This PDF file contains the front matter associated with SPIE Proceedings Volume 10933, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists.

First it will be shown that the nonlocal correlations of any hypothetical theory obeying two basic assumptions, namely, generalized uncertainty and relativistic causality, are bounded by the known quantum mechanical bounds. Then, novel bounds on quantum nonlocal correlations will be presented, including multipartitie and continuous variables scenarios. Experimental tests of some new bounds and concepts (performed with the Genovese and Karimi groups using SPDC photons) will be discussed. Finally, a few fundamental and practical implications on quantum entangled systems will be presented. This talk is based on past and on-going works with Avishy Carmi and the aforementioned experimental groups.

Two important challenges in quantum photonics are to generate useful states with high fidelity, and to detect them and verify their properties. Particularly valuable states are single photons and entangled photon pairs in well-defined optical modes, as they can be used in many quantum information protocols or used to build up more complex states. For sources, we employ integrated nonlinear optics (waveguides in lithium niobite and potassium titanyl phosphate) to maximize brightness and go beyond what is possible in bulk optics, showing simultaneously high state fidelity, heralding efficiency, and spectral purity across three experiments: first we show record heralding efficiency in a fully-fibered heralded single-photon source, and use it to probe the tradeoff between spectral purity and heralding efficiency in non-engineered sources. With an engineered source, we then herald up to 50 photons in a nonclassical state. The last source is for polarization-entangled photon pairs, with brightness of 3.5 million pairs/s·mW, fidelity to a Bell state of 96%, heralding efficiency of 43%, and HOM interference visibility of 82%. Once a complex state is constructed, it must also be verified. For this we employ a time-multiplexed detector consisting of a fibre loop and a single-photon detector. Surprisingly, we are able to extract information even in the saturation regime of the detector. We use the click statistics of the time-multiplexed detector to verify the non-classicality of quantum light, and we use its extremely high dynamic range (123 dB) to measure a macroscopic power level with a single-photon detector. Eliminating calibrated attenuators with this approach will allow direct standardization of quantum and classical optical power levels.

Light sources for applications in quantum information, quantum-enhanced sensing and quantum metrology are attracting increasing scientific interest. To gain inside into the underlying physical processes of quantum light generation, efficient photon detectors and experimental techniques are required to access the photon statistics. In this work, we employ photon-number-resolving (PNR) detectors based on superconducting transition-edge sensors (TESs) for the metrology of photonic microstructures with semiconductor quantum dots (QDs) as emitters. For the PNR analysis, we developed a state of the art PNR detection system based on fiber-coupled superconducting TESs. Our stand-alone system comprises six tungsten TESs, read out by six 2-stage-SQUID current sensors, and operated in a compact detector unit integrated into an adiabatic demagnetization refrigerator. This PNR detection system enables us to directly access the photon statistics of the light field emitted by our photonic microstructures. In this contribution, we focus on the PNR study of deterministically fabricated quantum light sources emitting single indistinguishable photons as well as twin-photon states. Additionally, we present a PNR-analysis of electrically pumped QD micropillar lasers exhibiting a peculiar bimodal behavior. Employing TESs our work provides direct insight into the complex emission characteristics of QD- based light sources. We anticipate, that TES-based PNR detectors, will be a viable tool for implementations of photonic quantum information processing relying on multi-photon states.

This paper will overview recent progress in time-dependent metrology. By applying coherent control to a quantum system, we will demonstrate that the amount of information about a parameter can be increased by typically a power law in the duration time of the experiment. Several examples will be given: measurement of an external oscillation frequency, the Landau-Zehner transition, and the details of a recent experiment realizing these physical predictions. A simple example will be analyzed in detail to illustrate the ingredients of the theory.

