This PDF file contains the front matter associated with SPIE Proceedings Volume 12911, including the Title Page, Copyright information, Table of Contents, and Conference Committee information.

Large-scale quantum photonics requires the integration of several elements on the same chip, including quantum emitters and memories, active photonics, and single-photon detectors. In this talk, I will report on i) our recent work integrating superconducting nanowire single-photon detectors (SNSPD) with mechanically reconfigurable integrated photonics, and ii) our recently developed method for integration of SNSPDs onto arbitrary photonic substrates.

Superconducting Nanowire Single Photon Detectors (SNSPD) have set leading performance benchmarks in terms of detection efficiency, low noise operation, speed, and timing accuracy. Besides scaling up the numbers of individually addressable nanowire detector channels, integration with nanophotonic functionalities as well as optical and electrical multiplexing will be key to advance single photon counting applications, such as quantum key distribution, optical quantum information processing and simulations of sampling problems. Here we show how integration of SNSPDs with nanophotonic circuit components gives rise to novel use cases. We combine highperformance waveguide-integrated SNSPDs with nano-electromechanical phase-shifters, high speed electro-optic modulators, sub-wavelength grating structures, directional coupler networks, on-chip delay lines and highly efficient and broadband 3D-interconnects to optical fibers, achieving crucial active and passive functionalities for integrated quantum photonics. We showcase the capabilities and practical relevance of photonic integrated SNSPD solutions for the example of high-rate quantum key distribution receivers.

We report on the design and development of a quantum backend for an optical ground station for space-based quantum communication and science experiments. The quantum backend will enable the Optical Communication Telescope Laboratory (OCTL) to establish links with quantum satellites in the future. We aim to test this quantum enabled ground station with upcoming satellite Quantum Key Distribution (QKD) missions. We present measurements of the ground station properties that are relevant for future quantum links. Specifically, we discuss the polarization disturbance imposed by the optical communication telescope and present mitigation strategies in the form of polarization control systems. In addition to the optical design, we also present an end-to-end QKD model that is used to guide the development.

NASA Glenn Research Center’s quantum metrology approach is to combine measurements and models. Measurement results and models are subsequently integrated with NASA’s aerospace competency needs to provide an understanding of how spaceflight components work together in quantum network architectures. Trade studies and device measurements are performed within NASA’s Quantum Metrology Laboratory (NQML) whereas dynamic quantum network modeling occurs via the NASA Quantum Communications Analysis Suite (NQCAS) simulation tool. In illustrating the synthesis of the network model and metrology for quantum network development, we have focused on the evaluation of a degenerate Spontaneous Parametric Down Conversion (SPDC) source. Here we present an overview of Hong-Ou-Mandel and Joint Spectral Intensity measurements of the degenerate SPDC source. Results of these experiments are input into NQCAS to evaluate source suitability for entanglement swapping. This demonstrates the technology development approach of coupling of quantum measurement and free space quantum network models.

Quantum networking at many scales will be critical to future quantum technologies and experiments on quantum systems. Photonic links enable quantum networking. They will connect co-located quantum processors to enable large-scale quantum computers, provide links between distant quantum computers to support distributed, delegated, and blind quantum computing, and will link distant nodes in space enabling new tests of fundamental physics. Here, we discuss recent work advancing photonic tools and protocols that support quantum networking. We provide analytical results and numerics for the effect of distinguishability errors on key photonic circuits; we considered a variety of error models and developed new metrics for benchmarking the quality of generated photonic states. We review a distillation protocol by one of the authors that mitigates distinguishability errors. We also review recent results by a subset of the authors on the efficient simulation of photonic circuits via approximation by coherent states. We study some interactions between the theory of universal sets, unitary t-designs, and photonics: while many of the results we state in this direction may be known to experts, we aim to bring them to the attention of the broader quantum information science community and to phrase them in ways that are more familiar to this community. We prove, translating a result from representation theory, that there are no non-universal infinite closed 2-designs in U(V) when dim V ≥ 2. As a consequence, we observe that linear optical unitaries form a 1-design but not a 2-design. Finally, we apply a result of Oszmaniec and Zimborás to prove that augmenting the linear optical unitaries with any nontrivial SNAP gate is sufficient to achieve universality.

Continuous variable quantum key distribution (CVQKD) is the sharing of a secret key between parties using the continuous quadratures of light, the phase and amplitude quadratures, which can infer the presence of an eavesdropper using fundamental quantum mechanics. A significant performance inhibitor of free-space CVQKD is turbulence which gives rise to optical wavefront distortions consisting of random phase and amplitude fluctuations (scintillation). In this work, we experimentally study the negative effects of turbulence on coherent states in a free-space channel for CVQKD. We demonstrate during coherent state transmissions from a continuous-wave laser in a turbulent channel, the interferometric visibility between a local oscillator and signal fluctuates and decreases with higher degrees of scintillation. This leads to the breaking of the phase-lock between the local oscillator and quantum signal and a decrease in the secret key rate (SKR). By incorporating an adaptive optics system, the degraded optical wavefront from turbulence can be corrected using a closed-feedback loop. This leads to the stabilisation and reduction in the decrease of the interferometric visibility in a turbulent channel. The improvement provided by adaptive optics leads to the increased performance of CVQKD, resulting in positive SKRs, which would have otherwise been infeasible in a turbulent channel.

