Reconfigurable photonic integrated circuits enable applications in a wide range of fields. They represent a powerful platform for signal processing due to the ability to perform parallel computations. In this work, we demonstrate a photonic processor based on a six-channel universal chip fabricated by femtosecond laser writing with an interlaced architecture comprising multiport beam splitters. We exploit and enhance a previously demonstrated calibration procedure for a single building block, and extend it to a complete six-channel interferometer, reconstructing the corresponding interferometer models over a broad spectral range. We validate them by measuring a set of 100 Haar-random unitary matrices, achieving mean amplitude fidelities of 95.9

Quantized vortices are ubiquitous in physics, spanning superconductivity, astrophysics, superfluid condensed matter systems, and nonlinear optics. Yet embedding vorticity into topologically protected nonlinear states has remained a major challenge, with all previously observed corner solitons in higher-order topological insulators (HOTIs) exhibiting only trivial phase distributions. Here, we report on the first realization of stable topological corner vortex solitons in a photonic fractal HOTI. Using an array of laser-written waveguides in the shape of Sierpiński gasket with a controllable distortion, we design linear topological vortex modes, from which nonlinear corner vortex solitons bifurcate. Moreover, we demonstrate that these solitons exhibit exceptional robustness across a broad power range and, unlike vortex solitons in topologically trivial lattices, form without a power threshold. Our results introduce the angular momentum degree of freedom into the physics of topological corner modes, opening prospects for topologically protected vortex-based photonics.

Nonlinear optical materials designed for mid‐infrared applications gain increasing interest in photonics and quantum‐optical research. However, many of these materials remain unexplored in the broad mid‐infrared range due to the limited availability of suitable equipment. Silver thiogallate (AgGaS2) is one of the widely known nonlinear crystals used in mid‐infrared frequency‐conversion applications. While its transmission and refractive indices are well characterized up to 10.6 μm, dispersion properties beyond this wavelength remain unreported. In this work, a quantum‐optical approach is employed to determine the refractive indices of AgGaS2 across an extended mid‐infrared wavelength range. Correlated photon pairs are generated in the crystal, where one photon in a pair is generated at a near‐infrared wavelength and the other in the mid‐infrared. Due to the correlations, detecting the spectrum of the near‐infrared photons allows inferring the crystal's dispersion properties in the mid‐infrared region up to 21 μm, without the need for infrared light sources or detectors. Sellmeier equations are further proposed that accurately describe both ordinary and extraordinary refractive indices across the entire transparency range of the crystal. The results are consistent with previously reported data below 10 μm and demonstrate the suitability of AgGaS2 for frequency‐conversion processes over an extended wavelength range.

Reconfigurable photonics have rapidly become an invaluable tool for information processing. Lightbased computing accelerators are promising for boosting neural-network learning and inference [X. Xiao Nat. Commun. 15, 6189 (2024)] and optical interconnects are foreseen as a solution to the information transfer bottleneck in high-performance computing [Y. Li et al., in 2021 58th ACM/IEEE Design Automa29, 1 (2022); A. Netherton et al., Photonics Res. 12, A69 (2024)]. In this study, we demonstrate the successful programming of a transformation implemented using a reconfigurable photonic circuit with a nonconventional architecture. The core of most photonic processors is an MZI-based architecture [M. Reck analytical connection between controllable parameters and circuit transformation. However, several archi124, 010501 (2020)] that are substantially more difficult to program have improved robustness to fabrication defects. We use two algorithms that rely on different initial datasets to reconstruct the circuit model of a complex interferometer, and then program the required unitary transformation. The first method is based on the global fitting of the experimental calibration data, while the second method is an ML-based approach introduced in Kuzmin et al. [Opt. Express 29, 38429 (2021)]. Both methods performed accurate circuit programming with an average fidelity greater than 99% and 97%, respectively. Our results provide a strong foundation for the introduction of nonconventional interferometric architectures for photonic information processing.

