his PDF file contains the front matter associated with SPIE Proceedings Volume 12993, including the Title Page, Copyright information, Table of Contents, and Conference Committee information.

Complete control of light-matter interactions at a single quantum level is critical for quantum science applications such as precision measurements and information processing. Nanophotonic devices, developed with recent advancements in nanofabrication techniques, can be used to tailor the interactions between single photons and atoms. One example of such a device is the optical nanofibre, which provides an excellent platform due to the strongly confined transverse light fields, long interaction length, low loss, and diverse optical modes. This facilitates a strong interaction between atoms and guided light, revealing chiral atom-light processes and the prospect of waveguide quantum electrodynamics. This work highlights recent advances on optical nanofiber mediated excitation of cold rubidium Rydberg atoms for the creation of a 1D Rydberg array.

We propose a novel quantum interferometer based on two stimulated Parametric Down-Conversion (PDC) crystals seeded by weak coherent idler fields to investigate the relationship between the nonclassicality of the signal photon state and the fidelity of the conjugate idler photon state. We show that the purity of the signal photon state is tightly bounded by the entanglement between the signal photon and its conjugate idler photon, i.e., the fidelity of the idler state in the bipartite system, resulting in a novel complementarity relation for wave-particle duality. Moreover, we find that the nonclassical measure of the signal photon state is identical to the fidelity of the conjugate idler photon states prepared in each crystal. In our novel interferometer, the input beam splitter of the conventional SU(2) interferometer is replaced with two seeded SU(1,1) PDC systems that emit only a single photon at once from two crystals, thus producing an entangled superposition state where the conjugate idler photon states act as path detectors with controllable fidelity. We demonstrate that the presented quantum interferometer consisting of two stimulated PDC crystals positioned in parallel is an ideal setup for investigating and quantitatively understanding the complementarity relation of wave-particle duality.

Superconducting Nanowire Single-Photon Detectors (SNSPDs) offer several advantages over traditional photodetectors. However, they have limitations related to their latching dead time and dark count rate. This research explores the integration of graphene as an on-chip thermal and electrical shunt in NbTiN superconducting detectors, marking the first application of a 2D material in this capacity. We investigate the impact of graphene on the performance of NbTiN detectors within the multiphoton regime. The NbTiN nanowires are produced via DC reactive magnetron sputtering with a thickness of 25 nm and optical or electron beam lithography onto SiOx/Si substrate. The monolayer graphene is grown by Chemical Vapor Deposition (CVD) and transferred on top of the nanowires. The characterization of the samples has been performed in a close-circuit cryostat setup (3.8 K). The experimental of nominally identical devices with and without graphene, reveal a shift in critical temperature (Tc), a broader superconducting transition, and a significantly reduced electrical hysteresis in the hybrid system. Preliminary results show a non-latching behavior of NbTiN meanders with graphene under pulsed laser illumination of 640 nm photons with a repetition rate up to 1 MHz, emphasizing its efficacy as a shunt resistor speeding up the detector and increasing the detection efficiency. The increased FWHM also highlights the role of graphene in the electrothermal feedback and dissipation. In conclusion, this pioneering work contributes valuable insights into the intricate interplay between 2D materials and superconducting detectors, paving the way for future advancements in this field.

Data protection and confidentiality have become a serious concern in today’s world. Their security is guaranteed by cryptographic protocols, which heavily rely on random numbers as a measure against predictability. Classically, randomness is generated via complex but deterministic algorithms, which are vulnerable to attacks. Quantum Random Number Generators (QRNGs) have emerged as a promising solution, as they provide true random numbers based on the intrinsic non-deterministic nature of quantum mechanics. However, critical challenges for QRNGs are the certification and quantification of their genuine randomness, especially in the presence of untrusted devices, and their compactness for systematic deployment. In this feasibility study, to face these challenges, we propose to use a silicon-photonic platform, leveraging on the concept of quantum contextuality for a semi-device independent generator. In particular, we use Klyachko-Can-Binicioglu-Shumovsky (KCBS) inequality to assess a fundamental property of quantum measurements: that their outcomes depend on the specific measurement context.

Recently, there has been a growing focus on quantum communication due to its inherent security advantages. However, there are still many challenges that need to be addressed, such as the distribution of entangled states over long distances, closing the Bell test loopholes , and increasing the key rate. In this work, we present an innovative device-independent Quantum Key Distribution (QKD) protocol based on the distribution of near-maximally entangled multiphoton states across long distances. Our proposed strategy employs the resources available within the current landscape of integrated quantum photonic technology, including the utilization of squeezed vacuum states and photon-number-resolving detectors. Besides, this protocol enables entanglement sharing and quantum communication in free space.