Photon entanglement has appeared to play a crucial role in the foundation of quantum physics and in the ever-increasing requirements of quantum information processing, quantum communication, quantum sensing and quantum computing. Thus, the industrial development and the characterization of practical entangled photon sources at telecom wavelengths are key element for the deployment of emerging quantum applications, such as Quantum Key Distribution. The photon source is based on a 775 nm pumped type II Periodically Poled Lithium Niobate wave guide (PPLN_WG) designed for generation of orthogonally polarized photon pairs at 1550 nm. Picometer tunability of the converted central wavelength is achieved by using an accurate temperature control of both the PPLN_WG and the laser diode. All optical and electronics elements are embedded in a compact module. This practical photon source is driven by a USB communication port compatible with computer. The main characteristics of the photon source, Coincidence-to-Accidental-Ratio (CAR), brightness, bi-photon spectrum, heralded efficiency, purity, and indistinguishability are measured by using integrated optical benches and a 2-channels very-low-noise InGaAs single photon avalanche counters (G-SPAD). The proposed photon source measurements protocol is also available for the characterization of other photon sources technologies, such as for the Quantum Dots and semiconductor photon sources. We performed two experimental Bell inequality violations by creating polarization entanglement or by using the natural frequency entanglement of our source. Violation by more than five standard deviations Bell inequalities with our setups demonstrate that our photon source is a promising tool for the realization of various distances quantum information experiments.

In this communication, we report on the design, fabrication, and testing of silicon-on-insulator (SOI) and silicon-nitrideon- insulator (SiNOI) photonic circuits for nonlinear and quantum optics applications. As recently demonstrated, the generation of correlated photons on Si platforms can be used for quantum cryptography and quantum computing. Concerning SiNOI waveguides, Kerr frequency combs have been proposed in many applications, such as atomic clocks, on-chip spectroscopy, and terabit coherent communications. Silicon is an attractive platforms for correlated photons sources because of its high nonlinearity, they can have several modes in telecom band with sharp line widths (tens of μeV) and its inherent complementary metal-oxide-semiconductor (CMOS) compatibility. Moreover, the SiNOI is an attractive platform for Kerr comb generation due to their large bandgap and consequently the low two-photon absorption in the telecommunication band. Furthermore, in all the previous SiNOI-based frequency combs, the silicon nitride film undergoes long and high-temperature annealing to reduce the absorption in the telecommunication band caused by the dangling N-H bonds, thus making such annealed Si₃N₄ films non-CMOS compatible. However, both in the case of correlated photons pairs generation and Kerr frequency combs, the source efficiency is related to the quality factor (Q), so that a high-Q resonator is required to get highly-efficient sources. Authors report here about the fabrication and the characterization of annealing-free CMOS-compatible SiNOI- and hydrogen-annealed silicon-based waveguides and microresonators featuring ultra-low losses (e.g., 0.6 dB/cm for single-mode Si waveguides) that can be used, respectively, as efficient sources for Kerr combs and correlated photon pairs sources.

For over a decade the field of quantum photonics has increasingly looked towards optical integrated platforms to perform more complex and sophisticated experiments. Silica integrated optics is an ideal material for this area, offering low propagation and fibre-coupling losses. To date many of the key on-chip experiments have been carried out in this platform, using bespoke monolithic devices. In this work we propose an alternative approach, implementing a linear network constructed from a number of identical reconfigurable modules. The modules are measured separately to produce an accurate model of the overall network. The cellular nature also allows the replacement of modules that are faulty or substandard. Each module comprises of an array of 10 Mach-Zhender interferometers. Forty thermo-optic phase shifters on each chip allows the control of both the amplitude and phase of the optical field within the devices. By cascading the modules any arbitrary NxN unitary network can be realised. The optical waveguides within the modules are fabricated by direct UV writing, where a scanning focused UV laser beam increases the local refractive index within a photosensitive germanosilicate glass layer. The resulting channel waveguides are engineered to have dimensions that are mode matched to standard optical fibre producing excellent coupling efficiency. Bragg gratings can also be simultaneously produced within the waveguides which greatly assists in the precise characterisation of the phase shifters, coupling ratios and optical losses within the modules. We will present our recent work in this area, demonstrating devices operating at telecom wavelengths for quantum information processing. We present a modular reconfigurable system for on-chip quantum optics experiments with excellent fibre compatibility and low propagation losses, implemented using direct-UV-written silica-on-silicon. The performance of fabricated devices in various configurations is reported.