We propose a new tomography protocol for reconstruction of the polarization qutrit state that does not require the conversion to a ququart. We show that a set of unitary two-mode polarization transformations is sufficient to perform a full set of measurements. In the experimental realization of the protocol, this transformation is implemented through a quarter-wave plate oriented at different angles. It is shown that this protocol allows one to reconstruct both pure and mixed states.

Higher-order topological insulators (HOTIs) are unique topological materials supporting edge states with the dimensionality at least by two lower than the dimensionality of the underlying structure. HOTIs are observed on lattices with different symmetries, but only in geometries, where truncation of HOTI produces a finite structure with the same order of discrete rotational symmetry as that of the unit cell, thereby setting the geometry of insulator edge. Here, a new type of 2D HOTI based on the Kekulé-patterned lattice is experimentally demonstrated, whose order of discrete rotational symmetry differs from that of the unit cells of the constituent honeycomb lattice, with hybrid boundaries that help to produce all three possible corners that support effectively 0D corner states of topological origin, especially the one associated with spectral charge 5/6. It is also shown that linear corner states give rise to rich families of stable hybrid nonlinear corner states bifurcating from them in the presence of focusing nonlinearity of the material. Such new types of nonlinear corner states are observed in hybrid HOTI inscribed in transparent nonlinear dielectric using fs-laser writing technique. The results complete the class of HOTIs and open the way to observation of topological states with new internal structure and symmetry.

In higher-order topological insulators (HOTIs), topologically nontrivial phases are usually associated with the shift of Wannier centers to topologically nontrivial positions on the edges of the unit cells, and the emergence of fractional spectral charges in the corners of the lattice upon its truncation that keeps the number of its unit cells integer. Here we propose theoretically and illustrate experimentally a different approach to the construction of HOTIs. This approach utilizes lattices with incomplete unit cells and achieves localized modes of topological origin across a broader parameter space. When truncation disrupts translational symmetry by cutting through the interior of multiple unit cells, boundary modes in our system emerge for both trivial and topologically nontrivial positions of the Wannier centers. We link these modes to the appearance of fractional Wannier centers. We also demonstrate that linear boundary states give rise to rich families of stable solitons bifurcating from them in the presence of focusing nonlinearity. Multiple types of thresholdless topological solitons with different internal symmetries are observed in waveguide arrays with triangular configurations featuring incomplete unit cells for any dimerization of waveguide spacings. Our results expand the family of HOTIs and pave the way for the observation of boundary states with different symmetries.

Broadband correlated photon pairs (biphotons) are valuable in quantum metrology, but current generation methods either involve complex nonlinear structures or lack sufficient bandwidth and brightness. In this work, we theoretically describe and experimentally demonstrate a novel technique for generation of a bright collinear biphoton field with a broad spectrum, achieved by using a tightly focused pump in a bulk nonlinear crystal. As the most straightforward application of the source, we employ Michelson interferometer-based quantum optical coherence tomography (QOCT). Utilizing the source enables the demonstration of record resolution and dispersion cancellation for this QOCT scheme.

We develop a method for generating a more squeezed than single-mode squeezed vacuum (SMSV) state by subtracting 2,4,6 photons from it. In general, the more photons are subtracted, the more gain of the squeezing (more of 3 dB) is observed in the measurement-induced continuous variable (CV) states of definite parity. However, the two-photon subtraction strategy is practically preferred. It can be implemented with higher success probability, wider squeezing gain width ~ 5 dB and least quadrature variance in the corresponding range of initial squeezing. We demonstrate the mitigating effect of a photon-number resolving (PNR) detector with non-unit quantum efficiency on the output characteristics of the measurement-induced CV states, resulting in their slight decrease compared to ideal photon subtraction. Use of a single photon in addition to the SMSV state at beam splitter (BS) input with subsequent registration of odd number of photons (say, 3 photons) allows the implementation of the measurement-induced even CV state which is several times brighter than the initial state and has lower quadrature noise.