In this work, we implement a proof-of-concept underwater free-space Quantum Key Distribution (QKD) system and analyze its performance in a controlled laboratory test environment. We implement a BB84 protocol with time-bin encoding operating at 520 nm. The quantum channel was composed of a five-meter-long tank equipped with the possibility of actively controlling the water turbulence. Finally, we measure the Quantum Bit Error Rate (QBER) in the various scenarios and we report the results together with those relating to the parameters of the considered channel.

The rapid growth of quantum technologies has driven the development of quantum defects for both single photon sources as well as for spin qubit and sensor applications. These artificial atoms in wide bandgap solid-state materials such as diamond and hBN offer the possibility of a truly quantum deterministic source. These single photon emitting quantum defects exhibit antibunching that can be quantified by performing the Hanbury-Brown Twiss (HBT) experiment. The HBT experiment allows one to evaluate the second-order autocorrelation (g⁽²⁾(τ )) function, which is a central measure in determining the quality of single photon emission. To enhance the various emission parameters such as intensity, wavelength, and directionality from these defects the development of photonic structures become essential. Here, we study single photon emission from a diamond nanopillar array containing Nitrogen Vacancy (NV) centers. We spatially map the g⁽²⁾(τ ) function and radiative lifetimes in these samples and correlate the two quantities. Crucially, we observe that the measured antibunching varies as a function of position of the objective relative to the nanopillar, even across the same pillar. This likely shows the dependence of the apparent antibunching to the angle of excitation and detection in such photonic structures. Our results highlight the importance of structural and geometric factors in g⁽²⁾(τ ) measurements of emitters, especially when embedded in photonic structures. These should be carefully understood in evaluating the quality of the single photon sources.

We investigate the optimization of direct measurement twin beam (signal-idler) quantum illumination. We focus on each of the three main components of such a system in turn: the detectors, the light beam parameters and the information processing protocol. Surprisingly, there can be an advantage to having a signal detector whose quantum efficiency is significantly less than perfect. This advantage does not vanish in the quantum lidar limits of low object reflectivities or signal strengths. We show that decreasing the pulse separation, while keeping the photon flux fixed to retain the same degree of covertness, can improve target detection performance dramatically. Finally, we show that post-selecting on the idler detector firing is sub-optimal for target detection.

We study the feasibility of polarization-entangled photon pairs to be applied for probing the density of living microorganisms in a microfluidic chamber. We investigate the polarization response of scattering samples via quantum state change and relate this to different density levels of microorganisms on example of yeast cells in water solution.

Programmable optical circuits form a key part of quantum technologies today. As the size of such circuits is increased, maintaining precise control over every individual component becomes challenging. Here we show how embedding an optical circuit in the higher-dimensional space of a large mode-mixer allows us to forgo control over individual elements, while retaining a high degree of programmability over the circuit. Using this approach, we implement high-dimensional linear optical circuits within a commercial multi-mode fibre placed between controllable phase planes. We employ these circuits to manipulate high-dimensional entanglement in up to seven dimensions, demonstrating their application as fully programmable quantum gates. Furthermore, we show how these circuits turn the multi-mode fibre itself into a generalized multi-outcome measurement device, allowing us to both transport and certify entanglement. Finally, we show how a high circuit fidelity can be achieved with a low circuit depth by harnessing the resource of a high-dimensional mode-mixer. Our work serves as an alternative yet powerful approach for realizing precise control over high-dimensional quantum states of light.

We present a promising solid-state system able to emit triggered single-photons in the blue-green range up to room temperature. The active element is a CdSe Quantum Dot (QD) embedded in a bottom-up core-shell ZnSe nanowire grown by molecular beam epitaxy. The nanowire shell acts as a waveguide and confines the fundamental optical mode HE11, channeling the photons emitted by the QD along the nanowire axis. The nanowires have a base radius of 70-90 nm and a length of 5-6 μm with a conical ending that allows to adiabatically expands the guided mode and reduces the divergence angle. Photo-correlation measurements show anti-bunching with g⁽²⁾(0) values down to 0.3 at room temperature. Charged excitons degrade the emission properties at room temperature and we found that a neutral nanowire-quantum dot would emit single-photons with a brightness of 0.17 photon per pulse.

We give a short insight into our work on single photon emitters hosted in hexagonal Boron Nitride (hBN) and photonic integrated circuits, together with technology demonstrations of this platform. We have recently shown the deterministic creation of near-ideal hBN single photon emitters at room temperature and have made important progress on the identification of their atomic origin, comparing their optical characteristics with density functional theory calculations. The quantum emitters can be employed in various applications, including tests on the fundamentals of quantum mechanics and quantum communication, for which we work on their integration into photonic integrated circuits based on laser-written waveguides.