Trapped-ion qubits promise certain fundamental advantages for quantum information processing (QIP), owing to their indistinguishability and relatively high isolation from noisy environments. Though these qualities have allowed demonstrations of the necessary primitives for quantum computation, the complexity of the optical apparatus required is a major impediment to implementation at scales where quantum systems offer a clear advantage over classical computers. Here, we build on previous work with trap-integrated waveguide optics, describing designs and simulations for commercial foundry-fabricated ion trap chips with integrated Si3N4 waveguides and grating couplers to implement multi-qubit operations. We detail a design intended to address and implement quantum logic gates between 5 ions in a single register, and a configuration which utilizes the stable on-chip path lengths of waveguide devices to enact a novel fast entangling two-qubit gate. The devices and approaches presented here could form elements of a scalable architecture for trapped-ion QIP.

To resolve the intrinsic trade-off between the deterministic nature and scalability of controlled phase gate implementation in the linear optics regime, we propose a novel deterministic two-photon controlled phase gate scheme in chiral quantum nanophotonics. The essential ingredient here is that, a non-linear π-phase shift is imprinted during the photonic molecule generations. On one hand, the gate operates in a deterministic way without the assistance of probabilistic ancilla qubit measurement. On the other hand, the gate implementation is of low complexity to be highly scalable. To date, the chiral coupling is readily underpinned by the advent of directional emission technique. Potential implementation platforms include QD, superconducting qubit, Rydberg atom, or N-V center that is coupled to a PhC waveguide, transmission line, or optical fiber.

In this presentation, we will introduce the quantum equivalent of white-light interferometry. However, instead of a classical light source with thermal or poissonian statistics, we use energy-time entangled photon pairs. This provides one with several important advantages, which are highly relevant for high-accuracy measurements of chromatic dispersion: 1.) The interferometer does not have to be balanced, which improves the set-up time, especially when comparing different fibres. 2.) Two data fitting parameters are cancelling out, which reduces systematic errors significantly. 3.) The wavelength at which dispersion is measured is not extracted from the data, but is rather inferred with arbitrary precision using a wavemeter or an atomic reference. 4.) Twice as many interference fringes are observed for the same spectral bandwidth, allowing to measure dispersion in standard telecom fibres down to about 3 cm with a 140 nm bandwidth source. After introducing the concept and highlighting the quantum advantages, we will demonstrate the performance of the quantum approach by comparing it to the best state-of-the-art approaches. Statistical analysis is performed by 2 times 100 measurements using either technique. In terms of precision, the quantum (classical) approach achieves a 1-sigmal precision of 21 fs/nm/km (51 fs/nm/km). In addition, the classical approach presents a systematic error of 12 fs/nm/km, which is unlikely to occur using the quantum approach, as the related fitting parameters cancel out automatically. In summary, we believe that combining fundamental and conceptual advantages enabled by quantum optics is a promising approach for the future development of applications requiring precise and accurate measurements.

In this work, we demonstrate two developments, in device and material engineering respectively, towards an efficient spin-photon device using colour centres in diamond for scalable quantum computing networks: firstly, we report the emission enhancement of the coherent zero-phonon-line transistion of an nitrogen vacancy centre in a diamond membrane on coupling to a tunable open-cavity; secondly, we present a new method for deterministic writing of NV arrays in bulk diamond with a 96% yield of single defects, a 50 nm positional accuracy in the image plane, and NV electron spin coherence (T2) times of up to 170 microseconds.

The low-latency requirements of a loophole-free Bell test prohibit time-consuming post-processing steps that are often used to improve the statistical quality of a physical random number generator (RNG). Here we demonstrate a postprocessing-free RNG that produces a random bit within 2.4(2) ns of an input trigger. We use the Allan variance as a tool for characterizing non-idealities in the RNG and designing a feedback mechanism to account for and correct long-term drift. The impact of the feedback on the predictability of the output is less than 6.4 × 10⁷ , and results in a system capable of 24 hour operation with output that is statistically indistinguishable from a balanced Bernoulli process.