Quality factor (Q-factor) is important parameter for surface ion trap. To improve the Q-factor, isolation dielectric thickness was increased from 2μm to 3μm, Si trench concept was applied in this study. To reduce Si substrate RF loss, glass substrate was used for surface ion trap metal plates fabrication. We report the results obtained from determining the Q-factor of the fabricated devices at room temperature and cryogenic temperatures.

A transducer capable of faithfully converting single quanta between microwave and optical frequencies would enable an optical network of superconducting quantum computers. A primary challenge in the ongoing effort to bridge these frequency scales is the detrimental effect that optical photons have on superconducting circuits. This manuscript details the membrane-optomechanical Fabry-P´erot architecture we employ in a high-efficiency electrooptomechanical transducer. We use a chemical bonding process to create an integrated membrane-mirror etalon assembly that is robust against thermal misalignment upon cryogenic cooling. Our choice of input coupling mirror and cavity geometry allow flexible operation of the transducer in the presence of any additional nonidealities in the optical cavity, such as additional scattering loss associated with one mirror. Our transducer is unique in its ability to operate with continuous laser illumination without substantially impacting the superconducting circuitry of the transducer or a superconducting transmon qubit linked to its microwave input. We quantify the effect of laser light on the superconductor, measuring the effective occupancy of the transducer’s superconducting circuit with varying optical power to be less than 0.15 photons even at powers greatly exceeding that needed for transducer operation. We also measure the coherence time T₂ of a qubit attached to the transducer for optical readout, and determine that continuous laser pumping of the transducer has no measurable effect.

Thin-film lithium niobate (TFLN) waveguides provide a flexible platform for classical and quantum photonics. Their tight optical confinement enables strong nonlinear coupling, dispersion engineering for broadband interactions, passive components such as filters and high-Q resonators, low-voltage high-speed electro-optic devices, and tight-radius bends for dense integration. The performance of TFLN devices will be compared with that obtained in conventional weakly guiding waveguides, with a focus on quantum frequency conversion, heralded single photon sources, and progress towards few-photon nonlinear interactions.

Continuous variable quantum key distribution (CVQKD) uses the continuous quadratures of light to share a secret key between a transmitter and receiver. The presence of an eavesdropper can be detected by fundamental quantum mechanics. Compared to the more established discrete variable QKD, CVQKD uses more cost-effective homodyne detectors which are more compatible with current telecommunication technologies. In a future global quantum network, CVQKD could be used to ensure the secure exchange of information. In this work, the feasibility of a variety of CVQKD protocols in different channels that could be used to establish a global quantum network are studied. These are the Gaussian modulated and discrete modulated CVQKD protocols for satellite to optical ground station downlinks and uplinks for Earth-space connections, inter-satellite links for space connections, satellite to ship and underwater links for maritime connections, and fibre network links for terrestrial connections. Secret key distribution between continents and various regions is simulated while accounting for the different adverse effects associated with each link. The combination of these connections with routing and scheduling ensures inter-continental high-speed information exchange with unconditional security that is required to achieve the goal of a global quantum network.

Higher Dimensional Quantum Key Distribution (HD-QKD) improves information capacity and security compared to binary QKD systems. Previous HD-QKD implementations have required d+1 (d refers to encoding dimension) single-photon detectors, limiting large-scale implementation. In contrast, another HD-QKD implementation using coherent one-way (COW) protocol hardware with two detectors achieved a secret key rate (SKR) of 155 kbps for an 8-D system and 2.7% QBER tolerance for a 16-D system with qubit scrambling across a time window. In this paper, we theoretically simulate and experimentally demonstrate time-bin encoded HD-QKD using COW protocol hardware with two single-photon detectors operating at standard telecom wavelengths, achieving a remarkable maximum encoding of 6 bits per photon (i.e., 56-d system). Our HD-QKD system incorporates a practical decoy-state technique to counter the photon number splitting attack, allowing greater flexibility in choosing higher mean photon number thereby achieving higher SKR and distances. This implementation demonstrates a remarkable ninefold increase in SKR for a 16-d system (210 kbps over 25 km of standard single-mode fiber) compared to qubit-based systems, with a QBER tolerance of 13.5% for d = 16. Notably, we achieved positive key rates for emulated loss up to 131 km of fiber length for d = 8. By optimizing detector deadtime and mean photon number using our theoretical model, we achieve enhanced key rates and security. Our findings highlight the superiority of our approach and lay the foundation for high throughput HD-QKD implementation, offering a secure and efficient method for transmitting sensitive information.

Today’s society heavily relies on secure communications, which can be guaranteed by Quantum Key Distribution (QKD), the most mature quantum technology. However, achieving long-distance links without relying on trusted nodes is still challenging. An important limitation is the non-ideality of detection systems, where intrinsic dark counts can hinder key extraction. This work proposes using state-of-the-art superconducting nanowire singlephoton detectors (SNSPD) with ultra-low dark count rates (<1 Hz) to reduce the quantum bit error rate (QBER) and achieve a higher secret key rate. Together with a high-rate QKD transmitter and a self-stabilizing receiver, we enabled a key exchange over 55 dB, corresponding to 340 km over an ultra-low-loss optical fiber.

