We present a historical review of the development and impact of spontaneous parametric down-conversion (SPDC) in Brazil, marking over three decades since the first twin-photon experiments were performed in the country. This article traces the pioneering efforts that initiated the field, highlighting key experiments, institutions, and researchers who contributed to its growth. We discuss seminal works that established SPDC as a fundamental tool in the Brazilian Quantum Optics community, including studies on spatial correlations, entanglement, and decoherence. By presenting a curated sequence of experiments, we offer an overview of how Brazilian research in twin-photon systems has explored profound concepts through fundamental demonstrations, leading to significant international impact. This review also highlights the formation of a strong scientific community and its ongoing efforts to turn fundamental knowledge into quantum applications.

We conduct an experimental investigation into the conservation of the twist phase in Twisted Gaussian Schell Model (TGSM) beams during both up- and down-conversion three-wave mixing nonlinear processes. Independently generated TGSM beams, prepared with varying twist parameters, are used to pump and seed the nonlinear interactions. The resulting beams are then analyzed to determine their twist properties. Our findings demonstrate that the twists of the up- and down-converted beams depend on those of the pump and seed beams. Additionally, the results indicate that the twist phase is conserved throughout the process, in qualitative agreement with theoretical predictions. This study is motivated by the increasing potential applications of TGSM beams in various fields.

In the ever-evolving landscape of quantum cryptography, Device-independent Quantum Key Distribution (DI-QKD) stands out for its unique approach to ensuring security based not on the trustworthiness of the devices but on nonlocal correlations. Beginning with a contextual understanding of modern cryptographic security and the limitations of standard quantum key distribution methods, this review explores the pivotal role of nonclassicality and the challenges posed by various experimental loopholes for DI-QKD. Various protocols, security against individual, collective and coherent attacks, and the concept of self-testing are also examined, as well as the entropy accumulation theorem, and additional mathematical methods in formulating advanced security proofs. In addition, the burgeoning field of semi-device-independent models (measurement DI-QKD, Receiver DI-QKD, and One-sided DI-QKD) is also analyzed. The practical aspects are discussed through a detailed overview of experimental progress and the open challenges towards the commercial deployment in the future of secure communications.

The use of multicore optical fibers is emerging as a key solution to implement space-division multiplexing, essential for overcoming the capacity limits of conventional single-mode fibers. However, next-generation high-capacity optical networks will require new devices compatible with these fibers. In this work, we present an all-fiber architecture for a high-speed core-selective switch, crucial for efficient signal distribution in multicore networks. The device leverages multicore interference to achieve rapid core-switching within 0.7 μs-three orders of magnitude faster than state-of-the-art micro-electromechanical system switches. It also maintains an average inter-core crosstalk below −18 dB, ensuring compatibility with diverse network tasks. We validated the device’s functionality by routing optical signals ranging from 1 to 600 Gbps and successfully switching signals over a field-installed multicore fiber network. These results demonstrate, for the first time, a multicore optical fiber switch operating under real-world conditions with speeds far surpassing existing commercial devices. Potentially compatible with standard multiplexing techniques, this switch represents a significant advancement in enabling high-capacity multicore telecommunication networks. Its performance and adaptability position it as a key technology for the development of next-generation optical communication systems. Gabriel Saavedra and colleagues introduce an all-fiber device for rapid core-switching in multi-core fiber systems, achieving speeds under 0.7 μs. This device can improve the integration of multi-core fibers into networking applications.

We propose an estimation procedure for d-dimensional unitary transformations. For d>2, the unitary transformations close to the identity are estimated saturating the quantum Cramér-Rao bound. For d=2, the estimation of all unitary transformations is also optimal with some prior information. We show through numerical simulations that, even in the absence of prior information, two-dimensional unitary transformations can be estimated with greater precision than by means of standard quantum process tomography.

With the advent of quantum technology, the interest in communication tasks assisted by quantum systems has increased both in academia and industry. Nonetheless, the transmission of a quantum state in real-world scenarios is bounded by environmental noise, so that the quantum channel is an open quantum system. In this work, we study a high-dimensional open quantum system in a multicore optical fiber by characterizing the environmental interaction as quantum operations corresponding to probabilistic phase-flips. The experimental platform is currently state-of-the-art for quantum information processing with multicore fibers. At a given evolution stage we observe a non-Markovian behaviour of the system, which is demonstrated through a proof-of-principle implementation of the Quantum Vault protocol. A better understanding of phase-noise in multicore fibers will improve several real-world communication protocols, since they are a prime candidate to be adopted in future telecom networks.

We investigate theoretically and experimentally the optical second harmonic generation (SHG) with a twisted Gaussian Schell model (TGSM) beam as the fundamental field. We use Type-II phase matching and analyze the cross spectral density (CSD) of the SHG output beam when the input fundamental is prepared with a TGSM structure. We analyze two synthetization methods for preparing the TGSM fundamental beam and we find that for one method the SHG is also a TGSM beam. For the other method, we find that the SHG is not a TGSM beam and presents an anomalous CSD possessing a dip instead of a peak in the transverse spatial structure. Moreover, we show that the dip depth is directly related to the twisted phase parameter, being absent for a non twisted GSM beam. Our results show that the SHG from a fundamental TGSM beam can result in a doubled frequency TGSM or in a non-TGSM beam depending on the synthetization method.

