This PDF file contains the front matter associated with SPIE Proceedings Volume 12633, including the Title Page, Copyright information, Table of Contents, and Conference Committee information.

Time-bin entangled states are a promising paradigm for quantum communication between nodes of a quantum network. In addition, high-dimensional time-bin states are easy to generate and could offer significantly improved transmission fidelity compared to standard qubits. However, the overall rate of these transmissions is necessarily diminished because successive higher-dimensional time-bin states must be delayed such that they do not overlap in time. We propose to alleviate this concern by introducing an optical frequency shift on each time bin, taking advantage of quantum wavelength division multiplexing to greatly increase the rate of communication possible within a quantum channel. Here we report frequency shifts over a range of ∼ 2 nm (∼ 240 GHz) of telecom pulses in two time-bins separated by ∼ 250 ps, consistent with the requirements for multiplexing.

Previously, we reported a framework capable of simulating classical transmission systems and QKD based on weak-coherent prepare-and-measure protocols. The framework’s modular architecture provides a rich library of models for various system components (e.g., lasers, modulators, fibers). QKD components include singlephoton detectors, transmitter- and receiver designs, and performance estimators. For realistic system designs, component models can account for various imperfections (e.g., laser model with linewidth, RIN, side modes). Here, we present further developments of the framework and its applications. We applied our simulation tool to investigate a satellite-based BB84 system in the downlink scenario. For fiber-based applications in a coexistence scenario, we studied the usage of multicore fibers to better separate classical and quantum channels, and various PON configurations. The system performance was estimated by analyzing the QBER dependence on the classical channel power due to Raman scattering. Furthermore, we discuss the simulation results of a Gaussian-modulated CV-QKD system with a realistic model of a true local oscillator. We studied how the utilization of DSP techniques improves the system’s performance. We evaluated the secret key rate for different transmission distances.

Quantum memories can substantially increase the efficiency of long-distance communications by synchronizing entanglement swapping operations in quantum repeater nodes. To build a quantum memory, electromagnetically induced transparency (EIT) in atomic vapors can be exploited to coherently store light pulses even at room temperatures. As a quantum source of light, semiconductor quantum dots (QD) offer bright on-demand single photons with high purity.4 Interfacing QDs with atomic vapors has been shown by “slow light” but a quantum memory for QDs is yet to be demonstrated. In this work, we develop an EIT quantum memory hosted in warm cesium vapor. Storage of faint coherent light pulses on the single photon level shows high storage efficiency. A measured bandwidth in the order of 200 MHz makes the memory compatible with the Fourier-limited emission of QDs embedded in micropillar cavities. We show the first attempts to interface the emission from a QD-micropillar with our quantum memory by finetuning the emission wavelength of the emitters to one of the hyperfine transitions in Cs, where the EIT memory takes place. This work sets the base for a hybrid quantum memory based on atomic ensembles for an on-demand semiconductor single-photon source.

Optically active spins in solid state materials are an important candidate for quantum communication and distributed quantum computation over a network. To increase the size of quantum networks, long-lived quantum memories in a network node and high-fidelity control of qubits in the network nodes are desired. We discuss how isotopically engineered diamond can offer long-lived nuclear spin qubits that are robust to the optical link operation of the NV center. Furthermore, we use gate set tomography to report single-qubit and two-qubit gate fidelities exceeding 99.9% for the electron and nitrogen-nuclear spin of an NV center diamond.

Genuine multipartite entanglement is crucial for quantum information and quantum technologies but quantifying it has been a long-standing challenge. Most existing measures do not meet a “genuine” requirement, making them unsuitable for many applications. Here, we present a surprising triangle measure for tripartite entanglement, and introduce the extension to four-qubit systems. We discuss potential avenues for further generalizations along this geometric path. This indicates that our work may pave the way for further advancements.

We lay down a general scheme to quantify the amount of genuine tripartite entanglement present in the spatial and energy-time degrees of freedom using the correlations naturally present in many such sources. To that end, we test our method using the three-photon states generated in three-party extensions of spontaneous parametric down-conversion, and demonstrate that a substantial amount of spatial and energy-time genuine tripartite entanglement can exist for reasonable experimental sources.