The performance of a continuous-variable quantum key distribution (CVQKD) protocol depends on the efficiency of the post-processing of measurement results. The post-processing methods extract statistical information from the raw data, establish the mutual knowledge between the parties, and produce a final key that provides absolute security. Here we define an optimization method for post-processing in continuous-variable quantum key distribution. The optimization framework allows significant performance improvements in the post-processing phase compared to other existing reconciliation approaches.

Quantum communication networks are formed of secure links, where information can be transmitted with security guaranteed by the quantum nature of light. An essential building block of such a network is a source of single photons and entangled photon pairs, compatible with the low-loss fibre telecom window around 1550 nm. Previous work based on semiconductor quantum dots (QDs), colour centres in diamond and single atoms has been limited by emission wavelengths unsuitable for long distance applications. Efforts have been made to use standard gallium arsenide based QDs by extending their operating wavelength range, however, electrically driven quantum light emission from quantum dots in this telecommunication window has not yet been demonstrated. In this work, indium phosphide based QD devices have been developed to address this problem. The industry favoured growth method, metalorganic vapour phase epitaxy (MOVPE), has been used to create droplet QDs with low fine structure splitting (FSS). This growth scheme allows us to produce the first optoelectronic devices for single and entangled photon emission in the 1550 nm telecom window. We show single-photon emission with multi-photon events suppressed to 0.11±0.02. Furthermore, we obtain entangled light from the biexciton cascade with a maximum fidelity of 0.87± 0.04 which is sufficient for error correction protocols. We also show extended device operating temperature up to 93 K, allowing operation with liquid nitrogen or simple closed-cycle coolers. Our device can be directly integrated with existing long distance quantum communication, cryptography and quantum relay systems providing a new platform for developing quantum networks.

In classical digital communications two main families of “M-ary” modulation schemes are generally distinguished: bandwidth limited and power limited. The canonical comparison of these modulation methods is based on normalized data rate (R/W) (bits per second per hertz of bandwidth) and the signal to noise rate per bit required to achieve a given error probability for different M. In a classical picture, the two families reside in two separate semi-plains R/W < 1 and R/W > 1, i.e. energy efficiency and bandwidth efficiency cannot be optimized at the same time. However, we find an alphabet family that can be paired with a quantum receiver to simultaneously optimize bandwidth and power efficiency of a communication channel. Particularly we found that coherent frequency shift keying (CFSK) gives rise to a family of communication protocols that are bandwidth limited in nature, but whose bandwidth usage can be optimized so that R/W>1 for a range of alphabet lengths M, while power sensitivity beats that of power-limited protocols. We will report our theoretical findings and experimental progress towards implementation of this protocol family.

Secure free-space data links can be established using conventional laser communication technology or, if necessary, they can be further enhanced with quantum encryption. Security features of these systems are based on the protocols that make use of the inherent properties of laser light. In this case, encryption does not rely on complex mathematical algorithms that add overhead to the communication stream, but instead it is based on physical-layer processes in the laser sources and other modulating components. One promising approach is based on polarization entanglement between correlated photon pairs to achieve data encryption in quantum communication systems. The foundation of security lies in the response of photons to polarization measurements. Additional degrees of freedom can be added to each “singleparticle” state by using hyper-entanglement. The situation can be visualized when several carrier waves are assigned specific frequencies in the 100 GHz International Telecommunication Union (ITU) grid. The two technologies that can be eventually integrated to achieve this task include hyperspectral quantum circuits and the entangled pair source and detection systems. This results in frequency/polarization hyper-entanglement, which can be processed with additional wavelength-division multiplexing (WDM) components to achieve efficient separation of the signals. It is important to understand that most of the previous work is theoretical and assumes ideal properties of all optical parts. In reality, many non-ideal features of the quantum circuits and their components can change the way the quantum states are processed, and this constitutes the main focus of our paper.

