Parametric Down Conversion

Nous présentons une méthode basée sur le codage spectral de la polarisation permettant d'accéder avec une sensibilité optimale à la biréfringence d'un échantillon en un temps compatible avec la microscopie à balayage laser. Ce concept est étendu à la microscopie plein champ associée à une caméra hyperspectrale.

Two methods for creating arbitrary two-photon polarization pure states are introduced. Based on these, four schemes for creating two-photon polarization mixed states are proposed and analyzed. The first two schemes can synthesize completely arbitrary two-qubit mixed states, i.e., control all 15 free parameters: Scheme I requires several sets of crystals, while Scheme II requires only a single set, but relies on decohering the pump beam. Additionally, we describe two further schemes which are much easier to implement. Although the total capability of these is still being studied, we show that they can synthesize all two-qubit Werner states, maximally entangled mixed states, Collins-Gisin states, and arbitrary Bell-diagonal states.

We report a femtosecond optical parametric oscillator (OPO) based on the new semiconductor gain material orientation patterned gallium phosphide (OP-GaP) and being the first example of a broadband OPO operating across the molecular fingerprint region. OP-GaP crystals with lengths of 1 mm and several patterning periods were diced, polished, and antireflection (AR) coated for near-to mid-infrared wavelengths. We configured a synchronously pumped OP-GaP OPO in a 101.2-MHz resonator with high reflectivity from 1.15-1.35 µm, pumped with 150-fs pulses from a 1040-nm femtosecond laser (Chromacity Spark). The coating of one spherical mirror was optimized for transmission at the pump wavelength of 1040 nm and for high reflectivity at the resonant signal wavelength in a range from 1.15-1.35 µm, while the other spherical mirror collimated the idler beam emerging from the OP-GaP crystal and was silver coated to provide high reflectivity for all idler wavelengths. This collimated idler beam was output-coupled from the cavity by transmission through a plane mirror coated with high transmission for the idler wavelengths (5-12 µm) and high reflectivity for the signal wavelengths (1.15-1.35 µm) on an infrared-transparent ZnSe substrate. Idler spectra centered from 5.4-11.8 µm and extending to 12.5 µm were collected. The maximum average power was 55 mW at 5.4 µm with 7.5 mW being recorded at 11.8 µm. Details of Fourier transform spectroscopy using water vapor and a polystyrene reference standard are presented.

We report a continuous-wave (cw) source of tunable mid-infrared radiation providing tens of milliwatt of output power in the 6460-7517 nm spectral range. The source is based on difference-frequency generation (DFG) in orientation-patterned (OP)-GaAs pumped by a Tm-fiber laser at 2010 nm and a 1064 nm-Yb-fiber-pumped cw optical parametric oscillator. Using a 25.7-mm-long OP-GaAs crystal, we have generated up to 51.1 mW of output power at 6790 nm, with >40 mW and >20 mW across 32% and 80% of the mid-infrared tuning range, respectively, which is to the best of our knowledge the highest tunable cw power generated in OP-GaAs in this spectral range. The DFG output at maximum power exhibits passive power stability better than 2.3% rms over more than 1 h and a frequency stability of 1.8 GHz over more than 1 min, in high spatial beam quality. The system and crystal performance at high pump powers have been studied.

Although the realization of useful quantum computers poses significant challenges, swift progress in emerging quantum technologies is making this goal realistically approachable. In this context, one of the essential resources is quantum entanglement, which allows for quantum computations outperforming their classical counterparts. However, the task of entanglement detection is not always straightforward for several reasons. One of the main challenges is that standardly-used methods rapidly become unfeasible when dealing with quantum states containing more than a few qubits. Typically, this is due to the fact that a vast amount of measurements is needed on many copies of the state. Generally, it is not unusual to deal with a very limited number of state copies in experimental settings -in fact, this may be the case for many large quantum systems. In this article, an overview is provided of a probabilistic approach that enables high-confidence genuine multipartite entanglement detection using an exceptionally low number of state copies. Additionally, a study is presented that shows that this protocol remains efficient also in the presence of noise, thus confirming the practicality of the method for near-term quantum devices and its suitability for complex experimental settings.