Detecting the faint emission of a secondary source in the proximity of the much brighter one has been the most severe obstacle for using direct imaging in searching for exoplanets. Estimating the angular separation between two incoherent thermal sources is a also challenging task for direct imaging. Here, we experimentally demonstrate two tasks for super-resolution imaging based on hypothesis testing, quantum state discrimination and quantum imaging techniques. We show that one can significantly reduce the probability of error for detecting the presence of a weak secondary source (e.g. a planet), especially when the two sources have small angular separations. We reduce the experimental complexity down to a single two-input interferometer: we show that (1) this simple set-up is sufficient for the state discrimination task, and (2) if the two sources are of equal brightness, then this measurement can super-resolve their angular separation, saturating the quantum Cramér-Rao bound. By using a collection baseline of 5.3 mm, we resolve the angular separation of two sources that are placed 15 m apart at a distance of 1.0 μm with an accuracy of 1:7% { this is between 2 to 3 orders of magnitudes more accurate than shot-noise limited direct imaging.

Non-destructive testing (NDT) techniques are important for the evaluation of properties in materials, such as crack initiation or heat stresses induced by weld seams, without causing further damage to the component itself. Optically pumped magnetometers (OPM) offer a high sensitivity in the range of 15 fT/√Hz to detect resulting localized changes of the magnetization in ferromagnetic materials. Here, the sensitivity of OPMs allows the detection of small variations in magnetic stray fields on the surface of the material originating from such stresses in a volume only 0.1 mm³. However, to measure stresses in larger devices for NDT applications, the volume in which the magnetization is measured must be controlled by special devices like flux guides. They are fabricated from soft magnetic ferrites to adapt its magnetic characteristics. This paper presents first results from a novel OPM flux guide design to effectively pick up local magnetic stray fields on the surface of specimen with an increased spatial resolution and directing them to the sensing cell of a commercially available OPM. The scanning system is shielded against environmental magnetic perturbations to exploit the OPM sensitivity even in industrial environments. It is demonstrated how the two components, OPM and flux guide, can be combined to build a system for high resolution non-destructive testing of ferromagnetic steel samples.

We present integrated sources of polarization entanglement built on portable platforms for quantum communication. We employ type-II degenerate spontaneous parametric down conversion processes in PPKTP waveguides to generate pairs of orthogonally polarized photons at near-infrared wavelengths. The photons are sent to a 50/50 non-polarizing beam splitter (NPBS) to generate post-selected polarization-entangled states via coincidence measurements. The small size of the waveguides allows the installation of several units on the same platform, and thereby the generation of multiple entangled states simultaneously. We use a computer-controlled tomography system to characterize the produced states and combine the photons from adjacent waveguides on another NPBS to perform Bell-state measurements, a prerequisite to entanglement swapping. The overall experiment is compact and requires little alignment when set, making it an attractive option for local quantum networking using mobile platforms, e.g., drones or satellites.

A feasibility study and the mission definition for a Quantum Key Distribution from a geostationary satellite have been successfully completed by a Spanish consortium led by HISPASAT under ARTES ESA funding. The system will be a hosted payload of a regular communication satellite mission. In this work, the conceptual optical design of the payload is presented. It includes the trade-off between an on-axis telescope with central obscuration and an off-axis unobscured telescope. Besides, the optical design of the transmission (i.e.: quantum signal and downlink beacon) and reception channels (i.e.: uplink beacon) are described and the critical subsystems identified. A polarization control system based on liquid-crystals, which avoids mechanisms, is used to correct the errors due to instrumental residual polarization, reference system rotation or long-term instability of the QKD source during operation.

In Quantum Key Distribution (QKD), the emitter and receiver need to share an optical quantum channel - which can be optical fibre, terrestrial free-space or space-based links- to exchange the quantum states. However, with the future aim to achieve a quantum global communication network, communications links between small satellites in constellations will be required. In this context, the experience of INTA in the ANSER (Advanced Nanosatellite Systems for Earth Observation Research) small satellite constellation program will be exploited. This program develops a set of missions that will include groupings of a minimum of three CubeSats (a leader and two or more followers) flying in formation and in coordinated operation for a common mission. Therefore, the only difference between ANSER and Q-ANSER program will be the payload of the satellite. In Q-ANSER, in which a prepare-and-measure B92 QKD protocol will be used to generate the secret key, two optical systems will be introduced. In the emitter this system will be capable of sending polarized weak coherent laser pulses, attenuated to single-photon level, to the receiver, which will also be an optical system capable of receiving and detecting these single photons. Prepare-and-measure QKD schemes with polarization encoding require the minimization of polarization degradation both in the transmitter and receiver designs. In particular, the polarization extinction ratio (PER) should be maintained as high as possible to reduce the quantum bit error rate (QBER) . This polarization control will be done with the polarization modulators based on liquid crystals developed by INTA.