The COST Action MP1403 “Nanoscale Quantum Optics” (NQO) has recently released a NQO Roadmap identifying research priorities that address both classical and quantum schemes in information and communication technology, sensing and metrology, and energy efficiency. Based on the Roadmap, the COST Action has been working on a Market Research Study (MRS) to assess the potential of NQO for selected applications and markets in a more quantitative manner. The MRS has been carried out with the cooperation of Tematys and it has been focused on two areas: (i) quantum sensing, imaging and measurement systems: achieve unprecedented sensitivity, accuracy and resolution in measurement and imaging by coherently manipulating quantum objects and (ii) quantum communications: guarantee secure data transmission and long-term security for the information society by using quantum resources for communication protocols. The MRS has primarily addressed near-term technologies, like quantum random number generators (QRNG) for secure key or token generation and point-to-point quantum key distribution (QKD) for secure key exchange in crypto systems. Mid/long-term technologies like QKD networks, quantum memories and repeaters have not been assessed. Likewise for quantum sensing, imaging and metrology the focus has been on near-term technologies like magnetic resonance imaging (MRI), magnetic/electric field detection for materials analysis and biosensing, precision metrology and gravimetric sensors for civil and defense applications. The MRS has also taken into account quantum enabling technologies, which are fundamental components for the construction of quantum-photonics systems.

Optical quantum memories are important components in the long-distance quantum communication based on quantum repeater protocol. To outperform the direct transmission of light with quantum repeaters, it is crucial to develop quantum memories with high fidelity, high efficiency and long storage time. Recently, we demonstrate that it is feasible to achieve a high storage efficiency of 92% for electromagnetically-induced-transparency (EIT)-based memory with weak coherent signal pulses in cold atomic ensembles [1]. To realize the highly-efficient memory with quantum light, we have built a bright and narrowband photon-pair source which can be locked to atomic transition based on the cavity-enhanced spontaneous parametric down conversion [2]. Here, we present our results on the storage of single photons generated by such a source in EIT-based memories. A storage efficiency of 36% is obtained in initial runs. Future improvements toward a high efficiency are discussed. Such a development paves the way for the applications of photon-pair-based quantum repeater and multi-photon synchronization.

We present a multiplexed quantum repeater protocol based on an ensemble of laser-cooled and trapped rubidium atoms inside an optical ring cavity. We have already demonstrated strong collective coupling in such a system and have constructed a multiplexing apparatus based on a two-dimensional acousto-optical deflector. Here, we show how this system could enable a multiplexed quantum repeater using collective excitations with non-trivial spatial phase profiles (spinwaves). Calculated entanglement generation rates over long distances reveal that such a multiplexed ensemble-cavity platform is a promising route towards long distance quantum entanglement and networking.

Optical quantum memories will enable technologies including long distance quantum communication and modular quantum computing. Rare earth ion doped crystals provide an excellent solid state platform for optical quantum memories. Among rare earths, erbium is particularly appealing due to its long-lived telecom-wavelength resonance, allowing integration with silicon photonics and with existing optical communication technology and infrastructure. We present an on-chip all-optical quantum memory at telecom wavelengths using a nanobeam photonic crystal cavity fabricated directly in erbium-167 doped yttrium orthosilicate. Using an atomic frequency comb protocol, we store coherent pulses for memory times as long as 10 µs, albeit with low efficiency. For shorter memory times, we achieve a memory efficiency of 0.4%, which is limited by the coupling rate between the resonator and the ensemble of ions. By working at dilution refrigerator temperatures, we are able to access a regime where the ions have long optical coherence times and good spectral holeburning properties using only a moderate magnetic field applied with permanent magnets. We characterize the multimode properties and fidelity of the quantum memory in this device, and outline a path toward higher efficiency.

