In conventional lasers, the optical cavity that confines the photons also determines essential characteristics of the lasing modes such as wavelength, emission pattern, directivity, and polarization. In random lasers, which do not have mirrors or a well-defined cavity, light is confined within the gain medium by means of multiple scattering. The sharp peaks in the emission spectra of semiconductor powders, first observed in 1999, has therefore lead to an intense debate about the nature of the lasing modes in these so-called lasers with resonant feedback. In this paper, we review numerical and theoretical studies aimed at clarifying the nature of the lasing modes in disordered scattering systems with gain. The last decade has witnessed the emergence of the idea that even the low-Q resonances of such open systems could play a role similar to the cavity modes of a conventional laser and produce sharp lasing peaks. We will focus here on the near-threshold single-mode lasing regime where nonlinear effects associated with gain saturation and mode competition can be neglected. We will discuss in particular the link between random laser modes near threshold and the resonances or quasi-bound (QB) states of the passive system without gain. For random lasers in the localized (strong scattering) regime, QB states and threshold lasing modes (TLM) were found to be nearly identical within the scattering medium. These studies were later extended to the case of more lossy systems such as random systems in the diffusive regime where it was observed that increasing the openness of such systems eventually resulted in measurable and increasing differences between quasi-bound states and lasing modes. Very recently, a theory able to treat lasers with arbitrarily complex and open cavities such as random lasers established that the threshold lasing modes are in fact distinct from QB states of the passive system and are better described in terms of a new class of states, the so-called constant-flux states. The correspondence between QB states and lasing modes is found to improve in the strong scattering limit, confirming the validity of initial work in the strong scattering limit.

Integrated optical components on lithium niobate play a major role in standard high-speed communication systems. Over the last two decades, after the birth and positioning of quantum information science, lithium niobate waveguide architectures have emerged as one of the key platforms for enabling photonics quantum technologies. Due to mature technological processes for waveguide structure integration, as well as inherent and efficient properties for nonlinear optical effects, lithium niobate devices are nowadays at the heart of many photon-pair or triplet sources, single-photon detectors, coherent wavelength-conversion interfaces, and quantum memories. Consequently, they find applications in advanced and complex quantum communication systems, where compactness, stability, efficiency, and interconnectability with other guided-wave technologies are required. In this review paper, we first introduce the material aspects of lithium niobate, and subsequently discuss all of the above mentioned quantum components, ranging from standard photon-pair sources to more complex and advanced circuits.

7 pages, 5 figuresInternational audienceGranting information privacy is of crucial importance in our society, notably in fiber communication networks. Quantum cryptography provides a unique means to establish, at remote locations, identical strings of genuine random bits, with a level of secrecy unattainable using classical resources. However, several constraints, such as non-optimized photon number statistics and resources, detectors' noise, and optical losses, currently limit the performances in terms of both achievable secret key rates and distances. Here, these issues are addressed using an approach that combines both fundamental and off-the-shelves technological resources. High-quality bipartite photonic entanglement is distributed over a 150 km fiber link, exploiting a wavelength demultiplexing strategy implemented at the end-user locations. It is shown how coincidence rates scale linearly with the number of employed telecommunication channels, with values outperforming previous realizations by almost one order of magnitude. Thanks to its potential of scalability and compliance with device-independent strategies, this system is ready for real quantum applications, notably entanglement-based quantum cryptography

The realization of an ultra-fast source of heralded single photons emitted at the wavelength of 1540 nm is reported. The presented strategy is based on state-of-the-art telecom technology, combined with off-the-shelf fiber components and waveguide non-linear stages pumped by a 10 GHz repetition rate laser. The single photons are heralded at a rate as high as 2.1 MHz with a heralding efficiency of 42%. Single photon character of the source is inferred by measuring the second-order autocorrelation function. For the highest heralding rate, a value as low as 0.023 is found. This not only proves negligible multi-photon contributions but also represents the best measured value reported to date for heralding rates in the MHz regime. These prime performances, associated with a device-like configuration, are key ingredients for both fast and secure quantum communication protocols.The reliable generation of single photon states is crucial for a wide variety of quantum optical technologies, ranging from quantum computation and communication [1,2] to quantum metrology and detector calibration [3,4]. As an example, the use of single photon states is essential in quantum key distribution (QKD) protocols, where the unintended presence of more than one photon per time window can be exploited by an eavesdropper to extract part of the information [5].Ideal sources should be able to emit indistinguishable single photons in a deterministic way, at an arbitrarily high repetition rate and with zero probability of multiphoton emissions [1]. In particular, the request of ultrafast photon sources is mandatory to speed up data exchanges in quantum communication protocols. In anticipation to such optimal cases, a pertinent alternative is represented by heralded single photon sources (HSPS), where the detection of one photon of two simultaneously generated is used to herald the emission time of the second one [1,6]. In such schemes, the produced single photons rate is proportional to the detected heralding photon one, R H , and to the heralding efficiency, P 1 , namely, the probability of observing one heralded photon per heralding event. We note that, in experiments, the value of P 1 is essentially determined by optical losses [6].In the original and most common implementations, pairs of simultaneous photons are generated in nonlinear crystals via spontaneous parametric down conversion (SPDC) of a pump beam [6]. In particular, an accurate choice of the phase matching can lead to the production of photons at telecom wavelength, as required for long distance transmission in optical fibers [7,8]. SPDC being a probabilistic process, a way to obtain high photon rates is to increase the probability of generating the photon pairs as well as the photon transmission after the SPDC crystal. Accordingly, in the last years, many papers have been focusing on the realization of bright SPDC sources [9,10] and much effort has been made towards optimizing paired photon collection, separation and prop- * Corresponding author: virginia.dauria@unice.fr agati...