A variant of QAE algorithm by Suzuki et al. called maximum likelihood amplitude estimation (MLAE) achieves the amplitude estimation by varying depths of Grover operators and post-processing for maximum likelihood estimation without the additional controlled operations and QFT. However, MLAE requires running multiple circuits of different depths of Grover operators. On the other hand, quantum multi-programming (QMP) is a computing method that executes multiple quantum circuits concurrently on a quantum computer. The quantum circuits executed concurrently can be different and even have different circuit depths. The main motivation of the QMP is that the number of qubits of NISQ computers is much greater than their quantum volume. In this work, using QMP in conjunction with MLAE makes it possible to run MLAE using a single circuit, thus requiring sampling much fewer times. We validate this algorithm for a numerical integration problem using NVIDIA’s open-source platform CUDA Quantum (simulator), Qiskit (simulator) and Quantinuum H2 device.

The linear superposition principle and the quantum entanglement phenomenon play crucial roles in the fields of quantum computing and information. Their current interpretations are not satisfactory for the need of quantum computing and information. To reduce the measurement-bias of the current interpretation of the quantum linear superposition principle, this paper presents an alternative interpretation: A physical quantity of a quantum object keeps oscillating between the allowed values of the physical quantity. Thus, a quantum system is inherently deterministic, but it appears to be probabilistic because of randomness in timings of measurements. Then, to show that the so-called quantum entangled objects need not interact or communicate with each other, the paper presents an alternative interpretation of the quantum entanglement phenomenon: Quantum objects appear to be entangled if and when each physical quantity of these objects undergoes synchronous oscillations. An experimental method is presented to validate this interpretation. Quantum entanglement due to synchronous oscillations can lead to more and better ways of quantum computers. The paper introduces Excel and Python quos package approaches to simplify and expedite designing and simulating quantum computing circuits.

Quantum-augmented networks aim to use quantum phenomena to improve detection and protection against malicious actors in a classical communication network. This may include multiplexing quantum signals into classical fiber optical channels and incorporating purely quantum links alongside classical links in the network. In such hybrid networks, quantum protocols based on single photons become a bottleneck for transmission distances and data speeds, thereby reducing entire network performance. Furthermore, many of the security assumptions of the single-photon protocols do not hold up in practice because of the impossibility of manufacturing single-photon emitters. Multi-photon quantum protocols, on the other hand, are designed to operate under practical assumptions and do not require single photon emitters. As a result, they provide higher levels of security guarantees and longer transmission distances. However, the effect of channel and device noise on multiphoton protocols in terms of security, transmission distances, and bit rates has not been investigated. In this paper, we focus on channel noise and present our observations on the effect of various types of noise on multi-photon protocols. We also investigate the effect of topologies such as ring, star, and torus on the noise characteristics of the multi-photon protocols. Our results show the possible advantages of switching to multi-photon protocols and give insights into the repeater placement and topology choice for quantum-augmented networks.

Quantum computers, hypothesized in 1980s, use concepts of superposition and entanglement phenomena. Although theoretical propositions and associated search algorithms for accurate measurements are being generated, the development of practical quantum computers themselves are advancing very slowly requiring enormous time and investments. The underlying concepts of a quantum computer are not new to the optical domain. However, the crucial enabling concepts of Entanglement and Superposition Principle are remaining clouded under the unresolved postulates, Wave-Particle Duality (WPD), and Wave Packet Reduction (WPR), implicating incompleteness in the interpretations of the mathematical formalism behind Quantum Mechanics. The WPD debate started during late1600 between Newton and Huygens. Young’s resolution of WPD through his double-slit experiment in 1802 was effectively overturned by Einstein’s interpretation of photoelectric effect as due to “indivisible light quanta”. However, Einstein disowned his “light quanta” postulate shortly before his death in1955, even though it had earned him the Nobel Prize. We resolve WPD by synthesizing Newton’s and Maxwell’s concepts and assume atoms do emit quanta but propagate as time-finite exponential pulses. This assumption also resolves WPR for light-matter interaction with the assumption that Schrodinger’s ψ represents atom’s internal dipolar amplitude stimulations. This over-turns Born’s interpretation that ψ only represents the abstract mathematical probability amplitude, rather than the physical “internal amplitude stimulation” of the quantum entity. However, our concept of atomic pulse emission forces us to re-derive the expression for the N-slit grating-spectrometer response since the classical derivation uses CW light, which does not exist. This pulsespectrometric response function strengthens our postulate since the grating response to the exponential pulse appears to be the convolution of a Lorentzian spectrum with the classical CW response function of the grating. The Fourier Transform of an exponential function is Lorentzian and QM predicts spontaneous emission line width to be Lorentzian. Then, conceptually one can extend the grating-expression (with N=2) to get the double-slit pattern. This approach preserves the classical causality that each of the two slits, like the N-signals out of a grating, are physically real and jointly stimulate the quantum detector array at the far field to generate the “Local” cosine fringes. The detector array executes the square modulus operation on its imposed dipolar amplitude stimulation and absorbs the necessary energy to fill up their quantum cups. Hence the double-slit pattern must also be “Local”, just as the N-slit grating spectrum is generated locally at the exit spectral-plane of the spectrometer. This removes the need to believe that “single photons” mysteriously generate the double slit pattern.