Multi-photon state generation is of great interest for near-future quantum simulation and quantum computation experiments. To-date spontaneous parametric down-conversion is still the most promising process, even though two major impediments still exist: accidental photon noise (caused by the probabilistic non-linear process) and imperfect single-photon purity (arising from spectral entanglement between the photon pairs). In this work, we overcome both of these difficulties by (1) exploiting a passive temporal multiplexing scheme and (2) carefully optimizing the spectral properties of the down-converted photons using periodically-poled KTP crystals. We construct two down-conversion sources in the telecom wavelength regime, finding spectral purities of > 91%, while maintaining high four-photon count rates. We use single-photon grating spectrometers together with superconducting nanowire single-photon detectors to perform a detailed characterization of our multi-photon source. Our methods provide practical solutions to produce high-quality multi-photon states, which are in demand for many quantum photonics applications.

We propose an experiment to observe the topological phases associated with cyclic evolutions, generated by local SU(2) operations, on three-qubit entangled states prepared on different degrees of freedom of entangled photon pairs. The topological phases reveal the nontrivial topological structure of the local SU(2) orbits. We describe how to prepare states showing different topological phases, and discuss their relation to entanglement. In particular, the presence of a π/2 phase shift is a signature of genuine tripartite entanglement in the sense that it does not exist for two-qubit systems.

TheWigner formalism in the Heisenberg picture constitutes a bridge that connects QuantumOptics to Stochastic Optics. The vacuum field appears explicitly in the formalism, and the wavelikeaspects of light are emphasised. In addition, the zeropoint intensity as a threshold for detection is acommon denominator in both theories. In this paper, after summarising the basic rules of the Wignerapproach and its application to parametric down-conversion, some new results are presented thatdelve into the physical meaning of the zeropoint field in optical quantum communication. Specifically,the relationship between Bell-state distinguishability and the number of sets of zeropoint modesthat take part in the experiment is analysed in terms of the coupling between the phases of thedifferent fields involved and the subtraction of the zeropoint intensity at the detectors. Additionally,the connection between the compatibility theorem in quantum cryptography and zeropoint fieldis stressed.

If it is the author's pre-published version, changes introduced as a result of publishing processes such as copy-editing and formatting may not be reflected in this document. For a definitive version of this work, please refer to the published version.

We demonstrate the generation of non-diffracting heralded single photons, i.e. which are characterized by a single-photon transverse intensity distribution which remains essentially unchanged over a significant propagation distance. For this purpose we have relied on the process of spontaneous parametric downconversion (SPDC) for the generation of signal and idler photon pairs, where our SPDC crystal is pumped by a Bessel-Gauss (BG) beam. Our experiment shows that the well-understood non-diffracting behavior of a BG beam may be directly mapped to the signal-mode, single photons heralded by the detection of a single idler photon. In our experiment, the heralded single photon is thus arranged to be non-diffracting without the need for projecting its single-photon transverse amplitude, post-generation, in any manner.

Environment induced decoherence, and other quantum processes, have been proposed in the literature to explain the apparent spontaneous selection -out of the many mathematically eligible bases -of a privileged measurement basis that corresponds to what we actually observe. This paper describes such processes, and demonstrates that -contrary to common belief -no such process can actually lead to a preferred basis in general. The key observation is that environment induced decoherence implicitly assumes a prior independence of the observed system, the observer and the environment. However, such independence cannot be guaranteed, and we show that environment induced decoherence does not work in general. We conclude that the existence of the preferred basis must be postulated in quantum mechanics, and that changing the basis for a measurement is, and must be, described as an actual physical process.