Efficient generation and detection of coherent single photons are key to advances in photonic quantum technologies such as quantum computation, quantum simulation, and quantum communication. Among many quantum emitters, semiconductor quantum dots are promising due to their deterministic and high-rate single-photon emission and the possibility of integration into nanostructures. However, poor quantum coherence between single photons created by independent emitters poses a major roadblock. Here, we present near-unity two-photon interference visibilities from two separate GaAs quantum dots. This high visibility (~93%) is achieved under rigorous conditions: there is no Purcell enhancement, no temporal post-selection, no narrow spectral filtering, nor frequency stabilization. One key component is the heterostructure, an n-i-p diode using material of excellent quality. The quantum dot charge is locked via Coulomb blockade; within a charging plateau, the exact emission frequency can be tuned via the bias applied to the gate; the charge noise is very low. A second key component is the quantum dot itself: the relatively large size confers multiple benefits. Our results suggest that GaAs quantum dots represent a versatile choice for generating identical photons from multiple emitters.

We investigate the effect of backscattering on the Hong-Ou-Mandel manifold (HOMM) that manifests in double-bus micro-ring resonators (MRRs). The HOMM represents higher-dimensional parameter solutions for the complete destructive interference of coincident detection in the HOM effect. To model the backscattering, we introduce a set of internal ‘beam splitters’ inside the ring that allow photons to ‘reflect’ into new counterpropagating modes inside the MRR. We find that the one-dimensional HOMM in MRRs investigated here is extremely robust against deterioration due to backscattering, even in a linear chain of identical MRRs. Further we find that a small amount of backscattering introduced into a chain of non-identical MRRs connected in parallel could be desirable, causing them to behave more like chain of identical MRRs.

Discrete variable quantum photonic circuits rely on the interference between indistinguishable photons to produce non-classical results. However, indistinguishability between photons is often spoiled due to timing delays, different spectral profiles, or the presence of unwanted spectral correlations. Additionally, variability in circuit components can introduce further errors. Here we present a method for simulating the frequency domain response of quantum photonic integrated circuits (PICs), allowing the fidelity and probability of success of realistic quantum circuits to be characterized. As an example, we first model the biphoton wavefunction produced by spontaneous four-wave mixing in a silicon nitride microring resonator, then use our methodology to simulate the interference between heralded signal photons from two such sources in the presence of spectral correlations and circuit component variability due to manufacturing imperfections.

Quantum technologies have the potential to revolutionize sensing, communication, and computation. Trapped ion qubits provide unparalleled coherence and are a leading platform for current small-scale quantum technology demonstrations. Optical addressing of individual ions with low crosstalk enables high-fidelity single and multi-qubit gates, and ions trapped in the same potential naturally allow for all-to-all connectivity. However, demonstrations of trapped ion quantum information processing have not gone beyond tens of qubits. In this talk I will lay out the optical engineering hurdles facing large-scale trapped ion quantum information processing and discuss a path forward.

Quantum technologies such as quantum information processing, quantum metrology and sensing rely on single-frequency, low-noise lasers in their core operations. Further development, scalability and commercialization of quantum technologies will be heavily dependent on the availability of affordable single-frequency lasers on a variety of application-specific wavelengths. Quantum applications manifest strict requirements for laser sources in terms of central wavelength, linewidth, long-term stability, polarization extinction ratio, side-mode suppression ratio, etc. Semiconductor lasers offer numerous significant advantages for quantum technology applications. Their broad wavelength coverage is made possible through bandgap engineering of light emitting active area. Intrinsic versatility of semiconductor lasers’ emission coupled with frequency control, which is implemented either through monolithic on-chip gratings such as in distributed Bragg reflector (DBR) and distributed feedback lasers (DFB), or external cavity optical elements, makes semiconductor lasers a very promising laser platform to address the acute need for small-size, mass-produced singlefrequency lasers. Modulight presents the development and characterization results of two narrow-linewidth laser systems incorporating a combination of in-house manufactured single-frequency lasers and internally developed low-noise driving electronics. The first laser system is designed for two-wavelength operation at cooling and repumping frequencies of Rb⁸⁷ D2 line utilizing near-IR 780.24 nm DBR lasers. Another system example includes frequency-doubled semiconductor laser in the green part of spectrum at 553 nm for Ba photoionization in trapped ion computing applications. Demonstrated exemplar platforms are viable and cost-effective tailorable alternatives to bulky and expensive Ti:Sapphire and legacy dye lasers, thus facilitating the advancement of quantum industry into real-world applications.