With the advent of quantum computing, which offers exponential computational speedup compared to classical computers, and the constantly expanding field of machine learning, which focuses on extracting patterns and insights from data. The paper comprises two comprehensive case studies: Network Traffic Analysis and Earthquake Magnitude Classification. We were able to perform an overview of previous studies in this field and acknowledge the research gap while building a Quantum Machine Learning model that provides accuracy over 60% while using 4 Qubits and keeping the loss around 20%.

The constant advancements in single-photon technologies have led to the development of detectors with amazingly low jitter, that can play an important role in quantum measurements. A major limitation to their full exploitation in practical applications is represented by the timing electronics that should possess both low jitter characteristics, as well as good speed, linearity, and full-scale range (FSR) performance. In this paper, we propose a new TACbased single-channel timing system that features a state-of-the-art timing jitter of 4.5 ps FWHM, along with a peak-to-peak DNL of 1.5% LSB and a speed of 12 Mcps, over a wide full-scale range of 12.5 ns. Thanks to the promising results achieved in experiments with SNSPDs, we are extending the system to eight channels, to leverage converter parallelization to further reduce timing jitter below 2 ps.

Many quantum applications will benefit significantly from photon number resolving detection. However, photon number resolving detectors have been largely experimental devices implemented in laboratory settings. Recent advances in superconducting nanowire device fabrication and readout techniques have enabled the implementation of photon number resolution in widely deployable commercial single-photon detection systems. We will discuss progress in implementing photon number resolved detection with commercially produced superconducting nanowire single photon detectors via spatial multiplexing of independently instrumented detector elements and photon number dependent rise-time changes in single-element nanowire devices.

We present an optical design concept to create a two-dimensional tweezer array for neutral atom quantum computing. The first part of the setup splits the four input laser beams into 20 individual beam lines that correspond to the columns of the tweezer array. A combination of an acousto-optic deflector (AOD) and a suitable imaging lens for each beam line creates a magnified image of the one-dimensional columns in the tweezer array with a spacing of less than 200 μm. The spacing between the 20 columns is reduced to this value as well to achieve a spot array with an equidistant spacing in both axes. This is done successively in four steps by using a combination of one discrete mirror cascade and three mirror cascades integrated into monolithic mirror arrays. The final tweezer array with a spacing of 3.5 μm is then created by a two-stage telecentric demagnifying lens group. Unlike other approaches to create a two-dimensional tweezer array (e.g. crossed AODs), the use of a separate AOD for each column allows for an independent adjustment of the spacings within each column and thus a flexible adjustment of the local interaction of neighboring atoms. A raytracing analysis demonstrates that the whole setup produces a telecentric and diffraction-limited image of 20 × 100 spots in the following vacuum chamber.

The frequency modes of photonic resonators offer tantalizing possibilities for analog simulators of Hamiltonians both in the quantum and classical realm. We show how synthetic frequency dimensions can be formed by coupling such modes through external drives, enabling experimental demonstrations of a wide variety of topologically nontrivial effects that are challenging in other platforms. Harnessing the long-range coupling, coherent phase control and unprecedented reconfigurability afforded by synthetic dimensions, we observe phenomena such as the 2D quantum Hall effect and topological phase transitions in 0D structures consisting of one or two resonators. Our work elucidates how higher-dimensional physics can be implemented in simpler, experimentally feasible lower-dimensional structures. We also highlight the potential of scalably constructing new phenomena such as topological energy pumps showing quantized transport that is robust to dissipation by introducing an additional Floquet dimension in coupled resonators.

The Quantum Approximate Optimization Algorithm (QAOA) is a promising candidate for solving combinatorial optimization problems more efficiently than classical computers. Recent studies have shown that warm-starting the standard algorithm improves the performance. In this paper we compare the performance of standard QAOA with that of warm-start QAOA in the context of portfolio optimization and investigate the warm-start approach for different problem instances. In particular, we analyze the extent to which the improved performance of warm-start QAOA is due to quantum effects, and show that the results can be reproduced or even surpassed by a purely classical preprocessing of the original problem followed by standard QAOA.

Recent advancements in quantum hardware have enabled the realization of high-dimensional quantum states. This work investigates the potential of qutrits in quantum machine learning, leveraging their larger state space for enhanced supervised learning tasks. To that end, the Gell-Mann feature map is introduced which encodes information within an 8-dimensional Hilbert space. The study focuses on classification problems, comparing Gell-Mann feature map with maps generated by established qubit and classical models. We test different circuit architectures and explore possibilities in optimization techniques. By shedding light on the capabilities and limitations of qutrit-based systems, this research aims to advance applications of low-depth quantum circuits.

