This PDF file contains the front matter associated with SPIE Proceedings Volume10547, including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

Brain diseases such as autism and Alzheimer’s disease (each inflicting >1% of the world population) involves a large network of genes displaying subtle changes in their expression. Abnormalities in intraneuronal transport have been linked to genetic risk factors found in patients, suggesting the relevance of measuring this key biological process. However, current techniques are not sensitive enough to detect minor abnormalities. Here, we report a sensitive method to measure changes in intraneuronal transport induced by brain disease-related genetic risk factors. It relies on the spontaneous internalization of optically active photostable nanoparticles by neurons in endosomes, and the subsequent tracking of their motion within neuronal branches through their optical labeling. For two-dimensional primary neuron cultures, we used fluorescent nanodiamonds, which provide high brightness, photostability and absence of cytotoxicity. For thicker samples (3D-culture, brain slices…) we took advantage of nonlinear properties (second-harmonic generation) of silicon carbide nanoparticles which allows tracking at depth of ≈100 µm thanks to laser excitation in the near-infrared transparency window of tissues. This technique revealed abnormal intraneuronal transport in transgenic mouse models of brain diseases.

The delivery of MOF-enzyme nanofactories (50-100 nm in diameter) into the cytosol of cells will be presented. Delivery is achieved by co-incubation with the peptide dfTAT, a dimeric construct of the HIV TAT peptide. Cytosolic targeting is highly efficient and dfTAT circumvents the endosomal entrapment of macromolecular cargos. The delivery protocol does not require interactions between dfTAT and nanoparticles, thereby eliminating complex labeling steps. Cytosolic access is rapid and cells recover from the delivery process in a matter of minutes. Overall, this approach should facilitate the monitoring of intracellular processes by nanoparticle probes.

Fluorescent nanodiamonds (FNDs) are emerging as a novel fluorescent probe due to their stable fluorescence showing no photobleaching, negligible toxicity, chemical inertness, and optically detected magnetic resonance property. Recently, several unique applications such as multimodal imaging, single particle tracking, long-term cell tracking have been reported. However, the use of FNDs for those applications is still limited because well-optimized surface coating methods have not been established so far. In this presentation, we report a novel surface coating method based on lipid coating and subsequent photo-initiated radical polymerization. FNDs are first coated with lipids containing diacetylene in the alkyl chain, and subsequently crosslinked by UV irradiation to enhance the stability of the FND-lipid hybrids (FND-PCL: Photo-crosslinked lipid-coated FND). The method does not require complex organic chemistry techniques, and the produced coating layer is thin (ca. 2-3 nm) but highly stable thanks to the covalent bonding. The FND-PCL, as well as the biotin-functionalized FND-PCL, can be prepared in a couple of hours without much difficulty. The FND-PCL has many favorable properties for bioapplications such as strong aggregation resistance and reduced nonspecific adsorption by biomolecules. We further demonstrated two improvements on FND-PCL: enhancement of the production yield and facile surface modification. The coating method enhances the unique fluorescence properties of FNDs and opens interesting pathways of novel bioimaging.

Fluorescent nanodiamonds have proven useful as nontoxic probes for optical tracking of subcellular processes. Here, we demonstrate that a resonant magnetic field can be used to transfer spin polarization from intrinsic defects at nanodiamond surfaces into surrounding solution, enabling imaging of nanodiamond via MRI. In combination with fluorescence techniques, this new MRI modality promises to enable nanodiamond as a bioprobe for noninvasive imaging over subcellular to whole body lengthscales.