Efficient and long-lived multimode quantum memories are crucial devices in the development of quantum technolgies. The reversible mapping of quantum states of light in rare earth doped crystals represents one of the most promising routes towards the realization of this goal. Such systems are also compatible with the miniaturization of quantum memories in integrated optics platforms, which offer unique features in terms of experimental scalability and enhanced light-matter interaction. Here, we fabricate single mode channel waveguides for 606 nm light in a praseodymium-doped yttrium orthosilicate crystal (Pr3+:Y2SiO5), that, thanks to its excellent coherence properties, is a widely studied material for light storage experiments. Waveguides are inscribed by femtosecond laser writing, adopting the so-called Type I configuration, where the core is directly obtained at the irradiated area. Remarkably, fabricating this kind of waveguides in crystals is a difficult task, as it requires to operate in a very narrow processing parameters window, if existing. We then use these waveguides for performing the storage and retrieval of single photons, implementing the atomic frequency comb protocol. We achieve a storage time of 5,5 µs, which is almost 2 orders of magnitude longer than previous realizations of quantum light storage in a waveguide. In addition, we investigate the potential information multiplexing capabilities of our system by performing the quantum storage of single photons delocalized over 14 different spectral modes. Our results show that laser written waveguides in rare earth-doped solid state systems are very promising for the development of efficient and long-lived multimode quantum memories.

Very recently, the interest for quantum technologies by the scientific community and industry has strongly increased. Different types of implementations have been proposed as a practical implementation for a quantum bit. In particular, quantum photonics is a strong candidate for such applications. We are interested in using single photons and single spins in diamond as a solid state host matrix (nanodiamonds or membranes). Integration of nanosources of light is currently a major bottleneck preventing the realisation of all-photonic chips for quantum technologies and nanophotonics applications. Ideally, one needs optical circuitry, on-chip photodetection and on-chip generation of quantum states of light (single photons, entangled photons…). Our recent work on a new platform for quantum photonics using integrated optics can offer an easier and robust way to create compact quantum circuits that can be on chip and scalable. In this context, the coupling between waveguides and single photon emitters is critical. The goal of our research is to efficiently couple single photon emitters with a new platform made of optical glass waveguides. These waveguides are based on the so-called ion-exchange glass technology and is know in the photonics industry for quite some time but has never been used in the context of quantum technologies. Efficient light-matter interface is of primordial importance in this system. To achieve this goal, several paths are undertaken such as the use of dielectric and plasmonic structuration in order to increase the light interaction with the waveguide or to develop fabrication techniques to insert the emitters directly inside the guide (for nanodiamonds). We will show what is our current state of the art for placing single emitters at the right place on our optical waveguides made of ion-exchange in glass and in particular what can be done to improve our first promising results in order to get near unity coupling between the optical bus and single photon emitters. We will show first results with semiconductor nanocrystals (NCs) but also using nitrogen-vacancy and silicon-vacancy defects in nanodiamond. The very first step in of our approach consists in the design of the structured waveguide using electromagnetic FDTD. We demonstrated that it is possible to achieve more than 90% coupling. In practice, before using coloured centres in diamond, we started working with CdSe/ZnS semiconductor nanocrystals. So far, we use straight waveguides defined to be single modes at the nominal wavelength of the emission line of the nanoemitters. The positioning of nanoemitters is still a challenge to be achieved. We developed an original technique based on photopolymerisation of light where the nanocrystals are grafted into a light sensitive polymer and can be placed at adequate positions.

Topological edge states draw their unique robustness against perturbations from a topological invariant of the bulk of the system. As long as the topological properties persist, the edge transport is not perturbed by static defects, which is referred to as the bulk-edge correspondence. In our work we demonstrate that local periodic perturbations of the interface can destroy the topological protection even if the bulk of the system stays unperturbed. As model system we consider the Su-Schrieffer-Heeger (SSH) model realized in coupled plasmonic waveguide arrays with alternating short and long separations. Interfacing two SSH chains with different dimerizations we induce the topological edge mode. The temporal perturbations are realized by periodically bending the waveguide at the interface. The spatial evolution of surface plasmon polaritons (SPPs) in the array is monitored by real- and Fourier space leakage radiation microscopy. In Fourier space we observe that time-periodic perturbations of the interface create Floquet replicas of the topological edge mode. If the driving frequency is in the range for which the first Floquet replicas cross the static bands, the topological edge state couples to bulk states and the topological protection is destroyed resulting in delocalization of SPPs in real space. Otherwise the topological protection is conserved and SPPs stay localized at the interface. Our experimental findings are in full agreement with the theoretical analysis based on Floquet theory and illuminates the generalization of the bulk-edge correspondence for Floquet systems for the special case of a static bulk.