Efficient entangled photon pair sources are the main component for several applications based on quantum imaging. Specifically for ghost imaging, different wavelengths of signal (imaging photons) and idler (interaction with the object) photons are desired. An efficient and narrowband generation of entangled photons exploiting spontaneous parametric down-conversion using periodically poled (pp) nonlinear crystals is therefore a fundamental preliminary requirement to achieve (the process of) ghost imaging. This work presents the design and implementation of a precise and efficient crystalheater as a variable photon pair source and compares the achieved experimental values of the SPDC-wavelengths with theoretical calculations. A periodically poled nonlinear crystal from potassium titanyl phosphate (ppKTP) can generate various non-degenerate wavelengths from a pump radiation of 405 nm by temperature changes and satisfaction of energy conservation and quasi-phase-matching conditions. For this purpose, the crystal is securely housed in a custom-built mechanical mount. A computation and adjustment of various control parameters, as well as a precise determination of the current temperature via two temperature sensors allow the heater to set the target temperature with an accuracy of 0.1 °C±0.015 °C. A method for the theoretical determination of the temperature-dependent shift of the nondegenerate wavelengths, provides a foundation from which experimental verification of achievable wavelengths and intensities can be compared. By experimental verification, the efficiency and functionality of the photon pair source and SPDC-process is verified. These presented investigations and the design of the crystal-heater provide the basis for a precise and effective photon pair source, for subsequent studies in the field of ghost imaging.

Lasers have multiple applications in the field of quantum. A few examples are cooling and repumping lasers for atoms and ions used in constructing qubits, frequency combs and atomic clocks. The performance required from a laser solution varies between the applications, however, single-mode operation, narrow spectral linewidth, and extreme frequency stability over operation lifetime are demanded for any of the applications. Use of semiconductor laser diode as a laser source offers multiple advantages, such as tunability by current and temperature, small size, and low energy consumption. Narrow spectral linewidths from semiconductor lasers can be achieved by the means of external cavities, or monolithic approaches such as distributed Bragg reflector (DBR) and distributed feedback (DFB) lasers. The selection of the most suitable solution depends on the required output power, linewidth, and mode-hop free tuning range requirements. The use of monolithic on-chip gratings for wavelength stabilization decreases system complexity and increases overall rigidity compared to external cavity solutions. In this work, we present device-level results for our 780 nm DBR laser design and compare them to the simulations. The 780 nm wavelength range is of particular interest for quantum computing due to the Rb atom D2 line. For optimization purposes, devices with varied grating designs, ridge geometries and gain area lengths were fabricated and measured. Future improvements related to device processing, design, and extending to other wavelengths are discussed.

We investigate the optical properties of two-dimensional monolayer films of MoSe₂ and WSe₂ (each 0.7nm thick) assembled on SiO₂/Si substrates (285nm/0.5mm thick). These films are interesting because they are direct band gap semiconductors that have large excitonic responses. However, due to numerous challenges, including the lack of a quick, contactless, and reliable method, obtaining the optical constants and exciton binding energies in-situ remains a difficult endeavor. Here, we report the optical properties based on contactless ellipsometry to retrieve the optical constants (n,k) and excitonic properties of both monolayers (MoSe₂ and WSe₂). The optical properties of these materials away from the exciton (~700 nm) are generally not well understood. In this work, we will explore the optical response of these films over a broad range that includes the UV/visible and near infrared (200-2000nm) in order to understand if there are other spectral regions with a strong or tunable refractive index. The current samples are intrinsic without doping. The SiO₂ on the Si substrate would be used as a gate capacitor which would allow to vary the density by ~10¹¹-10¹² cm^-2. These transition metal dichalcogenides (TMD) offer new possibilities for designing modern photonic and optoelectronic components.