Single-molecule magnetic resonance spectroscopy and imaging is one of the ultimate goals in magnetic resonance and will has great applications in a broad range of scientific areas, from life science to physics and chemistry. To achieve this goal, the spin of a single nitrogen vacancy (NV) center in diamond is proposed as a highly sensitive magnetic-field sensor. Our research keeps on the line from one-dimension spectroscopy to two-dimension spectroscopy at single molecule scale. We and co-workers have successfully done a series of work on one-dimension spectroscopy [1-4]. However, measurement of the magnetic dipole-dipole couplings of a molecule is a missing key step from one-dimension to two-dimension magnetic resonance spectroscopy at nanoscale. Through correlation methods, we succeed to detect the intra-molecule magnetic dipole interaction of (5 nanometer)3 volume samples. In the future, magnetic coupling of a single molecule could be achieved if the coherence property of shallow NV sensor is improved [5]. References: [1] Fazhan Shi, Qi Zhang, Pengfei Wang, Hongbin Sun, Jiarong Wang, Xing Rong, Ming Chen, Chenyong Ju, Friedemann Reinhard, Hongwei Chen, Joerg Wrachtrup, Junfeng Wang, and Jiangfeng Du. Single-protein spin resonance spectroscopy under ambient conditions, Science, 347, 1135 (2015) [2] C. Mueller, X. Kong, J. M. Cai, K. Melentijevic, A. Stacey, M. Markham, J. Isoya, S. Pezzagna, J. Meijer, J. F. Du, M. B. Plenio, B. Naydenov, L. P. McGuinness, and F. Jelezko. Nuclear magnetic resonance with single spin sensitivity. Nature Communications, 5:4703 (2014) [3] Tobias Staudacher, Fazhan Shi, S. Pezzagna, Jan Meijer, Jiangfeng Du, Carlos A. Meriles, Friedemann Reinhard, Joerg Wrachtrup. Nuclear magnetic resonance spectroscopy on a (5nm)3 volume of liquid and solid samples, Science, 339, 561 (2013) [4] Fazhan Shi, Qi Zhang, Boris Naydenov, Fedor Jelezko, Jiangfeng Du, Friedemann Reinhard, and Joerg Wrachtrup. Quantum logic readout and cooling of a single dark electron spin, Phys. Rev. B, 87, 195414 (2013) [5] Fazhan Shi, Xi Kong, Pengfei Wang, Fei Kong, Nan Zhao, Renbao Liu, and Jiangfeng Du. Sensing and atomic-scale structure analysis of single nuclear spin clusters in diamond, Nature Physics, 10, 21 (2014)

Cellular sensing using nanodiamond is an emerging field, giving access to monitoring of chemical reactions in biological environment at nanoscale. Currently used nanodiamonds are prepared by irradiation of N-containing milled HPHT synthetic diamond and subsequent annealing. These nanodiamond suffer form irregular sizes and low spin coherence time which makes the detection in cells very challenging. We come with novel method for nanodiamond fabrication by plasma enhanced CVD, which allows to control the crystal shape and size and spin characteristics. We discuss application of optimized nanodiamond for cellular sensing

Photonic entanglement is a fundamental resource in quantum information processing and its distribution between distant parties is a key challenge in quantum communications. Optical satellite links allow transmitting entangled photons over longer distances than currently possible on ground and could provide a path towards global quantum communication networks. The majority of current photonic free-space quantum communication systems, including two recent quantum satellite experiments, use two-dimensional polarization encoding, where each photon can carry at most a single bit of quantum information. Encoding additional qubits in the high-dimensional spatio-temporal degrees of freedom could not only increase channel capacities but also improve robustness with respect to noise and eavesdropping in quantum cryptography. Here, we give an overview of our efforts towards exploiting high-dimensional entanglement in long-distance free-space quantum communications. We report on the results of a first feasibility study, in which we used polarization / energy-time hyperentanglement to increase the dimensionality of the state space and transmit genuine 4-dimensional entanglement via an intra-city free-space link. We discuss how we intend to extend this approach to applications in noise-tolerant large-alphabet quantum key distribution via free-space links, and, ultimately, experiments with satellites.