The efficient transfer of a quantum state from photons to matter qubits in order to momentarily store information has become a central problem in quantum information processing. A quantum memory turns out to be an essential tool to achieve advanced technologies such as quantum networks, quantum repeaters, deterministic single photon sources or linear optics quantum computers. The realization of a quantum interface has been investigated in various forms, among which one can find solid-state atomic ensembles, color centers in crystal lattices, cold atomic gases, optical phonons in diamond and many others. Here we focus on a broadband quantum interface for high repetition rate (76 MHz) ultrafast optical pulses (250 fs) at telecommunication wavelength (1530 nm) based on the photon echo process occurring in semiconductor quantum dots. We evaluated the quantum state of photonic qubits in order to characterize the impact of the storage on the transmitted signal. Homodyne traces corresponding to projections of the Wigner function of the signal on rotated quadrature components were obtained using broadband balanced homodyne detection, i.e. mixing the ultrafast optical pulses to analyze with a high repetition rate pulsed local oscillator. The reconstruction of the Wigner function from the homodyne traces was performed using three algorithms: the inverse Radon transform, the minimax adaptive reconstruction and the maximum likelihood estimation. The three methods lead to similar results, concluding that for an input pulse in a coherent state, the reemitted photon echo is also in a coherent state.

The simultaneous production of a set of arbitrary hyperentangled states is crucial for quantum machines running variant quantum protocols concurrently, like universal quantum computers and quantum communication hubs. We present an experimental method to prepare a set of arbitrary path-polarization hyperentangled states concurrently using non-collinear spontaneous parametric down-conversion (SPDC). A cascaded pair of type-I crystals are pumped by 405-nm diagonally-polarized beam to produce a noncollinear stream of photon pairs. Compensation crystals are inserted to correct for the angular slope of the relative phase of the produced polarization entangled state. The pathentangled states are created over four pairs of slits positioned at conjugate locations to the pump beam. The local relative-phases of the path and polarization states can be independently tailored by intercepting the SPDC emission by tiltable birefringent and glass plates. The amplitudes of the polarization states is controlled by the manipulation of the polarization state of the pump. Also, the amplitudes of the path states is accessed by translating the slits over the SPDC cone. Here, while a pure state describes the whole SPDC emission, the produced states can be deemed an independent set by avoiding paths balanced to less than the coherence-length.

In this paper, we present results of numerical investigation of quantum walks (QWs) in different type arrays of single-mode waveguides: (i) regular arrays of identical waveguides (AIW), (ii) periodic arrays of two different waveguides (PAW), (iii) quasi-periodic arrays of waveguides with Fibonacci sequences (FAW), (iv) quasi-periodic arrays of waveguides with Thue-Morse sequences (TMAW). Our simulations show in contrast with randomly disordered systems, localized QWs in quasi-periodic arrays of waveguides FAWs and TMAWs are predictable and controllable due to the deterministic disorder nature of the quasi-periodic systems.

To enhance the spatial utility of typical two-port Hadamard gate, we propose a novel single-channel scheme through frequency conversion in Λ-atom-mediated single-photon Raman scattering process in chiral quantum nanophotonics. We demonstrate faithful and efficient gate operations, and quantitatively analyze gate performance. Moreover, by manipulating photon-emitter coupling and frequency detuning, it is confirmed that an arbitrary unitary single-qubit operations are achieved in the presented configuration, including most representative single-qubit Hadamard, X, Z, S, and T gates. Due to the advent of directional emission techniques, the chiral condition is readily experimentally feasible. In addition, we further propose an alternative N-type atom architecture to perform Hadamard operations that are enabled by hyperfine structure in the strong-coupling regime.