The use of multicore optical fibers is now recognized as one of the most promising methods to implement the space-division multiplexing techniques required to overcome the impending capacity limit of conventional single-mode optical fibers. Nonetheless, new devices for networking operations compatible with these fibers will be required in order to implement the next-generation high-capacity optical networks. In this work, we develop a new architecture to build a high-speed core-selective switch, critical for efficiently distributing signals over the network. The device relies on multicore interference, and can change among outputs in less than 0.7 us, while achieving less than -18 dB of average inter-core crosstalk, making it compatible with a wide range of network switching tasks. The functionality of the device was demonstrated by routing a 1GBs optical signal and by successfully switching signals over a field-installed multicore fiber network. Our results demonstrate for the first time the operation of a multicore optical fiber switch functioning under real-world conditions, with switching speeds that are three orders of magnitude faster than current commercial devices. This new optical switch design is also fully compatible with standard multiplexing techniques and, thus, represents an important achievement towards the integration of high-capacity multicore telecommunication networks.

South American optics research has seen remarkable growth over the past 50 years, with significant contributions in areas such as quantum optics, holography, spectroscopy, nonlinear optics, statistical optics, nanophotonics and integrated photonics. The research has driven economic development in sectors like telecom, biophotonics, biometrics, and agri-sensing. This joint feature issue between JOSA A and JOSA B exhibits cutting-edge optics research from the region, fostering a sense of community and promoting collaboration among researchers.

In parametric down-conversion, a nonlinear crystal is pumped by a laser and spontaneous emission takes place in signal and idler modes according to the phase-matching conditions. A seed laser can stimulate the emission in the signal beam if there is mode overlap between them. This also enhances the emission in the idler beam, affecting its coherence properties. While the degree of coherence of the idler field as a function of the seed power has already been studied, the transverse coherence length has not yet been properly investigated. The transverse coherence length is a key parameter of optical beams that determines the beam divergence, for example. Here, we present a theoretical and experimental investigation of the transverse coherence length in stimulated down-conversion. In addition, we make a connection between stimulated down-conversion and partially coherent sources like the Gaussian-Schell model beams, and show that in general the idler field cannot be described with this model.

The quantum switch is a quantum computational primitive that provides computational advantage by applying operations in a superposition of orders. In particular, it can reduce the number of gate queries required for solving promise problems where the goal is to discriminate between a set of properties of a given set of unitary gates. In this work, we use Complex Hadamard matrices to introduce more general promise problems, which reduce to the known Fourier and Hadamard promise problems as limiting cases. Our generalization loosens the restrictions on the size of the matrices, number of gates and dimension of the quantum systems, providing more parameters to explore. In addition, it leads to the conclusion that a continuous variable system is necessary to implement the most general promise problem. In the finite dimensional case, the family of matrices is restricted to the so-called Butson-Hadamard type, and the complexity of the matrix enters as a constraint. We introduce the ``query per gate'' parameter and use it to prove that the quantum switch provides computational advantage for both the continuous and discrete cases. Our results should inspire implementations of promise problems using the quantum switch where parameters and therefore experimental setups can be chosen much more freely.

In parametric down conversion, a nonlinear crystal is pumped by a laser and spontaneous emission takes place in signal and idler modes according to the phase matching conditions. A seed laser can stimulate the emission in the signal beam if there is mode overlap between them. This also enhances the emission in the idler beam, affecting its coherence properties. While the degree of coherence of the idler field as a function of the seed power has already been studied, the transverse coherence length has not yet been properly investigated. The transverse coherence length is a key parameter of optical beams that determines the beam divergence, for example. Here, we present a theoretical and experimental investigation of the transverse coherence length in stimulated down conversion. In addition, we make a connection between stimulated down conversion and partially coherent sources like the Gaussian Schell model beams, and show that in general the idler field cannot be described with this model.

Orbital angular momentum can be used to implement high capacity data transmission systems that can be applied for classical and quantum communications. Here we experimentally study the generation and transmission properties of the so-called perfect vortex beams and the Laguerre-Gaussian beams in ring-core optical fibers. Our results show that when using a single preparation stage, the perfect vortex beams present less ring-radius variation that allows coupling of higher optical power into a ring core fiber. These results lead to lower power requirements to establish fiber-based communications links using orbital angular momentum and set the stage for future implementations of high-dimensional quantum communication over space division multiplexing fibers.

Most of the fundamental characteristics of quantum mechanics, such as non-locality and contextuality, are manifest in discrete, finite-dimensional systems. However, many quantum information tasks that exploit these properties cannot be directly adapted to continuous-variable systems. To access these quantum features, continuous quantum variables can be made discrete by binning together their different values, resulting in observables with a finite number $d$ of outcomes. While direct measurement indeed confirms their manifestly discrete character, here we employ a salient feature of quantum physics known as mutual unbiasedness to show that such coarse-grained observables are in a sense neither continuous nor discrete. Depending on $d$, the observables can reproduce either the discrete or the continuous behavior, or neither. To illustrate these results, we present an example for the construction of such measurements and employ it in an optical experiment confirming the existence of four mutually unbiased measurements with $d = 3$ outcomes in a continuous variable system, surpassing the number of mutually unbiased continuous variable observables.