Quantum networks provide a versatile infrastructure for communication, computing, and sensing with quantum information. Novel sources and detectors for transmitting and receiving quantum states are critical elements in the development and eventual deployment of robust quantum networks. Alongside performance, the compatibility of quantum network devices with modern networking infrastructure is an important requirement for deployment. We present results on the integration of quantum communication using superdense coding transmitted over optical fiber links into network environments. Our approach takes advantage of a novel complete Bell-state measurement setup that relies on hyper-entanglement in the temporal and polarization degrees of freedom for a two-photon state emitted from a quantum light source. Using linear optics and common single-photon detectors, we record a single-qubit channel capacity of 1.665±0.018. We then demonstrate a full experimental implementation of hybrid, quantum-classical communication protocol for image transfer applications. Our devices integrate with existing fiber optical network and software-defined transmitters and receivers as part of a modular design to provide an extensible quantum communication system that can adapt to future quantum technology goals.

Entangled and hyper-entangled states of light are valuable tools of quantum information protocols. Here, we discuss entanglement generation in quantum dot systems and its extension to hyper entanglement. We review the current results and give a perspective for possible improvement.

Quantum entanglement is a fundamental resource for secure information processing and communications. The canonical optical quantum information processing encodes a single qubit per photon, with remarkable demonstrations of secure quantum communications, linear optical quantum computing amongst others. Often the polarization qubit, a discrete variable with entangled Bell states, is used. Continuous variables such as energy-time modes and spatial modes, however, present a dramatically larger Hilbert space for quantum information processing. The high-dimensional entanglement capture more qubits per photon, enabling dramatically higher secure key rates over longer distances and with better error resilience. Here I will describe our results on high-dimensional entanglement in quantum frequency combs for dense quantum communications. We first demonstrate revival of the Hong-Ou-Mandel interferences, long postulated by theorists more than a decade ago, up to 19-dimensions and with visibilities up to 96.5%. The mode-locked two-photon state in high-dimensions is further witnessed through a stabilized Franson interferometer, as a generalized Bell’s inequality test and hyperentangled through multiple degrees-of-freedom. Entanglement revivals of the non-local interference at discrete time-bins are uncovered, up to 97.8% visibility, as a fundamental resource for dense secure information processing.

Superdense Teleportation (SDT) is a suitable protocol to choose for an advanced demonstration of quantum communication in space. We have taken further steps towards the realization of SDT in such an endeavor. Our system uses polarization and time-bin hyperentanglement via non-degenerate spontaneous parametric downconversion to implement SDT of 4-dimensional equimodular states. Previously, we have shown high fidelity (>90%) SDT implementation and the feasibility to perform SDT on an orbiting platform by correcting the Doppler shift. Here we discuss new analysis of the received state reconstruction performance in the presence of high channel loss and multiple pair events. Additionally, initial characterization of a waveguide-based entanglement source intended for space will be presented.

Advanced data links (ADL) can be implemented using free-space optical channels with quantum encryption. The security features of these links are based on quantum communication protocols that rely upon the inherent properties of laser light and offer physical-layer encryption without the mathematical complexity of conventional cryptography. Traditionally, these links are organized using the point-to-point topology, but it is highly desirable to extend this approach to multi-access communication systems to service an array of ADLs connected into a hub-and-spokes network. To facilitate connectivity, the "hub" can use a single aperture to establish connections to several target platforms within its field-of-view. Multi-access high-capacity wavelength division multiplexing can be used as an enabling technology for establishing communication between hub and spatially separated spokes. The associated problem of discriminating between multiple signals arriving simultaneously from distinct spatial locations is addressed by optical circuits capable of processing non-classical features of quantum states, such as entangled photon. One of the key elements of this receiving systems is the Lyot filter that offers high selectivity while directing the signals along different optical path lengths while propagating through the quantum circuit components, which may affect their processing capabilities. In this paper, we present analysis and preliminary experimental results that demonstrate the effects of off-axis arrival on performance of the Lyot filter and the resulting limitations of this technology.

