structural health detection and biomedicine.

The unique structure of frequency combs, light sources whose lines are perfectly evenly-spaced, allows for new avenues in the measurement of optical signals. I will discuss our work on frequency comb ptychoscopy, a radiometric technique that allows the spectrum of passive sources to be measured by many comb lines at once.1 Because the heterodyne signals from many signals are folded on top of one another at intermediate frequencies, an inversion algorithm is demonstrated that is able to unravel these beatings. This technique combines the resolution and speed of heterodyne spectroscopy with the bandwidth of comb spectroscopy, and has particular promise in the domain of remote sensing.

Theoretical calculation of the cross-polarization coupling between whispering-gallery modes in a microresonator shows that this coupling is, in general, asymmetric. The spin-orbit induced coupling of light from a TE mode to a coresonant TM mode will have a different strength than that of the coupling from the TM mode to the TE mode. This coupling asymmetry is confirmed in coupled-mode induced transparency experiments. By monitoring the throughput spectrum in both polarizations when the input directly excites only modes of one polarization, the coupling strengths in both directions are determined simultaneously by fitting to a model. Some examples and implications are discussed here.

Backward Brillouin scattering in whispering-gallery-mode micro-resonators offers an exciting avenue to pursue both classical and quantum optomechanics applications. Our team—the Quantum Measurement Lab—together with our collaborators, are currently utilizing this regime and the favourable properties it affords for non- Gaussian motional state preparation of the acoustic field. In particular, the high mechanical frequencies, and low optical absorption and heating provide a promising route to overcome current hindrances within optomechanics. Three of our recent experimental results in this area include: (i) Brillouin optomechanical strong coupling, (ii) single-phonon addition or subtraction to a thermal state of the acoustic field, and (iii) performing phase-space tomography of non-Gaussian states generated by single- and multi-phonon subtraction. This SPIE presentation will cover these three results, what they enable, and the broader direction of our lab including the prospects of this platform for quantum-memory applications.

Inertial sensors are used in a variety of applications including inertial navigation and precision measurements. Optical measurement of test mass displacement in a resonator allows for the creation of compact accelerometer systems. Fused silica resonators allow for excellent acceleration sensitivities due to their high mechanical quality factor, Q, at room temperature, but this changes significantly at lower temperatures. The Q factor of crystalline silicon, however, remains high at low temperatures. We work with compact fused silica resonators that operate at room temperature and aim to fabricate compact comparable mechanical resonators from Si wafers. We will report on the fabrication progress of these resonators and results from ringdown and sensitivity measurements.

We show an estimation scheme which reaches the Heisenberg-scaling sensitivity in the estimation of the average of the optical phases along the two arms of a Mach-Zehnder interferometer, by using a single squeezed vacuum state and homodyne detection at a single output port. We show that, in order to achieve this quantum advantage, it is required only a classical prior knowledge about the two phases, namely obtainable with a classical estimation strategy with shot-noise limited precision.

Hyperfine effects in Rydberg atom-based sensing have been observed for sensing polarization. However, in most work to date, the hyperfine structure involved in the radio frequency transition has been ignored because the residual Doppler widths realized in experiments are larger than the hyperfine energy splittings of the Rydberg states. Recently, we have proposed and demonstrated a collinear three photon scheme for Rydberg atom-based electrometry that has a greatly reduced residual Doppler width, < 500 kHz. In these experiments, we observe the effect of optical pumping and the hyperfine structure of the Rydberg states. We compare the 42P_3/2 → 41D_5/2 and 42P_3/2 → 41D_3/2 sensing transitions to show that Rydberg atom hyperfine structure effects can be observed at our spectral resolution. Hyperfine structure and optical pumping can alter the effective transition dipole moments on the sensing transition and can be used to detect polarization of the radio frequency field so our work is important for practical Rydberg atom electric field sensing.

The AlGaAsOI material platform offers great promise as a source of quantum states of light that can be utilized for enhanced sensing and metrology beyond the standard quantum limit. Here we focus on preliminary results from AlGaAsOI microring resonator sources that demonstrate a 1000-fold improvement in the brightness of entangled photon pairs compared to state-of-the-art chip-scale sources. The source maintains high single photon purity < 99% and time-energy entanglement visibility < 95%.

Conventional superresolution of closely spaced point sources using image intensity data acquired by noiseless single-photon detectors suffers from Rayleigh’s diffraction curse that can only be overcome at photon numbers that scale quartically with the degree of required superresolution. By contrast, acquiring and processing suitably chosen mode projections of the wavefront, which contain dual information of its intensity and phase, can reduce the photon budget to one that is quadratic in the degree of superresolution and achieve quantum-limited performance. Such dramatic improvements enabled by the wavefront projection (WFP) approach have been realized both in computer simulation and laboratory experiments over the last five years. The invited talk will explore the fundamental, theoretically insuperable limits on superresolution imaging by presenting computations of quantum Fisher information and practical realizations of the novel WFP approach that can reach these limits for a variety of point and extended source geometries and operating conditions.