The next generation quantum computing experiments are either based on trapped ions or neutral atoms. In the latter experiment that atoms exhibit a laser-induced excitation to Rydberg states which is typically carried out in glass cells that must operate under ultra-high vacuum conditions. The experimental setup requires the entry of laser light of numerous wavelengths at different angles of incidence into the glass cell. The optical spectrum ranges from the UV to near infrared range (310-820nm) and the angles of incidence (AOI) can reach up to 60°. Especially at high AOI, the Fresnel reflections occurring at each glass interface can reach non-negligible levels. The paper presents an AR solution called AR-plas2, which is based on nanostructures with very low effective refractive indices that can be stacked on top of each other and allow the combination with classical interference coatings within a closed process. This opens great design opportunities with outstanding AR properties in terms of residual reflectance, AOI tolerance as well as polarization neutrality in comparison to conventional AR coatings. Processing is performed in a commercial box coater and is based on the nanostructuring of organic layers by an APS plasma source. Subsequent coating with a dielectric capping layer and post treatment steps that remove the organic compounds lead to overall dielectric nanostructures with very low absorption values in the range of <2ppm (1064nm). An AR coating with an R_ave(310-820nm)<0.2% for AOI=6° and R_ave(310-820nm)<0.3% for AOI=45° with extremely low polarization splitting that was realized on basis of the AR-plas2 technology will be presented.

The observation of extreme dynamics within quantum simulators based on photonic circuits is typically precluded by optical losses, exponentially increasing with the system depth or, equivalently, with the number of optical components. This is a natural consequence of the standard approach to photonic simulations of quantum dynamics, where the complexity of the setup grows with the extension of the evolution in time. By focusing on simple protocols of discrete-time quantum walks, we show that it is possible to compress homogeneous evolutions within only three liquid-crystal metasurfaces, encompassing up to a few hundreds of time steps. By exploiting spin-orbit effects, these devices implement space-dependent polarization transformations that mix circularly polarized optical modes carrying quantized transverse momentum, mimicking the target quantum dynamics with high efficiency and accuracy. Being extremely versatile, our compact platform will pave the way to the simulations of extreme regimes of more exotic dynamics.

Efficient generation of entanglement between a stationary matter-qubit and a propagating photonic-qubit is a scientific and technological challenge of utmost importance for the realisation of fully-fledged quantum networks. Solid-state quantum emitters in the telecom C-band are a promising platform due to the minimal absorption of photons at these wavelengths and deterministic generation of high purity single photon flying qubits. Here, we use an InAs/InP quantum dot to implement an optically active spin-qubit and demonstrate high fidelity spin initialisation and coherent spin control of the resident electron. Lastly, for the first time we demonstrate high-fidelity spin-photon entanglement in a solid-state system with direct emission into the telecom C-band and obtain a lower bound on the entanglement fidelity of 80.07 %.

Semiconductor quantum dots can generate single indistinguishable photons at telecom wavelengths for quantum networking applications. Integrating quantum dots into polarized microcavities is highly beneficial for advanced functionalities. In this work, we couple a quantum dot emitting in the telecom ‘O’ band to an elliptical bullseye resonator, demonstrating broadband polarization-selective enhancement and linearly polarized single-photon emission.

Single photons generated by spontaneous parametric down conversion are often utilized in heralded schemes, in which case the heralded photon purity must be maximized to preserve the indistinguishability of the photons that carry out quantum computing. In this work, we present software designed for accurate calculation of such purity, which was optimized to minimize computational resources and time, allowing for maximum purity searches in broad wavelength ranges. Lithium niobate and potassium titanyl phosphate (KTP) were studied in three phase-matching conditions, and an ultra-high purity configuration was found. The configuration was experimentally realized through an ion-exchanged KTP waveguide, exploiting flexibility in pump and phase matching temperature to compensate fabrication deviations, and purities as high as 0.923 were found, with very high generation rate of 40.7 MHz/mW. The experimental purity is significantly degraded by the multimode guiding at the desired wavelengths, which we propose to address by engineering with femtosecond laser writing, of which preliminary results are presented.

Entangled photon pairs are key to quantum optical applications like quantum computing and communication. They're typically generated through spontaneous parametric down-conversion (SPDC) in non-linear crystals, yet current lab equipment doesn't satisfy the need for robust, compact, and accessible devices. We report about various designs of efficient, ultra-bright entangled photon sources.

For quantum-technology applications, spin defects in solids are promising candidates but they are limited by their spin-coherence time. Hence, understanding the processes governing the spin-coherence decay is essential. We numerically calculate the spin-coherence time T2 of the NV−-centre electron spin in diamond layers of different thickness in presence of a strongly-coupled disordered electron-spin bath. We show that the spin-coherence time in two-dimensional spin baths is longer than in three-dimensional spin baths. Furthermore, we obtain the corresponding stretched-exponential parameter, which also depends on the spatial dimensionality. We use the partitioning-based cluster-correlation expansion (pCCE) method, which partitions the bath into local subsystems and applies the clustion-correlation expansion method to these subsystems instead of individual spins, thereby offering a better numerical accuracy, especially in two-dimensional geometries.

We present an ultra-compact, hybrid faint pulse source (FPS) at 850 nm, making use of a linear eight VCSELs array at spectral (<1 pm wavelength difference, and >90% overlap at FWHM) and temporal (<1 ps) indistinguishability, as well as polarization quality in the four H/V/D/A BB84 channels >20 dB. A common VCSEL array on a single substrate is used, at a pitch of 250 μm and with integrated polarizers, having a spectral indistinguishability if the substrate is temperature levelled < 0.5 K. Each VCSEL represents either faint or full amplitude signal for the H/V/D/A channels of the BB84 protocol. The temperature levelling heatsink is made of Molybdenium, integrated on LTCC board to host the emitter substrate and its respective DAC driving circuit at speeds up to 10 GHz. VCSEL integrated micro-lenses and two additional micro-lens arrays fully collimate the beams and refocus them into a waveguide combiner chip which realizes the polarization independent coupling of all eight VCSEL free-space beams with a pulse delay variation <0.2 ps.