The efficient use of communication channels motivates extensive research in novel communication protocols. Modern communication protocols use large alphabets contain up to a few thousand symbols, thus optimizing the use of power and available frequency space. To date, quantum receivers that discriminate up to approximately 20 symbols were theoretically investigated and receivers with as many as 4 nonorthogonal coherent states have been experimentally demonstrated. However, all heretofore explored quantum receivers suffer from the sensitivity degradation with the alphabet size. Particularly, their Helstrom Bound (HB) nearly reaches the classical standard quantum limit (SQL). Here we introduce an M-ary quantum receiver based on a coherent frequency shift keying (CFSK) protocol with a record power sensitivity, free of the above deficiency. The CFSK not only provides better accuracy for longer alphabets but also allows discrimination with a practically attainable symbol error rate (SER) situated much below the HBs of other encodings for large alphabets. Our receiver operates with a classical transmitter, and with any communication channel, including the existing global fiber network. It can be used to increase the amplification-free range in a network and/or reduce power requirements on the transmitter by more than 1000 times. In addition, the quantum measurement advantage can significantly optimize the use of the frequency space in comparison to classical frequency keying protocols. This advantage can be used in deep-space telecom links to enhance the satellite power budget. In existing fiber network links, quantum CFSK receivers can improve the amplification-free range by approximately the factor of 2.

Monolithic photonics architectures which enable the generation and processing of quantum states of light will be discussed. Some architectures are shown to exhibit characteristics that are unique to integrated architectures over their bulk-optics counterparts. If we use on-chip quantum interference as an example, one find it is often assumed that the 2x2 mode coupler maintains a 50:50 splitting ratio over the twin-photons’ entire joint spectrum. However, this is not necessarily the only case possible for integrated devices. For some designs the interferometer behaves as an ideal 50:50 beamsplitter (BS), while for others it behaves as an ideal wavelength de-multiplexer (WD). This interesting ramifications for two-photon interference, where dispersion can allow integrated 2x2 couplers to play a far more versatile role in quantum circuits than their bulk-optics counterparts.

We present results from the development of a fiber-coupled Acousto-Optic modulator (Fiber-Q®) operating at near-UV and blue wavelengths.

A conventional TeO₂ based Bragg diffraction design is introduced for short wavelengths optical input. Pure silica core single mode fibers (both Polarisation Maintaining (PM) and non-PM) are used as coupling fibers for their transmission at these wavelengths, and to avoid the possibility of photo darkening. The end of the fibers are fused with silica end-caps, lowering the power density on their fiber-air interfaces to achieve a higher power handling. The Fiber-Q® can be optimized for multiple wavelengths (including 397nm or 422nm) and can accept power levels of up to 100mW. A hermetically sealed package is selected to provide a clean in-package environment thus protecting the optics from damage caused by external contamination.

In this presentation we report details of the design, and test results of a fiber-coupled Acousto-Optic modulator that demonstrates the performance required for use in ion-trap quantum information processing applications.

Free-space optical communication channels offer unsurpassed advantages over traditional radio frequency systems. They can be implemented in a point-to-point topology and, if necessary, augmented with additional security features for privacy-critical applications. The adoption of quantum mechanical principles in optical communication networks became a foundation for quantum communication protocols that offer added security without the mathematical complexity of traditional cryptography. Encryption can be achieved at the physical layer by using quantized values of photons, which makes exploitation of such communication links extremely difficult. Additional challenges are encountered when the focus is shifted from point-to-point links to multi-access communication systems arranged in a hub-and-spokes topology. To facilitate connectivity, the “hub” can use a single aperture to establish connection to a target platform within its fieldof- view. Polarization entanglement is proposed for data encoding, and additional degrees of freedom can be achieved for each quantum state by using hyper-entanglement. For example, if carrier waves arriving at the same time from multiple transmitters (spokes) are assigned specific frequencies in the 200 GHz ITU grid, their messages can be processed simultaneously by a receiver (hub) that uses a hyperspectral quantum optical circuit. In this paper, we present analysis of the components in the optical trains and propagation of hyper-entangled states in the quantum circuits. The results obtained in this project can be used as the first step before physical implementation of the quantum communication systems.