This study provides insights into the current limitations of quantum machine learning compared to classical machine learning and identifies areas for future research. We present a novel approach that utilizes real IBM quantum computers to classify celestial objects within the extensive Sloan Digital Sky Survey Data Release 18 (SDSS-V DR18) dataset. Despite persistent challenges in both hardware and software, quantum computers are being explored as tools for enhancing machine learning performance in comparison to classical methods due to potential upside. This investigation delves into the untapped potential of quantum machine learning and quantum neural networks in tackling the complexities of processing vast telescope data. By leveraging quantum technologies, we aim to expedite the analysis of large complex data, unveiling hidden patterns, and propelling specialized fields such as astronomical research into the quantum era.

Quadrature Phase Shift Keying Continuous Variable Quantum Key Distribution (QPSK CV-QKD) can be used to encode bits of information on weak coherent states, enhanced with quantum scissors. In our experiment, we use discretely modulated QPSK CV-QKD across a 150 m long Free-Space Optical (FSO) link over the two buildings with a four-symbol constellation. Other CV-QKD deployments in the literature have demonstrated a Frame Error Rate of 20% with a 100 MHz repetition rate and a loss of 6 dB in the channel. Our setup has a QBER of 25% for a 35 dB channel loss. Our system has been tested for a link loss of upto 50 dB. The QPSK symbols generated at 100 MHz from a Zynq Ultrascale+ Field Programmable Gated Array and fed to an I/Q transmitter, which in turn modulated a 1550 nm laser. The FSO transmitter, connected to our lab 500 m away, consists of a collimator and a beam expander. The transmitted states were captured on the other side by a FSO receiver unit consisting of a collimator. The received signal was transported back to the lab via a fiber of length 1 km, which is then passed through a 90 degree hybrid coherent receiver, with a 1 GHz bandwidth. The captured quadratures are post processed with a k-means cluster algorithm allowing us to recover the bit values. Our experiments focus on understanding the change in BER in the presence of varying levels of attenuation over the hybrid FSO+fibre channel.

Optical quantum technologies, especially quantum communication, yield higher potential with a network of many integrated quantum systems. Compatibility among each component is then essential. A single quantum system that can be used for different building blocks is ideal, as it automatically ensures a highly efficient interface between the different components. Fluorescent defects in two-dimensional hexagonal Boron Nitride (hBN) have been demonstrated to be a promising candidate to fulfil this requirement. This work herein demonstrates the potential of hBN defects for being both quantum emitter and quantum memory. The emission wavelengths of a large number of defects have been characterized. Together with thorough photophysical properties, these defects can be directly compared with experiments. The performance of hBN quantum memory has also been evaluated and provided with an experimental condition to achieve 95% efficiency. For an efficient global quantum network, the rigorous comparison of compatibility between hBN defects and other quantum components has been investigated. This work, therefore, serves as a recipe for generating a universal solid-state quantum system applicable to several components in optical quantum technologies.

AlGaAs Bragg-reflection waveguides offer the potential for on-chip entangled photon sources which integrate the entangled photon source and the pump source in the same chip. We designed and experimentally realized an AlGaAs Bragg-reflection waveguide structure in which the effective refractive indices for TE and TM polarized down converted photons are the same to improve the degree of entanglement without external compensation. This was realized by appropriate choice of the ridge width of 1.8 μm, which offers lateral optical confinement. The difference in effective refractive indices has been reduced by more than a factor of 10 compared to waveguides of several micron width. Photon generation rates of 9,200,000 cps (single rates) and 1,700,000 cps (coincidence rate) and a coincidence accidence ratio of about 60 were determined for these waveguides for 5 mW pump power.

Polarization-Entangled Photon Sources (EPS) are an important enabling technology in the fields of quantum sensing, quantum communication, and quantum computing. Recently, a need has arisen for efficient sources of entangled photons with high brightness and phase stability, for use in free space and fiber-based quantum communication links. In this work, we present a prototype of EPS based on commercial bulk opto-mechanical components, generating photon pairs via type-0 parametric down-conversion (SPDC). The source is configured in a linear interferometer, where a dual beam displacement is performed by symmetrically disposed birefringent components. The pairs emission can be prepared as an N00N state for quantum sensing, or as a Bell state for entanglement-based Quantum Key Distribution (QKD) protocols. We show a maximal Bell inequality violation, on ⪆99% average visibility, proving the high quality of the generated entanglement. The unique geometry of this interferometer is intrinsically symmetric, thus completely removing any temporal walk-off and decoherence between the two components of the Bell state and enhancing its suitability for various on-field quantum applications.