We explore the possibility for implementing nanoscale quantum imaging based on the concept of undetected photons with a pumping beam at x-ray wavelengths. Our proposed scheme exploits the nearly four order of magnitude angular magnification that is a result of the process of the effect of extreme non-degenerate spontaneous down conversion from x-rays into optical radiation, which is used for the generation of entangled photon pairs with one x-ray photon and one visible photon. In our scheme the x-ray photons interact with the object and the visible do not interact with the object, but in contrast to other schemes like ghost imaging, only the visible photons are detected. The scheme is sensitive to both the amplitude and the phase variations and can provide resolutions down to a few nanometers, hence can be used as a powerful tool for nanoscale imaging. In the present configuration, the scheme requires very high temporal coherence of the input beam, which is a significant challenge with the available x-ray sources, however, it should be beneficial with the proposed oscillator x-ray laser.

Endoscopes and similar instruments use bundles of optical fibers to relay each pixel of an image from facet to facet. But even one of these multi-mode fibers supports enough modes to relay a complete image, the problem being that modal dispersion results in the image being scrambled. However, by treating the fiber as a complex aberration and applying corrective beam shaping it is possible to relay an image along the fiber length. Here we show that by beam-shaping of a pulsed laser we can produce a scanning spot at the distil end of the fiber and by measuring the time dependent intensity of the back-scattered light we can achieve 3D imaging. We demonstrate imaging up to 3m from the fiber with a lateral resolution of 60x60 pixels and a millimetric depth resolution. Such minimally invasive endoscopic 3D imaging has applications in healthcare and remote inspection.

It has been shown through quantum estimation theory that Rayleigh’s limit can be avoided for single-lens imaging, referred to as superresolution. The quantum estimation approach has recently been used to show superresolution is also possible for imaging based on interferometer arrays in the weak source limit. Following this line of discussion, we consider the resolution limit of estimating the separation between two point sources of arbitrary strength using interferometer arrays. By carefully designing the measurement, we find it is possible to overcome the well-known resolution limit of interferometer arrays as determined by the longest baseline. We construct an optimal measurement to achieve superresolution using linear beam-splitters and photon-number resolved detection.

We propose a model that can simulate the performance of a photonic-integrated FMCW Doppler LiDAR system with various target and system settings. The well-rounded system model can predict the measured target speed and velocity, system axial resolution, beat signal SNR, frequency modulation nonlinearity induced system error, with a new concept for swept-source laser frequency stabilization. Two sets of autonomous driving scenarios are simulated. Simulations confirm that our model yield system performance consistent with the theoretical calculations.

In this paper we propose a new concept for LiDAR Photonic integrated circuits that is based on tilted grating coupler that doesn’t require any phase modulators for beam steering. Our simulations for SOI waveguide structure show that the concept of tilted grating coupler is feasible with a FOV of 100^◦ by 35^◦

We demonstrate a technique for the accurate measurement of diffusion coefficients for alkali vapor in an inert buffer gas. The measurement was performed by establishing a spatially periodic density grating in isotopically pure ⁸⁷Rb vapor and observing the decaying coherent emission from the grating due to the diffusive motion of the vapor through N₂ buffer gas. We obtain a diffusion coefficient of 0.245 ± 0.002 cm² /s at 50°C and 564 Torr. Scaling to atmospheric pressure, we obtain D₀ = 0.1819 ± 0.0024 cm² /s. To the best of our knowledge, this represents the most accurate determination of the Rb-N2 diffusion coefficient. Our measurements can be extended to different buffer gases and alkali vapors used for magnetometry and can be used to constrain theoretical diffusion models for these systems

In a resonator fiber optic gyroscope (RFOG), a residual amplitude modulation (RAM) may appear with the light phase modulation used to generate the error signals, which are necessary for locking the frequency of the laser on the optical cavity resonance and measure the angular velocity. The RAM causes an unstable bias on the resonance frequency measurement and thus limits the gyro performances. A well-known method to suppress the RAM was introduced by Wong and Hall in 1985. The intensity of the modulated beam is monitored, before entering the resonator, to generate a continuous voltage controlling the natural birefringence of the modulator crystal. We show that this technique seems to be not adapted to setup with an imperfect polarizer (having a limited extinction ratio) at the output of phase modulator followed by component exhibiting polarization dependent losses. To counter this limit, we propose a new strategy and we illustrate its effectiveness with two types of phase modulator to probe a cavity resonance in transmission and reflection.