The necessity for scalable, reliable and reproducible quantum systems is raising in the time for appealing potentialities in application of this technology. On-chip photonic solutions are a leading competitor by combining established classical fabrication technologies, integrated photonics and quantum optical effects. Lithium Niobate on insulator (LNOI) is one of the main contenders for the integration of on-chip functionality. Rare Earth Ions (REI) already proved many different good properties in term of optical and spin features as well as good integration through implantation. In particular, ¹⁷¹Ytterbium showed to have clock transitions in the zero phonon line at zero field which would screen magnetic noise from high spin materials. Here the possibility to create a reconfigurable interferometer on-chip by the employment of fabrication techniques has been showed with the creation of LN structures, interconnecting infrastructure and far field coupling from SU8 resist. Moreover, doping Lithium niobate photonic crystal cavities has been investigated to be employed as switchable single photon sources.

Photonic wire bonds have been developed as an interface for the collection of single photon emission from quantum dots within a Bragg waveguide. When resonantly excited from the top of the waveguide via free space excitation a low multiphoton contribution in the quantum dot emission with g⁽²⁾(0) = (9.5 ± 1.4) × 10⁻² is shown. Our measurements demonstrate the ability to collect single-photon emission from a ridge waveguide into an optical fiber via photonic wire bonds at cryogenic temperatures. This allows for a seamless plug-and-play operation of the fiber-coupled single-photon source. Furthermore, the demonstrated approach allows for resonance fluorescence excitation without the need for any additional cross-polarization filtering.

Semiconductor quantum dots embedded into nanophotonic resonators are one of the best performing sources for single and entangled photons. For certain protocols a defined charge in the quantum dot is required and to increase the device yield, charge control is a desired feature of the nanophotonic structure. In this paper, we present our latest efforts towards such a highly efficient, charge tunable device. Numerical simulations optimizing a bridged circular Bragg grating are discussed and photoluminescence measurements of a charge tunable quantum dot are shown.

We report our recent progress in generating and verifying entangled photon pairs in a telecommunication-length few-mode fiber using spontaneous four-wave mixing in a non-degenerate pump configuration. We demonstrate through seeding the desired four-wave mixing effect for pair production in the presence of a strong mode-coupling when using integrated mode multiplexers known as photonic lanterns. The mode content and phase-matching requirements were validated through computations and experiments. We discuss the challenges and requirements for phase matching and photon visibility without modifying the fiber structure. This work represents our first steps toward an efficient tunable source of entangled photons for space division multiplexing applications.

We report the optical architecture, experimental performance, and simulated performance of polarization- maintaining CW and pulsed single clad Tm-doped fiber amplifiers designed to operate over a wavelength span of 1760—1960 nm. We highlight the potential applications of these amplifiers to quantum computing and quantum qubit experiments using 1762 nm light. Our amplifier exhibits 3 W CW output power and 20 W peak pulsed output power (2 MHz rep. rate, 10% duty cycle) at 1762 nm. Measurements of the wavelength response of the TDFA yield an experimental operating bandwidth extending from < 1750 nm to > 1920 nm. Simulations of the amplifier bandwidth indicate a 3 dB (50% FWHM) wavelength span of 1745 nm to 1980 nm (135 nm). Experimental output power and bandwidth results agree well with the simulations. The external noise figure for this amplifier ranges from 7.5 dB to 9.5 dB. No linewidth broadening was observed in a typical TDFA output when using a single frequency input laser source with a linewidth of 10 kHz. We discuss suitability and applications of the TDFA to 1762 nm enabled manipulation of optical qubits in trapped ¹³³Ba⁺ ions.

Laser systems are utilized in quantum for various applications. Multiple wavelengths and tailored solutions are required depending on the technology that the laser will be applied to. For instance, lasers can be used for controlling particles and molecules, including excitations of the quantum systems. Key performance requirements for lasers used in these applications include narrow linewidth, frequency stability, and single-frequency operation. This performance can be achieved with laser diodes with integrated gratings, such as distributed Bragg reflector (DBR) and distributed feedback (DFB) structures. Laser diodes offer benefits such as low power consumption, compact size, and easy integrability to photonic integrated circuits. In addition, on-chip integrated gratings have advantages over external cavity diode lasers: reduced complexity in systems, smaller size, and better robustness. In this work, we present narrow linewidth DBR laser diode operating in the 650 nm wavelength regime which is required for quantum applications such as repumping in trapped Ba⁺ ion computing. In-house epitaxial design is based on a GaAs/AlInP/AlGaInP structure, including GaInP quantum well. Grating region is implemented as surface grating, requiring electron beam lithography (EBL) and high-aspect ratio etching by inductively coupled plasma reactive ion etching (ICP-RIE). Results for multiple variants are presented to achieve optimal device performance and grating coupling efficiency, targeting narrow linewidth operation required for quantum applications such as trapped ion computing.

