Multiphoton interference reveals strictly nonclassical phenomena. Its applications range from fundamental tests of quantum mechanics to photonic quantum information processing, where a significant fraction of key experiments achieved so far comes from multiphoton state manipulation. The progress, both theoretical and experimental, of this rapidly advancing research is reviewed. The emphasis is given to the creation of photonic entanglement of various forms, tests of the completeness of quantum mechanics (in particular, violations of local realism), quantum information protocols for quantum communication (e.g., quantum teleportation, entanglement purification, and quantum repeater), and quantum computation with linear optics. The scope of the review is limited to ``few-photon'' phenomena involving measurements of discrete observables.

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Multiphoton entanglement and interferometry

Photonic quantum technologies represent a promising platform for several applications, ranging from long-distance communications to the simulation of complex phenomena. Indeed, the advantages offered by single photons do make them the candidate of choice for carrying quantum information in a broad variety of areas with a versatile approach. Furthermore, recent technological advances are now enabling first concrete applications of photonic quantum information processing. The goal of this manuscript is to provide the reader with a comprehensive review of the state of the art in this active field, with a due balance between theoretical, experimental and technological results. When more convenient, we will present significant achievements in tables or in schematic figures, in order to convey a global perspective of the several horizons that fall under the name of photonic quantum information.

https://iopscience.iop.org/article/10.1088/1361-6633/aad5b2/pdf

Photonic quantum information processing: a review

Entangled quantum systems have properties that have fundamentally overthrown the classical worldview. Increasing the complexity of entangled states by expanding their dimensionality allows the implementation of novel fundamental tests of nature, and moreover also enables genuinely new protocols for quantum information processing. Here we present the creation of a (100 × 100)-dimensional entangled quantum system, using spatial modes of photons. For its verification we develop a novel nonlinear criterion which infers entanglement dimensionality of a global state by using only information about its subspace correlations. This allows very practical experimental implementation as well as highly efficient extraction of entanglement dimensionality information. Applications in quantum cryptography and other protocols are very promising.

https://www.pnas.org/content/pnas/111/17/6243.full.pdf

Generation and confirmation of a (100 × 100)-dimensional entangled quantum system

As quantum information processing continues to develop apace, the need for integrated photonic devices becomes ever greater for both fundamental measurements and technological applications. To this end, Crespi et al. demonstrate a high-fidelity photonic controlled-NOT gate on a glass chip.

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Integrated photonic quantum gates for polarization qubits

We present a novel optical device based on an integrated system of microlenses and single-mode optical fibers. It allows us to collect and direct into many modes two photons generated by spontaneous parametric down-conversion. By this device multiqubit entangled states and/or multilevel qudit states of two photons, encoded in the longitudinal momentum degree of freedom, are created. The multipath photon entanglement realized by this device is expected to find important applications in modern quantum information technology.

Multipath entanglement of two photons.

Franson's Bell experiment with energy-time entanglement [Phys. Rev. Lett. 62, 2205 (1989)] does not rule out all local hidden variable models. This defect can be exploited to compromise the security of Bell inequality-based quantum cryptography. We introduce a novel Bell experiment using genuine energy-time entanglement, based on a novel interferometer, which rules out all local hidden variable models. The scheme is feasible with actual technology.

Proposed bell experiment with genuine energy-time entanglement.

Drug resistance of BRAF-mutant melanoma: Review of up-to-date mechanisms of action and promising targeted agents.

Objectives To evaluate the geographical and socioeconomic differences in mortality and in life expectancy in Italy; to evaluate the proportion of mortality in the population attributable to a medium-low education level through the use of maps and indicators. Design Longitudinal design of the population enrolled in the 2011 Italian Census, following the population over time and registering any exit due to death or emigration. Setting and participants The study used the database of the Italian National Institute of Statistics (Istat) developed by linking the 2011 Census with the Italian National Register of Causes of Death (2012-2014) for 35 groups of causes of death. Age, sex, residence, and education level information were collected from the Census. Main outcome measures Life expectancy at birth was calculated by sex, Italian region, and education level. For the population aged 30-89 years, the following items were developed by sex: 1. provincial maps showing, for each cause of death, the distribution in quintiles of smoothed standardized mortality ratio (SMR), adjusted for age and education level and estimated with Bayesian models for small areas (spatial conditional autoregressive model); 2. regional maps of population attributable fraction (PAF) for low and medium education levels, calculated starting from age-standardized mortality ratios; 3. tables illustrating for each region standardized mortality rates and standardized years of life lost rate by age (standardized YLL rate), and mortality rate ratios standardized by age (MMRs). Results Males with a lower education level throughout Italy show a life expectancy at birth that is 3 years less than those with higher education; residents in Southern Italy lose an additional year in life expectancy, regardless of education level. Social inequalities in mortality are present in all regions, but are more marked in the poorer regions of Southern Italy. Geographical differences, taking into account the different population distributions in terms of age and education level, produce mortality differences for all causes: from -15% to +30% in women and from -13% to +26% in men, compared to the national average. Among the main groups of causes, the geographical differences are greater for cardiovascular diseases, respiratory diseases, and accidents, and lower for many tumour sites. A clear mortality gradient with an excess in Southern Italy can be seen for cardiovascular diseases: there are some areas where mortality for people with higher education level is higher than that for residents in Northern Italy with low education level. The gradient for "All tumours", instead, is from South to North, as it is for most single tumour sites. Population attributable fraction for low education level in Italy, taking into account the population distribution by age, is 13.4% in women and 18.3% in men. Conclusions The study highlighted important geographical differences in mortality, regardless of age and socioeconomic level, with a more significant impact in the poorer Southern regions, revealing a never-before-seen health advantage in the regions along the Adriatic coast. A lower education level explains a considerable proportion of mortality risk, although with differing effects by geographical area and cause of death. There are still mortality inequalities in Italy, therefore, representing a possible missed gain in health in our Country; these inequalities suggest a reassessment of priorities and definition of health targets. Forty years after the Italian National Health Service was instituted, the goal of health equity has not yet been fully achieved.

Italian Atlas of mortality inequalities by education level.

We report on the implementation of an interferometric scheme that allows the generation of photon pairs entangled in the time-energy degree of freedom. This scheme does not require any kind of temporal postselection on the generated pairs and can be used even with lasers with short coherence time.

Alessandro Rossi

Papers

Multipath entanglement of two photons.

Proposed bell experiment with genuine energy-time entanglement.

Drug resistance of BRAF-mutant melanoma: Review of up-to-date mechanisms of action and promising targeted agents.

Italian Atlas of mortality inequalities by education level.

Generation of time-bin-entangled photons without temporal postselection