Recent breakthroughs in silicon photonics technology may soon lead to mass-producible chip-scale tactical-grade (or better) gyroscopes by using a CMOS-compatible fabrication process to print highly integrated high-sensitivity optical gyroscopes. This paper reports our progress on designing and building an optical gyro out of an SiN racetrack resonator of 37-mm perimeter with 1270 finesse (10⁸ intrinsic quality factor) using off-the-shelf fiber components (circulators, splitters, and modulators) and a semiconductor laser to achieve an angular random walk (ARW) of 80 deg/h/Hz, or 1.3 deg/h. To our knowledge, it is a record by a factor of 2 for the ARW per footprint area of a Sagnac-effect-based gyroscope on a chip. A balanced-detection scheme is employed to cancel 18 dB of gyroscope noise caused by laser phase noise converted into amplitude noise by residual backscatterers in the resonator. The backscattering coefficient was found to be very sensitive to wavelength, and therefore to the resonance used to probe the resonator. The lowest backscattering coefficient was measured to be more than 1,000 times lower than the mean. The use of this resonance, as well as an asymmetric phase-modulation scheme, greatly reduced the gyroscope’s backscattering noise. Achieving this gyro’s theoretical minimum ARW of 16 deg/h/Hz will likely require a lower backscattering coefficient or better means of cancelling backscattering noise. Further improvements to tactical-grade performance (and better) will likely require a larger resonator area, further reduction of backscattering, and/or a laser with reduced frequency noise.

Bias stability is a critical performance parameter in navigation applications. We investigate the possibility of enhancing the bias stability in a navigation grade Resonant Fiber Optic Gyroscope (RFOG) through the use of dual frequency comb source. In a conventional RFOG, a single wavelength laser source is used to generate counter propagating waves in a ring resonator whose phase difference is measured to obtain the rotation rate. However, the primary limitation of the RFOG performance is the bias drift observed due to non-reciprocal effects such as Kerr nonlinearity, Rayleigh backscattering, and environmental fluctuations. To enhance the bias stability, we have investigated an alternative approach based on a frequency comb source. By using different set of frequencies (3, 5, 7, 9 etc) for the counter propagating waves, the above limitations can be mitigated since the uncertainty in the demodulated phase is diminished compared to a single frequency measurement leading to enhanced accuracy in the rotation rate determination. Using a theoretical model of the frequency comb-based RFOG, we have carried out simulations in Matlab and investigated the bias stability enhancement with respect to the number of comb lines used. Our simulation results shows that a bias stability of 0.01°/hr can be achieved using frequency comb source with 5 fundamental modes in ring resonator.

We demonstrated the high sensitivity and mechanical strength temperature and strain sensor based on liquid-infiltrated Fabry-Perot interferometer (FPI). The sensor is fabricated by splicing a short section of single-mode fiber (SMF) between two sections of SMF with a large intentional lateral offset forming open-cavity. For its high thermo-optic coefficient (TOC), isopropanol leads to a huge wavelength variation of the reflection spectrum while external temperature changes. The sensor was used to monitor the change of temperature and obtained a sensitivity of up to -655.0pm/°C in the range of 20 to 45 °C. In addition, the sensor presents a high sensitivity to a strain of 20.8pm/με. The sensor is expected to be used in high-sensitivity temperature and strain monitoring environments.

With the technique here proposed, we exploit the temperature variation of the photoluminescence of a direct bandgap semiconductor to achieve a full contact-less thermometry. The proposed method is based on the spectral measurement of the PL signal emitted from a red LED chip when excited by means of a 532 nm solid state laser. Both PL emission peak wavelength and FWHM are used to improve the temperature estimation. The proposed method has been demonstrated on an extended temperature range, from cryogenic (90 K) to 460 K; a ±1 K of uncertainty due to calibration, ±0.7 K of measurement accuracy and ±0.3 K of precision are demonstrated on a temperature range of 300 K.

In this paper, we proposed and demonstrated a parallel optical fiber Fabry–Perot interferometer (FPI) for temperature and strain sensor based on an optimized 3×2 steps fabrication process, aiming to improve the quality of the sensitivity by use of the Vernier effect. The sensor consists of two FPIs with different propagation mediums in parallel connection, which is formed by pairs of built-in plate reflectors, another is fabricated by splicing a three-hole fiber (THF) supported by a suspension core, the higher thermal-optical coefficient can improve the temperature sensitivity. In temperature and strain sensing, two different combinations of FPIS are used as sensing units and matching reference units respectively. The amplification is matched to the wavelength range of the light source by accurately controlling the cavity length between the two FPIs. The temperature and strain sensitivities achieved in the experiment are 182.15 pm/°C and 201.47 pm/µε, respectively. The separate settings of temperature and strain sensing amplification increase the flexibility applications of dual-parameter amplification sensors.