Environment induced decoherence, and other quantum processes, have been proposed in the literature to explain the apparent spontaneous selection -out of the many mathematically eligible bases -of a privileged measurement basis that corresponds to what we actually observe. This paper describes such processes, and demonstrates that -contrary to common belief -no such process can actually lead to a preferred basis in general. The key observation is that environment induced decoherence implicitly assumes a prior independence of the observed system, the observer and the environment. However, such independence cannot be guaranteed, and we show that environment induced decoherence does not work in general. We conclude that the existence of the preferred basis must be postulated in quantum mechanics, and that changing the basis for a measurement is, and must be, described as an actual physical process.

Photon loss is destructive to the performance of quantum photonic devices and therefore suppressing the effects of photon loss is paramount to photonic quantum technologies. We present two schemes to mitigate the effects of photon loss for a Gaussian Boson Sampling device, in particular, to improve the estimation of the sampling probabilities. Instead of using error correction codes which are expensive in terms of their hardware resource overhead, our schemes require only a small amount of hardware modifications or even no modification. Our loss-suppression techniques rely either on collecting additional measurement data or on classical post-processing once the measurement data is obtained. We show that with a moderate cost of classical post processing, the effects of photon loss can be significantly suppressed for a certain amount of loss. The proposed schemes are thus a key enabler for applications of near-term photonic quantum devices.

In this work, the entanglement of a superposition of bipartite qubit coherent states with non-phased coherent parameters is studied. We use Generalized-concurrence as the measure to quantify the entanglement and drive analytical results in terms of the effective parameters involved. Analyzing the results, we conclude that such states may attain maximum entanglement or no entanglement at all, depending on the choice of the parameters involved.

Quantum spectroscopy was performed using the frequency-entangled broadband photon pairs generated by spontaneous parametric down-conversion. An absorptive sample was placed in front of the idler photon detector, and the frequency of signal photons was resolved by a diffraction grating. The absorption spectrum of the sample was measured by counting the coincidences, and the result is in agreement with the one measured by a conventional spectrophotometer with a classical light source.

A polarization mixing OPO was demonstrated, that emits a single, linearly polarized narrow linewidth beam, fixed at degeneracy, independent of crystal temperature. The frequency locking is explained in terms of balanced roundtrip phase-matching condition.

We build a time series of single photons with quantum chaos statistics, using a version of the Grangier anti-correlation experiment. The criteria utilized to determine the presence of quantum chaos is the frame of the Fano factor and the power spectrum. We also show that photons with chaotic statistics are in a balanced superposition of photons with both wave-like and particle like behaviors. To support the presence of quantum chaos, we study both Shannon’s entropy, and the complexity of single photons time series.

We present the design and experimental proof of principle of a low threshold optical parametric oscillator ͑OPO͒ that continuously oscillates over a large bandwidth allowed by phase matching. The large oscillation bandwidth is achieved with a selective two-photon loss that suppresses the inherent mode competition, which tends to narrow the bandwidth in conventional OPOs. Our design performs pairwise mode locking of many frequency pairs, in direct equivalence to passive mode locking of ultrashort pulsed lasers. The ability to obtain high powers of continuous and broadband down-converted light enables the optimal exploitation of the correlations within the down-converted spectrum, thereby strongly affecting two-photon interactions even at classically high power levels, and opening new venues for applications such as two-photon spectroscopy and microscopy and optical spread spectrum communication.

A novel approach for an optical direct-sequence spread spectrum is presented. It is based on the complementary processes of broad-band parametric down-conversion and up-conversion. With parametric down-conversion, a narrow-band continuous-wave (CW) optical field is transformed into two CW broad-band white-noise fields that are complex conjugates of each other. These noise fields are exploited as the key and conjugate key in optical direct-sequence spread spectrum. The inverse process of parametric up-conversion is then used for multiplying the key by the conjugate key at the receiver in order to extract the transmitted data. A complete scheme for optical code-division multiple access (OCDMA) based on this approach is presented. The salient feature of the approach presented in this paper is that an ideal white-noise key is automatically generated, leading to high-capacity versatile code-division multiple-access configurations.

