Quantum technologies play a central role in establishing new ways of quantum-secured communication. This work investigates free-space quantum communication and explores the advantage of implementing quantum key distribution (QKD) with weak coherent states produced by a light source in the mid-infrared (). Time-bin encoded quantum key distribution is simulated, and it is demonstrated that a free-space QKD link operating in the mid-infrared outperforms configurations based on conventional near-infrared wavelengths under various weather scenarios, with a particular significance in adverse meteorological conditions.

Laser sources, since their invention, have proved to be the right solution in practically all conceived applications. Recently, the so-called second quantum revolution and quantum technologies like sensing, computing, simulation or communication are triggering a new generation of sub-classical sources to tackle such novel and challenging applications. First concepts and experimental results aimed to endow quantum cascade lasers and other infrared sources with truly quantum properties will be shown.

Background: Optical microtraps at the focus of high numerical aperture (high-NA) imaging systems enable efficient collection, trapping, detection and manipulation of individual neutral atoms for quantum technology and studies of optical physics associated with super- and sub-radiant states. The recently developed “Maltese cross” geometry (MCG) atom trap uses four in-vacuum lenses to achieve four-directional high-NA optical coupling to single trapped atoms and small atomic arrays. This article presents the first extensive characterisation of atomic behaviour in a MCG atom trap. Methods: We employ a MCG system optimised for high coupling efficiency and characterise the resulting properties of the trap and trapped atoms. Using current best practices, we measure occupancy, loading rate, lifetime, temperature, fluorescence anti-bunching and trap frequencies. We also use the four-directional access to implement a new method to map the spatial distribution of collection efficiency from high-NA optics: we use the two on-trap-axis lenses to produce a 1D optical lattice, the sites of which are stochastically filled and emptied by the trap loading process. The two off-trap-axis lenses are used for imaging and single-mode collection. Correlations of single-mode and imaging fluorescence signals are then used to map the single-mode collection efficiency. Results: We observe trap characteristics comparable to what has been reported for single-atom traps with one- or two-lens optical systems. The collection efficiency distribution in the axial and transverse directions is directly observed to be in agreement with expected collection efficiency distribution from Gaussian beam optics. Conclusions: The multi-directional high-NA access provided by the Maltese cross geometry enables complex manipulations and measurements not possible in geometries with fewer directions of access, and can be achieved while preserving other trap characteristics such as lifetime, temperature, and trap size.

The Second Quantum Revolution facilitates the engineering of new classes of sensors, communication technologies, and computers with unprecedented capabilities. Supply chains for quantum technologies are emerging, some focused on commercially available components for enabling technologies and/or quantum-technologies research infrastructures, others with already higher technology-readiness levels, near to the market. In 2018, the European Commission has launched its large-scale and long-term Quantum Flagship research initiative to support and foster the creation and development of a competitive European quantum technologies industry, as well as the consolidation and expansion of leadership and excellence in European quantum technology research. One of the measures to achieve an accelerated development and uptake has been identified by the Quantum Flagship in its Strategic Research Agenda: The promotion of coordinated, dedicated standardisation and certification efforts. Standardisation is indeed of paramount importance to facilitate the growth of new technologies, and the development of efficient and effective supply chains. The harmonisation of technologies, methodologies, and interfaces enables interoperable products, innovation, and competition, all leading to structuring and hence growth of markets. As quantum technologies mature, the time has come to start thinking about further standardisation needs. This article presents insights on standardisation for quantum technologies from the perspective of the CEN-CENELEC Focus Group on Quantum Technologies (FGQT), which was established in June 2020 to coordinate and support the development of standards relevant for European industry and research.

A novel study on harmonic frequency combs emitted by quantum cascade lasers (QCLs) is presented here, demonstrating the presence of intensity correlations between twin modes characterizing the emission spectra. These originate from a four‐wave mixing process driven by the active medium's third‐order nonlinearity. The study of such correlations is essential for the engineering of a new generation of semiconductor devices with the potential of becoming integrated emitters of light with quantum properties, such as squeezing and entanglement. Starting from experimental results, the limits of state‐of‐the‐art technology are discussed as well as the possible methodologies that could lead to the detection of nonclassical phenomena, or alternatively improve the design of QCLs, in the compelling perspective of generating quantum correlations in mid‐infrared light.