We develop a method to generate nonclassical light with a larger quadrature squeezing than one of the single-mode squeezed vacuum (SMSV) states by subtracting 2,4,6 photons from it. In general, the more photons are subtracted, the more gain of the squeezing (more than 3 dB) is observed in the measurement-induced continuous variable (CV) states of definite parity. However, the two-photon subtraction strategy is practically preferred. It can be implemented with higher success probability, wider squeezing gain width similar to 5 dB, and less quadrature variance. We demonstrate the mitigating effect of a photon-number resolving (PNR) detector with nonunit quantum efficiency on the output characteristics of the measurement-induced CV states, resulting in their slight decrease compared to ideal measurement of photons in the auxiliary mode. Use of a single photon in addition to the SMSV state at beam splitter (BS) input with subsequent registration of an odd number of photons (say, 3 photons) in auxiliary mode allows the implementation of the measurement-induced even CV state, which is several times brighter than the initial state, has lower quadrature noise, and is generated with greater probability. (c) 2025 Optica Publishing Group. All rights, including for text and data mining (TDM), Artificial Intelligence (AI) training, and similar technologies, are reserved.

Progress in observation of solitons in photonic topological insulators is discussed. Results are presented of experiments with nonlinear topological states in Su-SchriefferHeeger arrays fabricated using the femtosecond writing technique that are static, i.e., invariable in the direction of light propagation, and dynamically modulated (primarily periodically) in the direction of light propagation. Such objects are one of the simplest models of a topologically nontrivial structure. Solitons in topological insulators bifurcate with increasing laser beam power from linear edge states in the topological bandgap, inheriting their topological protection. The spatial localization of the soliton and the position of its propagation constant in the topological bandgap depend in a nonlinear medium on peak power and can be effectively controlled. Experimental observation of the switching of the edge topological modes in the bandgap between two closely spaced dimerized Su-Schrieffer-Heeger arrays is presented. The switching, whose rate depends on radiation intensity, can be completely arrested in a strongly nonlinear regime. In trimer waveguide arrays, whose spectrum in the topological phase features two simultaneously emerging topological bandgaps with edge states of different symmetries, two coexisting types of topological solitons exhibiting different degrees of stability were observed. We also discuss experimental observations of TE-solitons nonlinear topological Floquet states periodically reproducing their profiles in 1D- and 2D-dimensional Su-Schrieffer-Heeger arrays modulated in the direction of propagation of radiation.

An experimental implementation of a quantum random number generator has been proposed. A new method for the extraction of provably random bit sequences from correlated sequences of photocounts, which are Markov chains, has been implemented experimentally for the first time. The reached generation rate of bits 0 and 1 is 154.5 Mbit/s. Fundamental natural constraints on the achievement of perfect true randomness are also discussed.

We study quantum computing technology based on neutral rubidium atoms. Effect of Rydberg blockade is use in order to achieve entanglement of two qubits. Rydberg states are obtained by shining on the array of cold atoms with two lasers. Stability of the driving lasers and noise reduction are required for high fidelity of quantum gates. Theoretical analysis and experimental setup of PDH-locking scheme with additional filtering resonator will be presented.

We demonstrate compensation-free approach to the realization of multi-mode delay interferometers, mainly for use in phase encoded quantum key distribution (QKD). High interference visibility of spatially multimode beams in unbalanced Michelson or Mach-Zehnder interferometers with a relatively wide range of delays is achieved by the appropriate choice of the transverse size of the beam. We provide a simple theoretical model that gives a direct connection between the visibility of interference, the delay, and the beam parameters. The performed experimental study confirms our theoretical findings and demonstrates the measured visibility of up to 0.95 for the delay of 2 ns. Our approach's simplicity and robust performance make it a practical choice for the implementation of QKD systems, where a quantum signal is received over a multimode fiber. The important application of such configuration is an intermodal QKD system, where the free-space atmospheric communication channel is coupled into a span of the multimode fiber, delivering the spatially distorted beam to the remote receiver with minimal coupling loss.