Quantum information processing (QIP) approach based on trapped ions is a promising technology that exploits different ion species as individual qubits and qubit gates, while featuring long coherence times, high fidelity of operations, fast readout, and ion-ion gate entanglement. The practical usefulness of trapped-ion quantum computer depends on its further scaling to a large number of qubits. Meanwhile, trapped-ion quantum computer relies on a set of single-frequency lasers, utilizing them for different functions such as photoionization, atom cooling, optical pumping and qubit operations. Thus, a combination of narrow linewidth lasers is required, with the emission wavelengths precisely defined by the corresponding electronic transitions of the selected ion species. Further development, scalability and commercialization of quantum information technologies is interwind with the availability of affordable and robust single-frequency lasers on a variety of application-specific wavelengths. QIP manifests strict requirements posed to lasers in terms of central wavelength, linewidth, long-term stability, beam quality, polarization extinction ratio, side-mode suppression ratio, etc. In this work we present the development and characterization of multiwavelength narrow linewidth system designed for the Ba+ trapped ion computing. The laser system emits single-frequency, sub-MHz emission at 553 nm, 493 nm and 614 nm based on second-harmonic generation of two separate semiconductor laser platforms. Two-step photoionization of neutral Ba is implemented resonantly through 6s2 1S0 → 6s6p 1P1 transition corresponding to 553 nm wavelength, whereas Ba ion is Doppler cooled on 6S1/2 →6P1/2 transition corresponding to 493 nm. Such single-frequency greenyellow and cyan lasers are based on intracavity frequency doubled vertical-external-cavity surface-emitting laser (VECSEL), serving as an effective replacement of the legacy dye laser, previously used to address these Ba transition. On the other hand, frequency doubled external cavity diode laser (ECDL) is employed to generate 614 nm orange emission needed for 5D5/2→ 6P3/2 transition. The laser system incorporates modular laser engines together with low-noise driving electronics, mechanical and acoustic concealment and provide laser locking interfaces to high-finesse optical cavity or a wavemeter.

Infleqtion's quantum matter service, "Oqtant," equips users with remote access to quantum hardware for performing sophisticated research and enriching scientific education. Users can dynamically control reconfigurable optical fields applied to ultracold atoms, expanding their exploration of quantum mechanical phenomena. The platform offers tutorials, demos, and simulations to empower users to study and develop applications. In this paper, we introduce Oqtant and explore how users can leverage the platform for education and quantum research.

Ion-trapped-based quantum computers offer long coherence times of over 2 seconds with high fidelity and hence constitute a promising architecture for realizing quantum computer systems beyond the NISQ classification. Utilizing a fiber-coupled laser source at a wavelength of 396 nm our setup is able to manipulate any combination of 40Ca+ ions inside a coherent chain of 10 ions simultaneously, while also being able to collect the readout of each individual ion. Our setup achieves an ion-sided mode field diameter of less than 2 μm and a pointing accuracy of 250 nm for one chain of ions. A micro-lens integrated waveguide carrier chip with linear optics is used to create 10 separate laser beams at the actual ion dimensions. The modularity of the systems allows the system to be arranged onto additional linked ion trap chains, pathing the way for a scalable quantum computer.

The quantum phase estimation (QPE) is one of the fundamental algorithms based on the quantum Fourier transform. It has applications in order-finding, factoring, and finding the eigenvalues of unitary operators. The major challenge in running QPE and other quantum algorithms is the noise in quantum computers. In the present work, we study the impact of incoherent noise on QPE, modeled as trace-preserving and completely positive quantum channels. Different noise models such as depolarizing, phase flip, bit flip, and bit-phase flip are taken to understand the performance of the QPE in the presence of noise. The simulation results indicate that the standard deviation of the eigenvalue of the unitary operator has strong exponential dependence upon the error probability of individual qubits. However, the standard deviation increases only linearly with the number of qubits for fixed error probability when that error probability is small.

Precise control of system parameters and extensive optimization play a crucial role in enabling quantum information technologies. As a further challenge, when targeting practical manufacturable systems, the presence of manufacturing variations in components necessitates individual optimization for each system. To address this challenge, we develop a generalisable optimisation framework based on deep reinforcement learning (RL). By applying our method to real-world quantum transmitters based on optical injection locking (OIL), we demonstrate that our RL agent can autonomously identify the optimal operating regions, and generalise its knowledge for new quantum transmitters of the same type. This work presents a new avenue for efficient optimisation of complex systems using modern RL algorithm.

In this Invited Paper, we will summarize our vision for quantum networks based on arrays of individual neutral atoms. With a photonic interface such as an optical cavity, some atoms in the array – the “communication” qubits – can establish remote entanglement while others – the “data” qubits – are used for large-scale quantum information processing or for quantum-enhanced metrology via an atomic clock transition. Deterministic gates within the array will enable operations and algorithms that are distributed among all the qubits in two or more nodes. Additionally, we will discuss our efforts to realize this vision with arrays of ytterbium-171 atoms in optical tweezers, in which telecom-band photons can be directly entangled with nuclear spin qubits without the need for wavelength conversion.