The classification of big data usually requires a mapping onto new data clusters which can then be processed by machine learning algorithms by means of more efficient and feasible linear separators. Recently, Lloyd et al. have advanced the proposal to embed classical data into quantum ones: these live in the more complex Hilbert space where they can get split into linearly separable clusters. Here, we implement these ideas by engineering two different experimental platforms, based on quantum optics and ultra-cold atoms respectively, where we adapt and numerically optimize the quantum embedding protocol by deep learning methods, and test it for some trial classical data. We perform also a similar analysis on the Rigetti superconducting quantum computer. Therefore, we find that the quantum embedding approach successfully works also at the experimental level and, in particular, we show how different platforms could work in a complementary fashion to achieve this task. These studies might pave the way for future investigations on quantum machine learning techniques especially based on hybrid quantum technologies.

A quantum embedding protocol embeds classical data into quantum ones living in the more complex Hilbert space where, for instance, they can get split into linearly separable clusters (data classification) or processed for an hybrid quantum machine learning application. The cover image represents the result of this embedding on the Bloch sphere via a parameterized quantum circuit. In article number 2100140 by Filippo Caruso and colleagues, the successful experimental implementations have been realized on two complementary, atomic and photonic, platforms, and compared with a similar analysis running on the cloud-accessible Rigetti superconducting quantum processor. (Image created by I. Mastroserio)

We present the characterization of a novel balanced homodyne detector operating in the mid-infrared. The challenging task of revealing non-classicality in mid-infrared light, e. g. in quantum cascade lasers emission, requires a high-performance detection system. Through the intensity noise power spectral density analysis of the differential signal coming from the incident radiation, we show that our setup is shot-noise limited. We discuss the experimental results with a view to possible applications to quantum technologies, such as free-space quantum communication.

We describe optical methods for trapping, cooling, and observing single $^{87}$Rb atoms in a four-lens "Maltese cross" geometry (MCG). The use of four high numerical-aperture lenses in the cardinal directions enables efficient collection of light from non-collinear directions, but also restricts the optical access for cooling and optical pumping tasks. We demonstrate three-dimensional atom localization with sub-wavelength precision, and present measurements of the trap lifetime, temperature and transverse trap frequency in this geometry. We observe a trap performance comparable to what has been reported for single-atom traps with one- or two-lens optical systems, and conclude that the additional coupling directions provided by the MCG come at little cost to other trap characteristics.

We are developing an atom-based platform for investigating the non-classical nature of mid infrared light emitted by Quantum Cascade Laser frequency combs, based on four wave mixing 87 Rb.

We demonstrate a novel geometry for strong coupling of light and matter in free space, i.e., without the use of optical cavities. Guided by optical metrology tools, we use a manual pick-and-place technique to precisely and stably position four high numerical aperture (NA=0.5) aspheric lenses along the four cardinal directions with their foci at a single central point. The geometry immediately doubles the available solid angle and thus the light-atom coupling relative to two-lens schemes, and will enable new trapping, excitation, and collection methods. We test the system by trapping a single 87Rb at the common focus and observing its fluorescence simultaneously from four directions. The fluorescence signals indicate both sub-Poissonian atom number statistics and photon anti-bunching, showing suitability for cold atom quantum optics.

We report on the simultaneous observation from four directions of the fluorescence of single 87 Rb atoms trapped at the common focus of four high numerical aperture (NA = 0.5) aspheric lenses.We use an interferometrically-guided pick-and-place technique to precisely and stably position the lenses along the four cardinal directions with their foci at a single central point.The geometry gives right angle access to a single quantum emitter, and will enable new trapping, excitation, and collection methods.The fluorescence signals indicate both sub-Poissonian atom number statistics and photon anti-bunching, showing suitability for cold atom quantum optics.