Using correlated photons from parametric downconversion, we extend the boundaries of experimentally accessible two-qubit Hilbert space. Specifically, we have created and characterized maximally entangled mixed states (MEMS) that lie above the Werner boundary in the linear entropytangle plane. In addition, we demonstrate that such states can be efficiently concentrated, simultaneously increasing both the purity and the degree of entanglement. We investigate a previously unsuspected sensitivity imbalance in common state measures, i.e., the tangle, linear entropy, and fidelity.

Chirped quasi-phasematching (QPM) optical devices offer the potential for ultrawide bandwidths, high conversion efficiencies, and high amplification factors across the transparency range of QPM media. In order to properly take advantage of these devices, apodization schemes are required. We study apodization in detail for many regimes of interest, including low-gain difference frequency generation (DFG), high-gain optical parametric amplification (OPA), and high-efficiency adiabatic frequency conversion (AFC). Our analysis is also applicable to secondharmonic generation, sum frequency generation, and optical rectification. In each case, a systematic and optimized approach to grating construction is provided, and different apodization techniques are compared where appropriate. We find that nonlinear chirp apodization, where the poling period is varied smoothly, monotonically, and rapidly at the edges of the device, offers the best performance. We consider the full spatial structure of the QPM gratings in our simulations, but utilize the first order QPM approximation to obtain analytical and semianalytical results. One application of our results is optical parametric chirped pulse amplification; we show that special care must be taken in this case to obtain high gain factors while maintaining a flat gain spectrum. © 2013 Optical Society of America OCIS codes: (190.4360) Nonlinear optics, devices; (230.7405) Wavelength conversion devices; (190.4970) Parametric oscillators and amplifiers; (320.7080) Ultrafast devices.

Quantum correlation, such as entanglement and squeezing have shown to improve phase estimation in interferometric setups on one side, and non-interferometric imaging scheme of amplitude object on the other. In the last case, quantum correlation among a pair of beams leads to a subshot-noise readout of the image intensity pattern, where weak details, otherwise hidden in the noise, can be appreciated. In this paper we propose a technique which exploits entanglement to enhance quantitative phase retrieval of an object in a non-interferometric setting, i.e only measuring the propagated intensity pattern after interaction with the object. The method exploits existing technology, it operates in wide field mode, so does not require time consuming raster scanning and can operate with small spatial coherence of the incident field. This protocol can find application in optical microscopy and X-ray imaging, reducing the photon dose necessary to achieve a fixed signal-to-noise ratio.

Herein the synthesis, structural characterization and photoluminescence behavior of Er/Yb activated Gd2O3
phosphor is presented. Yb and Er-doped Gd2O3 phosphor samples were synthesized through the conventional
solid-state-reaction (SSR) method and their structural aspects were examined by the X-ray diffraction (XRD)
analysis. Morphology of the prepared phosphor samples was examined by Scanning Electron Microscopy (SEM)
and Transmission Electron Microscopy (TEM). The emission and excitation spectra of the prepared phosphor
samples were systematically recorded. They displayed intense emission in the visible region. The excitation
spectra recorded at 613 nm emission showed two prominent excitation peaks at 254 and 278 nm. Similar to that
excitation with various emission peaks were also observed correspondingly at 467, 545 and 613 nm. The present
study gives basic information about the transition of (Yb, Er) activated rare earth as well as the spectroscopic
parameters of the prepared phosphor samples. The CIE (Commission Internationale de-l’Eclairage) coordinate for
the prepared phosphor samples was calculated and the color rendering index [CRI] parameters were also
calculated using CIE. The prepared phosphor samples are proposed to be very useful in numerous optoelectronic
applications.