We review recent progress in the field of optical quantum metrology, with a focus on the analysis of the current level of theoretical and experimental research on the generation, transformation, and measurement of nonclassical states of light, such as NOON, squeezed, and hybrid states, which com- bine transformations of both discrete and continuous variables of a quantized light field. We show how such states can be used to improve the measurement accuracy and to estimate unknown phase parameters in both linear and nonlinear metrology. Sig- nificant attention is paid to the description of actual quantum metrology schemes that take the loss of particles, the limited fidelity of photon detectors, and other factors into account. We therefore identify both the ultimate (fundamental) bounds im- posed by quantum mechanical uncertainties of the quantities being measured and the bounds due to the effect of classical noise on the propagation and measurements of a quanti Of special importance are quantum metrology options spontaneous parametric light scattering, which, for m 50 years, has been an indispensable tool for key acc ments in quantum optics and related areas of photoni tum cryptography, quantum computing, and quantum In this regard, we analyze the current status of the u well-known Hong-Ou-Mandel photon anticorrelati and biphoton interference in various quantum metro proaches in measuring temperature, length, material co tion, and so on. We also discuss the use of biph photometry, radiometry, and sensing for the absolute tion of modern photon-count detectors, as well as for ments of the brightness temperature of hot radiation The quantum metrology phenomena, methods, aproaches discussed here in light of the most recent progress on sources and detectors of quantum radiation will be an important tool in developing and practically implementing new schemes and algorithms for quantum processing and information transmission.

A quantum metrology problem of simultaneous measurement and estimation of several phase parameters was considered in the framework of current tendencies of development of alternative navigation. The fundamental limits of both linear and nonlinear metrology were studied. The effect of losses on the accuracy of quantum metrology of several phase parameters was analyzed. A method to prepare three-mode N00N- states using atomic bright solitons was proposed.

We report low-loss multiscan waveguides fabricated in fused silica using femtosecond-laser-writing technology. The multiscan principle allows the writing regime to be tailored to excel at key features of any integrated photonic platform: coupling losses and propagation losses. We optimized the writing parameters for different sizes of square-shaped waveguides and reached the mode overlap value with a standard single- mode optical fiber of above 98.8% and demonstrated very low coupling losses of 0.2 dB/facet on average. Propagation losses in the fabricated waveguides amounted to 0.07 dB/cm. We applied the developed recipe to the fabrication of a fiber-coupled 25-channel interferometer with total insertion losses below 1 dB. The findings of this work are of interest for broad range of applications and in particular for optical information processing and quantum photonics.

In this paper, the possible negative effects of the influence of turbulent disturbances in atmospheric communication channels for entanglement-based quantum key distribution systems have been considered. In order to decrease these effects an active tracking system has been developed. We demonstrate the results of its evaluation in a quantum key distribution system.

Quantum key distribution systems with an untrusted intermediate node described by the so-called measurement device independent (MDI) protocol have been actively studied in the last decade. In early works, it was only argued why such a quantum key distribution system ensures the security of distributed keys mentioning that the security proof of the MDI protocol, which was not presented, is similar to that for the basic Bennett‒Brassard 84 (BB84) protocol. For this reason, despite the existing experimental implementations of the MDI quantum key distribution system, physical reasons for the protocol security are still questionable. Such quantum key distribution systems provide a common key between two network nodes connected through the intermediate untrusted node, which does not require protection of the equipment on it, and an eavesdropper sees the entire operation of the equipment, including the results of the operation of photodetectors. In this work, the MDI protocol has been analyzed. It has been shown that the physical reasons for the protocol security are based on fundamental properties such as the interference of photons from different sources, monogamy of entanglement, and nonorthogonality of states. A simple and explicit derivation is given showing the equivalence of the MDI and BB84 protocols and physical reasons for the identity of the corresponding expressions for the length of the final key.