The ability to synchronize remote clocks plays an increasingly important role in our infrastructure, from maintaining coherence in the electrical grid to allowing precise positioning and navigation for civilian and military applications. However, many of the techniques to establish and maintain this time synchronization have been shown to be susceptible to interference by malicious parties. Here we propose a protocol that builds on techniques from quantum communication to provide a verified and secure time synchronization protocol. In contrast with classical protocols aimed at increasing the security of time distribution, we need not make any assumptions about the distance or propagation times between the clocks. In order to compromise the security of the protocol, an adversary must be able to perform quantum non-demolition measurements of the presence of a singe photon with high probability. The requirement of such quantum measurements raises a serious technological barrier for any would-be adversary

Quantum interconnects allow disparate quantum systems to be entangled, leading to more powerful integrated quantum technology and increases in scalability. The foundation for such technology, including photonic quantum memories and coherent microwave-to-optical (M2O) transducers, have already been developed in rare-earth ion (REI) crystals. Here we demonstrate improved REI quantum device functionality in an on-chip platform that dramatically strengthens the ions’ interactions with optical fields and integrates with planar microwave technology. Using a photonic crystal nanobeam fabricated in a Nd-doped yttrium vanadate (YVO) crystal, we harness the enhanced ion-photon interactions that create single photon Rabi frequencies as large as 60 MHz. In particular, the large AC Stark shift is used to control an ensemble of approximately 4000 ions for photonic quantum memory applications. We demonstrate AC Stark shift control of the storage time in the atomic frequency comb protocol as well as the possibility of memories based on an all-optical variation of the hybrid photon echo rephasing protocol. The spin state of the REIs can also be addressed directly through the integration of microwave striplines and coplanar waveguide cavities. The achievement of optically detected magnetic resonance in on-chip waveguides and nanophotonic cavities in Nd:YVO will be presented along with the initial progress of achieving coherent M2O conversion using Raman heterodyne spectroscopy. With photonic quantum memories and sources, single ion qubits, and quantum M2O all feasible in the one integrated platform, REI technology is a promising platform for enabling large scale integration of diverse quantum resources.

There are a variety of technologies used for single-photon detection, and within each of these technologies there are multiple device designs. While they differ radically in their nature and operation, in all cases their response to and recovery from a detection event is a complex and temporally evolving process. This makes the state of the detector at any given time dependent on the device's prior history. For many types of measurements, and particularly high-precision measurements, a detector's complex history dependence can lead to systematic errors that must be accounted for in analysis, and this sort of accounting requires a comprehensive knowledge of the detection system. For a typical non-photon-number-resolving detector accurate characterization includes multiple parameters beyond detection efficiency (afterpulsing, recovery time, etc.) We show that all heretofore explored properties can be described in a unified way with a generalized second-order model of a detector. While empirically proven effective, there are no experimental attempts that check the validity of this approach. We propose and experimentally demonstrate a simple validity test based on calculation of 2nd and 3rd order correlation functions from a single list of detection timestamps. We also accurately calibrate detectors used for this test.

Generating entangled photons on monolithic chips is a significant progress towards real-life applications of optical quantum information processing such as quantum key distribution and quantum computing. Here we present our recent achievements in generating polarization entangled photons on monolithic III-V semiconductor chips without any off-chip component. We demonstrate the direct generation of broadband polarization entangled photons from a semiconductor chip for the first time with a record degree of entanglement. We also show an alternative approach for polarization entangled photon generation on the same epitaxial structure, which enabled a single chip generating both co-polarized and cross-polarized entangled photons. With recent progress on pump laser integration, our results pave the way for fully integrated entangled photon sources in the foreseeable future.