Higher penalties may apply, and higher damages may be awarded, for offences and infringements involving the conversion of material into digital or electronic form. ^^Mf i te a^» «gK^ pa 9« UNI VERSI TY OF WOLLONGOWS A thesis submitted in fulfilment of the requirements for the award of the degree of Master of Science (Honours) from UNIVERSITY OF WOLLONGONG

A single trapped ion is converted into a pseudo-two-photon source by splitting its resonance fluorescence, delaying part of it and by recombining both parts on a beam splitter. A destructive twophoton interference is observed with a contrast reaching 83(5)%. The spectral brightness of our twophoton source is quantified and shown to be comparable to parametric down-conversion devices.

The negativity of the Wigner function is discussed as a measure of the non classicality and the quantum interference pattern obtained therein as a possible measure of the entanglement between the two modes of the vortex states. This measure of entanglement is compared with the results obtained from concurrence.

Photon modes have an important role in characterizing the quantum sources of light. Proper coupling of various photon modes obtained in spontaneous parametric down conversion (SPDC) process in optical fibers is essential to generate an effective source of entangled photons. The two main pre-detection factors affecting the biphoton mode coupling in SPDC are the pump beam focusing parameter and the crystal thickness. We present the numerical and experimental results on the effect of pump focusing on conditional down-converted photon modes for a Type-I BBO crystal. We experimentally verify that biphoton coupling efficiency decreases asymptotically with pump beam focusing parameter. We attribute this behaviour to (a) the asymmetry in the spatial distribution of down-converted photons with the pump beam focusing parameter and (b) the ellipticity of biphoton modes introduced due to the focusing of the pump beam. We also show experimentally the ellipticity as well as quantify the asymmetry with the pump focusing parameter. These results may be useful in selecting optimum conditions in the down-conversion process for generating efficient sources of entangled photons for quantum information applications.

In this article we report that the entanglement produced by single photon subtraction is maximum, by studying the entanglement of multi photon subtraction from two mode squeezed states. We argue that the single photon subtraction produces maximum entanglement as it is more non-classical in nature than many photon subtractions.

We show that one can use a single optical cavity as a simplest possible resonator in order to ascertain the presence of an object with an arbitrarily low portion of an incoming laser beam. In terms of individual photons, each photon non-repeatedly tests the object with an arbitrary high probability of detecting its presence without interacting with it.

All Bell experiments carried out so far have had ten or more times fewer coincidence counts than singles counts and this, in effect, means a detection efficiency under 10%. Therefore, all these experiments relied only on coincidence counts and herewith on additional assumptions. Recently, however, Santos devised hidden variable models which do not obey the assumptions and thus made the experiments inconclusive. This, as well as recent improvements in detectors efficiencies, prompted an increasing interest in the loophole-free Bell experiments which do not rely on additional assumptions and which originally stem from the idea of the event-ready detectors (introduced by J.S. Bell) which would preselect Bell pairs ready for detection. Till recently it was assumed that such detectors would distort the pairs. Here we devise those that would not do so and propose an experiment which can realistically improve the detection efficiency and visibility up to over 80%. The setup uses two nonlinear crystals of type-II both of which simultaneously downconvert a singlet-like pair. We combine one photon from the first singlet with one from the second singlet at a beam splitter and consider their coincidence detections. Detectors determine optimally narrow solid angles for the downconverted photons. However, for their two companions (from each singlet) we use five times wider solid angles or even drop pinholes altogether and resort to frequency filters. So, we are able to realistically collect close to 100% of them. The latter pairs-preselected by coincidence detection at the beam splitter-appear entangled in (non)maximal singlet-like states, i.e., detectors at the beam splitter act as event-ready detectors for such Bell pairs.

We present a fiber based source of polarization-entangled photon pairs that is well suited for quantum communication applications in the 1550 nm band of standard fiber-optic telecommunications. Polarization entanglement is created by pumping a nonlinear-fiber Sagnac interferometer with two time-delayed orthogonally-polarized pump pulses and subsequently removing the time distinguishability by passing the parametrically scattered signal and idler photon pairs through a piece of birefringent fiber. Coincidence detection of the signal and idler photons yields biphoton interference with visibility greater than 90%, while no interference is observed in direct detection of either the signal or the idler photons. All four Bell states can be prepared with our setup and we demonstrate violations of the CHSH form of Bell's inequality by up to 10 standard deviations of measurement uncertainty.