The twisted Gaussian Schell Model describes a family of partially coherent beams that present several interesting characteristics, and as such have attracted attention in classical and quantum optics. Recent techniques have been demonstrated to synthesize these beams from a coherent source using a discrete set of ‘pseudo-modes’, where the phase of each mode is randomized so that they are mutually incoherent. Here we investigate this technique and evaluate the resulting beam parameters, such as divergence, coherence length and twist phase. We show that for a finite set of modes there is also some residual coherence, which can have an observable effect. A theoretical model is developed for the output field that includes residual coherence and agrees very well with experimental data. In addition, we demonstrate a simple method to measure the twist phase using double slit interference.

The most common form of measurement in quantum mechanics projects a wavefunction onto orthogonal states that correspond to definite outcomes. However, generalized quantum measurements that do not fully project quantum states are possible and have an important role in quantum information tasks. Unfortunately, it is difficult to certify that an experiment harvests the advantages made possible by generalized measurements, especially beyond the simplest two-level qubit system. Here we show that multiport beamsplitters allow for the robust realization of high-quality generalized measurements in higher-dimensional systems with more than two levels. Using multicore optical fibre technology, we implement a seven-outcome generalized measurement in a four-dimensional Hilbert space with a fidelity of 99.7%. We present a practical quantum communication task and demonstrate a success rate that cannot be simulated in any conceivable quantum protocol based on standard projective measurements on quantum messages of the same dimension. Our approach, which is compatible with modern photonic platforms, showcases an avenue for faithful and high-quality implementation of genuinely non-projective quantum measurements beyond qubit systems.

The problem of non-orthogonal state discrimination underlies crucial quantum information tasks, such as cryptography and computing protocols. Therefore, it is decisive to find optimal scenarios for discrimination among quantum states. We experimentally investigate the strategy for the optimal discrimination of two non-orthogonal states considering a fixed rate of inconclusive outcomes (FRIO). The main advantage of the FRIO strategy is to interpolate between unambiguous and minimum error discrimination by solely adjusting the rate of inconclusive outcomes. We present a versatile experimental scheme that performs the optimal FRIO measurement for any pair of generated non-orthogonal states with arbitrary a priori probabilities and any fixed rate of inconclusive outcomes. Considering different values of the free parameters in the FRIO protocol, we implement it upon qubit states encoded in the polarization mode of single photons generated in the spontaneous parametric down-conversion process. Moreover, we resort to a newfangled double-path Sagnac interferometer to perform a three-outcome non-projective measurement required for the discrimination task, showing excellent agreement with the theoretical prediction. This experiment provides a practical toolbox for a wide range of quantum state discrimination strategies using the FRIO scheme, which can significantly benefit quantum information applications and fundamental studies in quantum theory.

A vector vortex beam presents local polarization states that are spatially modulated in the plane transverse to propagation. By employing a two-crystal-sandwich source allowing for nonlinear interactions between input fields of any polarization, we analyze experimentally the process of stimulated parametric down-conversion and observe unique effects when the pump and seed fields are vector vortex beams. We reconstruct the transverse polarization pattern of the generated idler beam by Stokes polarimetry. The experimental results obtained are in good qualitative agreement with theory and exhibit manifold polarization transverse distributions for different combinations of pump and seed vector vortex beams, which may find applications in both classical and quantum optics.

Certification of quantum nonlocality plays a central role in practical applications like device-independent quantum cryptography and random number generation protocols. These applications entail the challenging problem of certifying quantum nonlocality, something that is hard to achieve when the target quantum state is only weakly entangled, or when the source of errors is high, e.g. when photons propagate through the atmosphere or a long optical fiber. Here we introduce a technique to find a Bell inequality with the largest possible gap between the quantum prediction and the classical local hidden variable limit for a given set of measurement frequencies. Our method represents an efficient strategy to certify quantum nonlocal correlations from experimental data without requiring extra measurements, in the sense that there is no Bell inequality with a larger gap than the one provided. Furthermore, we also reduce the photodetector efficiency required to close the detection loophole. We illustrate our technique by improving the detection of quantum nonlocality from experimental data obtained with weakly entangled photons.

Fourth-order interference is an information processing primitive for photonic quantum technologies, as it forms the basis of photonic controlled-logic gates, entangling measurements, and can be used to produce quantum correlations. Here, using classical weak coherent states as inputs, we study fourth-order interference in 4 × 4 multi-port beam splitters built within multi-core optical fibers, and show that quantum correlations, in the form of geometric quantum discord, can be controlled and maximized by adjusting the intensity ratio between the two inputs. Though these states are separable, they maximize the geometric discord in some instances, and can be a resource for protocols such as remote state preparation. This should contribute to the exploitation of quantum correlations in future telecommunication networks, in particular in those that exploit spatially structured fibers.