We present a fabrication method to obtain freestanding optical microcavities in Single Crystal Diamond (SCD), based on a combination of Reactive Ion Etching (RIE) and multidirectional Focused Ion Beam (FIB) milling, and we report for the first time experimental optical characterization of freestanding diamond optical microdisk resonators obtained by this fabrication method. Patterning of the optical microcavities is achieved by contact photolithography on single crystal CVD diamond plates (3 mm x 3 mm x 0.15 mm), using a SiO₂ hard mask and optimized O₂ diamond plasma etching, resulting in multiple circular pillars in a single etch step. Individual pillars are subsequently undercut by multi-directional FIB milling from two orthogonal directions, shaping the anchor to the bulk substrate. Sequential FIB thinning and smoothing of the disks allows obtaining freestanding optical microcavities. During FIB milling, an Al/Cr layer (50 nm/75 nm) is used to ground the diamond substrate, simultaneously limiting ion implantation and reducing FIB induced edge rounding. We experimentally probe the cavities by a tunable laser, coupled to the resonator by a tapered single mode fiber. The spectral response of a typical microdisk (diameter 5.9 μm, thickness 800 nm) in transmission over the tuning range of the laser (1485 nm to 1550 nm) reveals multiple optical resonances with a Free Spectral Range of 52.5 nm and optical Q-factors attaining up to 1500 (at 1496 nm). To our knowledge, this is the first time that freestanding optical microdisk resonators are demonstrated in Single Crystal Diamond by a combination of RIE and multidirectional FIB milling, providing a path for high-Q optical cavities in diamond.

We investigate by theory ultra broadband photon pair generation in graded index multimode optical fibers (GIMFs). It has been shown how the unique dispersive properties of GIMFs provide an opportunity to generate state engineered photon pairs. Our findings hint on interesting features such as: - GIMFs provide can be used as an ultra tunable source of state engineered photon pairs. - Quantum correlations are independent of fiber mode and group number and only depend on spectral separation from the pump. - It is possible to simultaneously generate uncorrelated and correlated photon pair in the same fiber.

We perform the experimental generation of pairs of photons on a solid-state rare-earth ion doped crystal of Eu³⁺:Y₂SiO₅, by using a DLCZ-like protocol designed for inhomogeneously broadened media. The idea relies on the use of the atomic frequency comb technique, in order to rephase the atoms for the emission of the photons. A specificity of this protocol is its high temporal multimode capacity, as many pairs of photons can be emitted at different instants in time. A Cauchy-Schwarz inequality violation of 2.88>1 is witnessed, proving the non-classical correlations of the photon pairs that we produce. A detailed analysis of the source and detection imperfections is conducted, revealing ways of increasing the quality of the pairs that are produced.

Formation of optical solitons in self-induced transparency (SIT) regime, where light pulses propagate virtually without loss in nominally strongly absorbing medium, is one of the most striking coherent transient phenomena in optics. Here we study experimentally and by numeric simulations how a square shape pulse gradually transforms into a smooth sech shape pulse of well-defined pulse area, depending on the parameters such as the pulse amplitude, duration, propagation distance etc. The SIT experiments for circularly polarized light are performed in the R1−3∕2) line of a 30 ppm ruby (α-Al2O3:Cr3+) at 1.7 K in a magnetic field of BIIc = 4.5 T, which corresponds to effective absorption coefficient, αL =14.5. In such a magnetic field and temperature range, a 30 ppm ruby is in the so-called superhyperfine limit resulting in a very long decoherence (phase memory) time, TM = 50 μs. We show, in good quantitative agreement with the simulations, how SIT soliton is formatted and how this results in extremely slow pulse peak propagation velocity of ∼300 m∕s, which is to date, the slowest pulse propagation ever observed in a SIT experiment. We also show that for accurate quantitative description of the observed SIT and soliton pulse shapes, the simulations need to account for variation of the incident pulse amplitude across the beam spatial profile. Potential implications of the SIT effect on classical- and quantum information storage will be discussed.