We demonstrate an optical-fiber based source of polarization entangled photon-pairs with improved quality and efficiency, which has been integrated with off-the-shelf telecom components and is, therefore, well suited for quantum communication applications in the 1550 nm telecom band. Polarization entanglement is produced by simultaneously pumping a loop of standard dispersionshifted fiber with two orthogonally-polarized pump pulses, one propagating in the clockwise and the other in the counterclockwise direction. We characterize this source by investigating twophoton interference between the generated signal-idler photon-pairs under various conditions. The experimental parameters are carefully optimized to maximize the generated photon-pair correlation and to minimize contamination of the entangled photon-pairs from extraneously scattered background photons that are produced by the pump pulses for two reasons: i) spontaneous Raman scattering causes uncorrelated photons to be emitted in the signal/idler bands and ii) broadening of the pump-pulse spectrum due to self-phase modulation causes pump photons to leak into the signal/idler bands. We obtain two-photon interference with visibility > 90% without subtracting counts caused by the background photons (only dark counts of the detectors are subtracted), when the mean photon number in the signal (idler) channel is about 0.02/pulse, while no interference is observed in direct detection of either the signal or the idler photons.

Un réseau de communications par paquets IP (un ‘net’) et a fortiori l’interconnexion de tous ces réseaux de communications par paquets (Inter-net) posent au XXIème siècle un certain nombre de défis déjà bien identifiés.
L’un concerne la croissance, toujours exponentielle, du volume d’informations traitées et transmises entre les utilisateurs d’Internet et les plateformes des géants (GAFAM, BATX, NATU) . Un second défi sera le besoin de rendre intelligent le réseau, pour qu’il s’auto-organise (Self-Organizing Network, Deep Learning), tant sa complexité et le délai requis à toute prise de décision rendront l’intervention humaine de moins en moins possible ou adaptée.
Le troisième enjeu concerne la protection des données personnelles transmises ou stockées contre les attaques de tout genre.
Ces défis voient le jour alors qu’a lieu l’avènement de la seconde révolution quantique. Des solutions peuvent-elles être apportées par cette dernière ? C’est l’objet de cet ouvrage.
Rappelons ce que sont ces révolutions quantiques. Qu’apportent-elles ? La physique quantique permet de décrire le comportement des systèmes à l’échelle des atomes, ses effets débordent du seul domaine de l’infiniment petit. La première révolution quantique, qui a accompagné les progrès technologiques du XXème siècle, a vu naître des technologies comme le transistor, pièce maîtresse de nos ordinateurs et smartphones, le laser ou le Global Positioning System (GPS) fondé sur les horloges atomiques. La seconde révolution quantique, qui est donc en train d’arriver, combine la physique quantique et la théorie de l'information, pour fonder la Science de l’Information Quantique (Quantum Information Science ou QIS). Elle n’est pas nouvelle, ses racines remontant aux années 1960. Ces dernières années, les chercheurs, ingénieurs ont réalisé des progrès dans l'acquisition, le traitement et la transmission de l'information quantique, ce qui sera abordé dans cet ouvrage sous les noms de détection, calcul et communication quantiques, trois ingrédients indispensables à l’Internet quantique.
Vis-à-vis des défis identifiés plus haut et que devra relever l’Internet actuel ou « classique », la seconde révolution quantique et son lot de propriétés (comme celles liées à la superposition quantique, à l’intrication ou aux nouveaux algorithmes) ouvrent notamment la voie à la cryptographie quantique et post-quantique, à des capacités de calcul inédites pour certains problèmes, et peut-être à l’utilisation de ressources physiques moins gourmandes en énergie. D’où la liesse récente des chercheurs et des ingénieurs à travailler sur l’Internet quantique. Notons bien que de tels réseaux de communications quantiques n’ont pas vocation à remplacer Internet, mais à le compléter.