We describe a cavity-enhanced spontaneous parametric down-conversion (CE-SPDC) source for narrowband photon pairs with filters designed such that 97.7% of the correlated photons are in a single mode of 4.3(4) MHz bandwidth. Type-II phase matching, a tuneable-birefringence resonator, MHz-resolution pump tuning, and tuneable Fabry-Perot filters are used to achieve independent signal and idler tuning. We map the CE-SPDC spectrum using difference frequency generation to precisely locate the emission clusters, demonstrate CE-SPDC driven atomic spectroscopy, and measure a contribution from unwanted modes of 7.7%. The generated photon pairs efficiently interact with neutral rubidium, a well-developed system for quantum networking and quantum simulation. The techniques are readily extensible to other material systems.

The generation of nonclassical states of light in miniature chips is a crucial step towards practical implementations of future quantum technologies. Semiconductor materials are ideal to achieve extremely compact and massively parallel systems and several platforms are currently under development. In this context, spontaneous parametric down conversion in AlGaAs devices combines the advantages of room temperature operation, possibility of electrical injection and emission in the telecom band. Here we report on a chip-based AlGaAs source, producing indistinguishable and energy-time entangled photons with a brightness of $7.2\times10^6$ pairs/s and a signal-to-noise ratio of $141\pm12$. Indistinguishability between the photons is demonstrated via a Hong-Ou-Mandel experiment with a visibility of $89\pm3\%$, while energy-time entanglement is tested via a Franson interferometer leading to a value for the Bell parameter $ S=2.70\pm0.10$.

We demonstrate postselection free heralded qubit amplification for Time-Bin qubits and single photon states in an all-fibre, telecom-wavelength, scheme that highlights the simplicity, stability and potential for fully integrated photonic solutions. Exploiting high-efficiency superconducting detectors, the gain, fidelity and the performance of the amplifier are studied as a function of loss. We also demonstrate the first heralded single photon amplifier with independent sources. This provides a significant advance towards demonstrating device-independent quantum key distribution as well as fundamental tests of quantum mechanics over extended distances.

We report the realization of a fibred polarization entangled photon-pair source at 1560nm based on a type-II nonlinear interaction and working in the picosecond regime. By taking advantage of a set of fibre filters, we deterministically separate the photons and project them into wavelength separable states. A standard entanglement measurement with a net interference visibility close to 1 proves the relevance of our approach as an enabling technology for entanglement-based quantum communication.

[2] J. Barrett, L. Hardy, and A. Kent, Phys. Rev. Lett. 95, 010503 (2005). [3] T. C. Ralph and A. P. Lund, in Proceedings of 9th ICQMC, edited by A. Lvovsky (AIP, New York, 2009), p. 155. [4] F. Ferreyrol, M. Barbieri, R. Blandino, S. Fossier, R. Tualle-Brouri, and P. Grangier, Phys. Rev. Lett. 104, 123603 (2010). [5] G. Y. Xiang, T. C. Ralph, A. P. Lund, N. Walk, and G. J. Pryde, Nature Photon. 4, 316 (2010). [6] N. Gisin, S. Pironio, and N. Sangouard, Phys. Rev. Lett. 105, 070501 (2010). Bibliography Heralded Qubit Amplification for Device-Independent Quantum Key Distribution

We report on the generation of indistinguishable photon pairs at telecom wavelengths based on a type-II parametric down conversion process in a periodically poled potassium titanyl phosphate (PPKTP) crystal. The phase matching, pump laser characteristics and coupling geometry are optimised to obtain spectrally uncorrelated photons with high coupling efficiencies. Four photons are generated by a counter-propagating pump in the same crystal and anlysed via two photon interference experiments between photons from each pair source as well as joint spectral and g((2)) measurements. We obtain a spectral purity of 0.91 and coupling efficiencies around 90% for all four photons without any filtering. These pure indistinguishable photon sources at telecom wavelengths are perfectly adapted for quantum network demonstrations and other multi-photon protocols.