Single photons are the key ingredient for many photonic quantum technologies including quantum key distribution and measurement-based quantum computing. However, it remains difficult to create devices with the appropriate specifications for use in non-laboratory environments. The optical microcavity platform provides an attractive route towards a room temperature single photon source device. Our ultra-small focused ion beam (FIB) milled open-access cavities offer enhancement of the spontaneous emission rate, tunability of the emission spectrum and increased light collection. The embedment of solid-state emitters within these cavities enables us to create a robust room temperature single photon source device, with the potential for high efficiencies and single photon purities. Defects such as the nitrogen-vacancy (NV) centre in diamond have been shown to be stable room-temperature sources of single photons. There are new single emitters emerging in two-dimensional materials such as hexagonal boron nitride (hBN). Here we present developments in room-temperature coupling of single defects to open-access microcavities of a planar-hemispherical geometry with mode volumes down to λ3. We report enhancements in the spectral density of photons into a single cavity mode, combined with improved single photon purities. It will be shown that the NV-cavity system provides a ~3% single photon emission efficiency with purities of up to 94%. The hBN-cavity system provides count rates >1Mcts/s into a single cavity mode with purities up to 96%. With these high single photon purities, such devices would be robust against photon number splitting attacks making them attractive for applications in quantum cryptography.

Entangled photons generation is an interesting field of research, since progress in this area will directly affect the development of photonic quantum technologies, including quantum computing, simulation and sensing. Several methods have been sifted to increase the performances of entangled photon sources and the integrated optics approach represents a promising strategy. In particular, integrated waveguide sources represent a robust tool, thanks to their stability and the enhancement of nonlinear light-crystal interaction provided by waveguide field confinement. Here, we show the versatility of a hybrid approach, realizing an integrated optical source for the generation of entangled photon-pairs at telecom wavelength. The nonlinear active medium used is lithium niobate, while the routing and manipulation of the generated signal is performed in aluminum-borosilicate glass photonic circuits. The system is composed of three cascaded devices. First, a balanced directional coupler at the fundamental wavelength equally splits the pump in the lithium niobate waveguides, which generate single-photon pairs through type 0 spontaneous parametric down-conversion process. A third chip, encompassing directional couplers and waveplates, closes the interferometer and recombines the generated photons, thus giving access to different quantum states of light: path-entangled or polarization-entangled states. A thermal phase shifter, which controls the relative phase between the interferometer arms, gives an additional degree of freedom for engineering the output state of the presented photon pairs source. All these components are entirely fabricated by femtosecond laser micromachining, a direct and very versatile technique that allows to process different kind of materials and realize high quality optical circuits.

In quantum repeater networks, the varying stability of entangled quantum links makes dynamic topology resilience an emerging issue. Here we define an efficient topology adaption method for quantum repeater networks. The model assumes the random failures of entangled links and several parallel demands from legal users. The shortest path defines a set of entangled links for which the probability of stability is above a critical threshold. The scheme is utilized in a base-graph of the overlay quantum network to provide an efficient shortest path selection for the demands of all users of the network. We study the problem of entanglement assignment in a quantum repeater network, prove its computational complexity, and show an optimization procedure. The results are particularly convenient for future quantum networking, quantum-internet, and experimental long-distance quantum communications.

Experimental results on the transient optical nutation effect inside an acetylene-filled hollow-core photonic crystal fiber (HC-PCF) are reported. The experiments used 15 ns optical pulses with peak powers up to 5 W. The light wavelength was centered at 1530.37 nm, which corresponds to the P9 acetylene (¹²C₂H₂) vibrational-rotational absorption line. The gas pressure inside the PCF, with hollow core diameter of ∼10.3 μm, was kept around 0.12 Torr. Comparison of the experimental data with numerical simulations using the Maxwell-Bloch equations allowed us to evaluate the characteristic longitudinal and transverse relaxation times around 10 ns, as well as the transition dipole moment (1.36 × 10⁻³² Cm).