Pour bien des raisons donc, Internet va aussi évoluer pour intégrer une facette quantique. Par exemple, alors qu'Internet repose sur des bits classiques pouvant être dans un état de 0 ou 1, sa facette quantique utilisera des bits quantiques, également appelés qubits, qui peuvent exister dans plusieurs états simultanément en raison du phénomène de superposition quantique. Et l'évolution d’Internet vers le quantique permettra d'exploiter des propriétés nouvelles comme les communications intrinsèquement sécurisées ; en effet, le protocole de Distribution de Clés Quantiques (Quantum Key Distribution ou QKD) permettra d'échanger des clés de chiffrement de manière sécurisée en exploitant les propriétés de la physique quantique. Si une tentative d'interception de l’information quantique est faite, elle perturbe l'état des particules quantiques utilisées dans la communication, alertant ainsi les utilisateurs sur toute tentative d'espionnage.
L'internet quantique est encore en phase de recherche et de développement ; plusieurs défis technologiques doivent être relevés afin qu'il puisse devenir une réalité pratique à grande échelle. Cependant, il suscite un intérêt croissant en raison de son potentiel : i) renforcer la sécurité des communications dans un monde de plus en plus connecté, ii) mieux détecter les informations transmises par notre environnement ou iii) résoudre des problèmes algorithmiques que même le calcul de haute puissance (High Performance Computing ou HPC) sur les superordinateurs actuels mettrait un temps extrêmement long à résoudre.
Pour vous accompagner dans la découverte de l’Internet Quantique, cet ouvrage débutera par la présentation de quelques notions de physique quantique (chapitre 1), utilisant la notation de Dirac (c’est-à-dire des kets) pour décrire les états quantiques.
Ainsi dans le chapitre 2, nous pourrons aborder la seconde révolution quantique : l’avènement de l’informatique quantique. Qu’est-ce qu’un élément d’information quantique ou qubit ? A quoi correspond l’intrication dans cette Science de l’Information Quantique (QIS)? Quels types de calcul pouvons-nous effectuer sur un ou plusieurs qubits ? Et quels protocoles de communications quantiques sont envisageables ?
Forts de ces deux parties introductives, nous pourrons aborder dans le chapitre 3 les applications phares qui vont inciter au développement de l’Internet Quantique : les communications intrinsèquement sécurisées, le calcul quantique distribué et la détection de seconde génération, améliorée des fonctionnalités des réseaux quantiques.
Nous prendrons ensuite le temps, dans le chapitre 4, d’énumérer les éléments nécessaires d’un tel réseau de communications quantiques : les sources générant et distribuant l’intrication, les répéteurs quantiques, les mémoires quantiques et les serveurs centraux orchestrant tout ça.
Le chapitre 5 fera le point sur le modèle en couches qu’il sera nécessaire de développer pour conserver un Internet transparent vis-à-vis des choix technologiques et matériels sur chacun de ses nœuds. L’équivalent quantique du modèle OSI ou du modèle TCP-IP.
Comme les sites utilisateurs d’un tel Internet Quantique ne connaissent pas les chemins qui les relient, à travers l’un de ces réseaux de communications quantiques comme à travers leur interconnexion, nous exposerons dans le chapitre 6 les nouveaux protocoles de routage qu’il faudra mettre en place et la raison pour laquelle les techniques actuelles de routage dans Internet ne conviennent pas (en l’état).
Enfin avec le chapitre 7, nous ferons un exposé non exhaustif des premiers réseaux de communications quantiques qui voient le jour, tant en Chine, qu’aux Etats-Unis et en Europe.

We report on recent developments of widely tunable, continuous-wave (CW) laser sources based on difference-frequency generation (DFG) in nonlinear optical materials. The state-of-the-art of CW DFG technology will be reviewed. Applications of the DFG-based laser source to high-resolution molecular spectroscopy and trace gas detection will be presented. New development trends of DFG will be discussed. To cite this article: W.