Entanglement is a unique resource for quantum information processing, which allows for device-independent (DI) quantum communication protocols, such as Quantum Key Distribution and Quantum Random Number Generation. Loophole-free violation of Bell inequalities is a necessary condition for DI protocols. While timebin entanglement is promising for long-distance distribution over fiber optics, its most common implementation suffers from a post-selection loophole (PSL) which invalidates Bell non-locality tests. We present an implementation of a postselection-free “genuine” time-bin entanglement scheme working with 1550 nm biphotons with an estimated Bell violation of S = 2.56.

Quantum networking holds tremendous promise in transforming computation and communication. Entangled-photon sources are critical for quantum repeaters and networking, while photonic integrated circuits are vital for miniaturization and scalability. In this talk, we focus on generating and manipulating frequency-bin entangled states within integrated platforms. We encode quantum information as a coherent superposition of multiple optical frequencies; this approach is favorable due to its amenability to high-dimensional entanglement and compatibility with fiber transmission. We successfully generate and measure the density matrix of biphoton frequency combs from integrated silicon nitride microrings, fully reconstructing the state in an 8 × 8 two-qudit Hilbert space, the highest so far for frequency bins. Moreover, we employ Vernier electro-optic phase modulation methods to perform time-resolved measurements of biphoton correlation functions. Currently, we are exploring bidirectional pumping of microrings to generate indistinguishable entangled pairs in both directions, aiming to demonstrate key networking operations such as entanglement swapping and Greenberger–Horne–Zeilinger state generation in the frequency domain.

In the field of cryptography, it is crucial that the random numbers used in key generation are not only genuinely random but also private, meaning that no other party than the legitimate user must have information about the numbers generated. Quantum random number generators can offer both properties - fundamentally random output, as well as the ability to implement generators that can certify the amount of private randomness generated, in order to remove some side-channel attacks. In this study we introduce perovskite technology as a resilient platform for photonics, where the resilience is owed to perovskite’s ease of manufacturing. This has the potential to mitigate disruptions in the supply chain by enabling local and domestic manufacturing of photonic devices. We demonstrate the feasibility of the platform by implementing a measurement-device independent quantum random number generator based on perovskite LEDs.

Quantum memories will be an essential part of many quantum information protocols, including distributed quantum computing, quantum sensing, and the synchronization of repeater nodes. Most efforts toward developing such a quantum memory have been focused on matter-based storage systems, which convert the energy and information from an optical state to an atomic state of matter to be retrieved later. These matter memories, while capable of achieving great storage times, have several limitations; namely, they are inherently narrow bandwidth, they typically do not operate at key telecommunications wavelengths, and often require costly overhead in the form of cryogenics. In this work, we have developed a quantum memory that operates in free space at room temperature, allowing us to avoid all the previously mentioned limitations, and achieve a record-high time-bandwidth product.

Spin defects in diamond are attracting interest because of their possible application in quantum information processing. One group of such defects are group 4 defects (G4V). They posses an inversion symmetry, which makes them (in first order) invariant under electric fields, allowing them to be placed inside nanophotonic structures. One of those defects is the tin-vacancy (SnV) center. It has an atom like energy structure with the zero phonon line (ZPL) at 620 nm and a Debye-Waller factor of 0.6. The lower two branches are separated by 850 GHz [1], which means that phonon induced dephasing can be neglected at 2 K.

Time-bin entangled photons have previously been utilized for advanced quantum communication protocols such as quantum state teleportation or entanglement swapping, thus showing the potential of realizing a quantum internet. For such protocols, intricate feedback systems are essential to preserve quantum information during distribution, and require different forms of state preparation that can incorporate the feedback in quantum state generation. We use different methods of state preparation via arbitrary waveform generators and PLC MachZehnder type interferometers, and entangle the states by SPDC using commercial lithium niobate waveguides to create time-bin entangled photon pairs. Quantum state tomography is conducted on each method to evaluate their effectiveness in generating states suitable for quantum communication protocols.

Many quantum entanglement generation protocols require phase stabilization between the nodes. For color centers that are embedded in a solid immersion lens (SIL) often a reflection from the SIL’s surface is input to an interferometer where it is mixed with a reference beam. However, the beam reflected beam by the SIL does not travel colinear with the photons that are emitted by the color center, which ultimately leads to a reduction of the interferometer’s signal-to-noise ratio (SNR). Additionally, imperfections of the SIL surface introduce aberrations into the reflected light, thereby further reducing the SNR. Through several design-iterations and extensive experience realizing phase stabilization on many different SIL’s we have come to an approach that significantly improves the SNR and enhances the operability of the quantum node. In this paper we report on our optical design and provide useful guidelines for the operation thereof.

In recent years, significant progress has been made in building quantum computers by several companies. Despite the progress, these noisy intermediate-scale quantum (NISQ) computers still suffer from several noises and errors such as measurement errors, multi-qubit gate errors, and worse, short decoherence times. Though quantum computer vendors are releasing better quantum computers in terms of Quantum Volume, the quantum device still remains far from quantum supremacy in practical problems. The Quantum Approximate Optimization Algorithm (QAOA) was suggested to potentially demonstrate a computational advantage in combinatorial optimization problems on NISQ computers. In this paper, we optimize the QAOA circuits and to scale the problem size on IBM quantum processors. In addition, we study the effect of the length of the QAOA ansatz on IBM quantum processors and discuss optimal implementation methods for scalable QAOA. We test our implementations on the MaxCut problems.