Photonic Doppler Velocimetry (PDV) systems are developed to measure velocities in physics experiments such as ramp compression experiment on novel materials. Better time resolution, in the fast acceleration phase, is required when the velocity reached is limited to few tens of m/s. This resolution being proportional to the wavelength, a green PDV system provides a factor 3 gain compared to a telecom based PDV. Furthermore, a Triature design allows to further increase the time resolution thanks to the phases signal processing. A 532-nm Triature PDV system was realized with singlemode fibers (4.5 µm core diameter) and compared to three standard PDV systems in reproducible ramp compression experiments. To minimize the optical losses, most of the fibers components were fused. The velocity profile measured has oscillations between 0 and 10 m/s. The first velocity peak is reached in 1.0 µs. The 1550-nm homodyne PDV system provides only 6 fringes and it clearly limits the time resolution. The 1550-nm heterodyne PDV system provides hundreds of fringes but variations of about 0.1 m/s are observed. The green homodyne PDV system provides almost the correct velocity profile by Short Time Fourier Transform. The 250-ns long acceleration phase is better resolved applying a signal processing with at least two phases.

A fully integrated and mass producible platform for laser cooling has the potential to revolutionize the growing field of quantum technologies and atomic sensors. Recent advancements in the micro-fabrication of components at the heart of cold atom systems have laid the foundations for the amalgamation of a simple, stackable solution to laser cooling. In this talk we will highlight our recent progress towards a fully chip-scale, cold-atom platform, outlining our approach for on-chip wavelength referencing, examining a solution for imaging atoms in a planar stacked device, and finally discussing the limitations to passively pumped vacuum longevity.

Induced transparency in a microresonator can result from cross-polarization coupling of two coresonant orthogonally polarized whispering-gallery modes of very different Q. The coupling creates supermodes that are superpositions of the two modes. Mode superpositions that result from simultaneous excitation of two orthogonally polarized modes can also show induced transparency, even in the absence of cross-polarization coupling. Induced transparency is accompanied by pulse delay, and it is also possible to observe induced attenuation with pulse advancement or delay. These effects are proposed theoretically, modeled numerically, and confirmed experimentally; a summary is presented here

Energy-time entangled photon pairs (EPPs), which are at the heart of numerous quantum light applications, are commonly generated in nonlinear crystals. Some highly sensitive quantum applications require the use of ultra-broadband entangled photons that cannot be generated in nonlinear crystals due to phase-matching requirements. Here, we investigate the possibility of using metallic nanoparticles (MNPs) as a means for generating entangled photons through spontaneous parametric down-conversion (SPDC). MNPs are known for their strong light-matter coupling at their localized surface plasmon resonance, and since the propagation length through them is negligible relative to optical wavelengths, we consider them as excellent candidates to serve as non-phase matched sources of ultra-broadband entangled photons. To that end, we report experimental results of classical-light second-harmonic generation in silver nanotriangles and nanocubes embedded in polyvinyl alcohol. Based on the results of our experiments, performed using the reference-free hyper-Rayleigh scattering method [A. Ashkenazy et al., J. Phys. B 145401, 2019], we present an estimation of the characteristics of SPDC in MNPs. We show that, despite of the metals' centrosymmetric structure, MNPs exhibit second order nonlinearity that is mainly of an electric dipole nature, and so they are suitable for SPDC. Moreover, we show that localized surface-plasmon resonance can play a significant role in enhancing the generated EPPs flux. Finally, we compare the SPDC capabilities of MNPs to that of commonly used nonlinear crystals and show that the expected EPPs flux from MNPs is weaker but the EPPs have very large bandwidth, which could be helpful for advanced quantum sensing, spectroscopy and communication applications.

We describe the development and applications of a single-photon source based on a quantum dot embedded in a semiconductor nanowire, which can be precision-tuned to emit ∼1ns long photons at wavelengths that match the transitions of caesium D1 line. We discuss interfacing such single-photon source with atomic ensembles and present our experimental results demonstrating a new method of tuning the emission of the quantum dot by condensing inert gas (N2) on the nanowire. Next, we describe how these single photons at ∼895nm can be efficiently converted to wavelength suitable for satellite QKD links (∼794 nm) and optical fiber links (∼1469 nm) using a laser-cooled atomic ensemble that is loaded and confined inside a hollow-core optical fiber. Lastly, we inroduce our proposal of integrating the semiconductor nanowire with a lensed fiber to create a compact single-photon source with improved photon-collection efficiency compared to conventional setups.