Over the last two decades the cold-atom physics has matured from proof-of-principle demonstrations to a versatile platform for precision measurements and study of quantum phenomena. Ultra-cold atomic ensembles have been used both for technological and fundamental science applications. To fully exploit their potential, a precise measurement and control of the atom number in the ensemble is crucial. We report on a precise, minimally destructive measurement technique that can be used to prepare an atomic ensemble with a desired atom number. The measurement relies on the dispersive light-atom interaction; thus it is expected to have a negligible effect on the ensemble temperature and to induce minimal decoherence in the atomic quantum state. As a result, it can be used to perform quantum-enhanced measurements and prepare the atom-number state at the start of an interferometer sequence.

This work presents a developed Wolfram Mathematica software tool based on the Quantum extension package. This tool is able to simulate the output state for any linear optical circuit based on Beam Splitters (BSs) and Phase Sifters (PSs) by using as input, quantum states of light. The input of the software is described via the use of photon number states capable of emulating Fock states or coherent states, whereas the output provides the probability of detecting a photon at each output port. This photonic circuit may include any number of linear optical elements, including beam splitters with variable splitting ratios, as well as phase shifters. By using the unitary transformation of any photonic circuit (e.g., photonic chip), the output states can be computed, and detection probabilities of the chip can be obtained. The unitary transformation of the Controlled NOT (CNOT) quantum gate through a linear optical layout is presented, validating the functionality of the proposed software tool. In conclusion, the proposed software aims to enable testing of simple photonic circuits in the quantum regime.

Due to today's communication demands, we require a highly secure key rate. However, there is still a significant room for improvement. This paper proposes a method to enhance the decoy state BB84 Quantum Key Distribution (QKD) on the transmitter side. Since some studies have indicated that the switching speed of optical switches can reach the attosecond level, we aim to utilize optical switches to generate more qubits per unit of time. Due to the need for proper experimental equipment and single photon detectors, we could only conduct a proof-of-concept experiment to validate our idea. We also generate the electrical signals using Python Productivity for ZYNQ (PYNQ) to control the intensity modulator for creating decoy states and our switch design for generating polarization states. Our experimental results clearly distinguish between decoy states and polarization states. Furthermore, it also aligns with our simulation results. The advantages of our transmitter design include: a) Using an optical switch to increase the critical rate. b) Enhancing system stability by employing a single laser and a fixed polarization generation instrument. c) FPGA is easily integrated with other systems. d) User-friendly key generation and comparison are done through Python. In addition, while this paper focuses on the study of decoy-state BB84, it is also feasible to consider the utilization of optical switches in other QKD protocols.

We discuss nonclassical light properties, in particular, focused on multi-photon quantum correlations in quantum optics. We analyze two-mode fields from a nondegenerate optical parametric oscillator below threshold using criteria introduced by Agarwal1 and Lee. Additionally, we identify that system parameters like pump and seed coherent amplitudes influence the nonclassicality of the output quantum state.

The rubidium two-photon optical atomic clock is a promising technology for applications that require a compact yet stable timekeeping device. However, its long-term frequency stability is often limited by the light shift (or AC Stark effect). In a rubidium two-photon frequency standard, we demonstrate in experiment that the light shift of the 778.1 nm wavelength probe beam can be mitigated with another, 1556.2 nm wavelength beam.

In this article, we present and discuss a board game called QTris that uses a concise set of rules to describe the quantum mechanics of nine qubits through the conceptual framework of preparation, operations, and measurement. Indeed, there is no quantum experiment on nine qubits that is not a game of QTris. Similarly, every game of QTris describes a quantum mechanical experiment. We show how, through its gameplay mechanics and strategies, QTris clearly emphasises the differences between statistical mixtures and coherent superpositions. As a result, the game mechanics of QTris provide an interpretation of quantum mechanics in the form of game strategies. This interpretation is clear-cut and free of metaphors.

We report a quantum diamond microscope for rapid, spatially resolved magnetic imaging across a wide-field of view. We use it to measure optically detected magnetic resonance spectra of randomly oriented single crystal nanodiamonds with dense NV- ensembles, serving as local sensors of magnetic fields. A novel planar microwave resonator, able to produce arbitrary in-plane polarizations is used for polarization-sensitive control of electron spins. The resonator exhibits more than 200 MHz bandwidth, extended homogeneous region, and magnetic field strength surpassing 29 A/m at 1 W input power. Circularly polarized microwaves are used to identify selected spin-transitions in order to retrieve the direction of magnetic field. The presented system holds potential for applications in diverse fields, including material science, biology, and space research.

Within this work we demonstrate on-chip magnetometry on a commercial 4H-silicon carbide power electronics device. For this purpose, we perform optically detected magnetic resonance measurements on silicon vacancies within the epi-layer. To this end, a dedicated home-made microscope was built. Spectra from wide range of magnetic field strength and angles were recorded and magnetic field parameters were robustly retrieved using a differential evolution algorithm. The approach presented here shows promise in robust retrieval of magnetic field parameters applied to on-chip power device magnetometry.