We display the results of the numerical simulations of a set of Langevin equations, which describe the dynamics of a degenerate optical parametric oscillator in the Wigner representation. The scan of the threshold region shows the gradual transformation of a quantum image into a classical roll pattern. An experiment on parametric downconversion in lithium triborate shows strikingly similar results in both the near and the far field, displaying qualitatively the classical features of quantum images.

The Born rule, a foundational axiom used to deduce probabilities of events from wavefunctions, is indispensable in the everyday practice of quantum physics. It is also key in the quest to reconcile the ostensibly inconsistent laws of the quantum and classical realms, as it confers physical significance to reduced density matrices, the essential tools of decoherence theory. Following Bohr's Copenhagen interpretation, textbooks postulate the Born rule outright. However, recent attempts to derive it from other quantum principles have been successful, holding promise for simplifying and clarifying the quantum foundational bedrock. A major family of derivations is based on envariance, a recently discovered symmetry of entangled quantum states. Here, we identify and experimentally test three premises central to these envariance-based derivations, thus demonstrating, in the microworld, the symmetries from which the Born rule is derived. Further, we demonstrate envariance in a purely local quantum system, showing its independence from relativistic causality.

The study of Orbital Angular Momentum (OAM) of electromagnetic fields is an exponentially growing scientific field, with many potential applications. One of its key issue is the characterization, the sorting, and the detection of this orbital angular momentum. Several methods to analyze it have been developed. They can be listed in four different general technics. The first one consists in transforming the OAM electromagnetic field into a plane wave using optical elements, and then in analysing this plane wave. The second one is based on the observation of interferences fringes. These fringes arise either from the interference between the beam carrying OAM and a plane wave, or from self-interferences, or from diffraction. The third one looks for frequency shifts of a transmitted beam carrying OAM through a rotating medium using the beat frequency with an unshifted beam. It takes advantage of the so-called rotational Doppler shift. The fourth one is related to the true real mechanical nature of the OAM. It is a direct measure of the torque exerted by the electromagnetic field on an object. In this chapter, these different technics are reviewed and discussed, describing their main advantages and disadvantages, and giving key ready to use issues to detect OAM.

High-dimensional entanglement with spatial modes of light promises increased security and information capacity over quantum channels. Unfortunately, entanglement decays due to perturbations, corrupting quantum links which cannot be repaired without a tomography of the channel. Paradoxically, the channel tomography itself is not possible without a working link. Here we overcome this problem with a robust approach to characterising quantum channels by means of classical light. Using free-space communication in a turbulent atmosphere as an example, we show that the state evolution of classically entangled degrees of freedom is equivalent to that of quantum entangled pho- tons, thus providing new physical insights into the notion of classical entanglement. The analysis of quantum channels by means of classical light in real time unravels stochastic dynamics in terms of pure state trajectories and thus enables precise quantum error-correction in short and long haul optical communication,...

Spin-entaglement has been proposed and extensively used in the case of correlated triplet pairs which are intermediate states in singlet fission process in select organic semiconductors. Here, we employ quantum process tomography of polarization entangled photon-pairs resonant with the excited state absorption of these states to investigate the nature of the inherent quantum correlations and to explore for an unambiguous proof for the existence of exciton entanglement.

Istituto Nazionale di Ricerca Metrologica, Torino, Italy National Laboratory for Ultrafast and Ultraintense Optica l Science C.N.R.-I.N.F.M., Como, Italy 3 Dipartimento di Fisica e Matematica, Università degli Stu di dell’Insubria, Como, Italia Consiglio Nazionale delle Ricerche, Istituto Nazionale pe r la Fisica della Materia (C.N.R.-I.N.F.M.), Como, Italia 5 Dipartimento di Fisica, Università degli Studi di Milano, I-20133 Milano, Italia I.S.I. Foundation, I-10133 Torino, Italia (Dated: April 27, 2021)

The Schmidt decomposition is an important tool in the study of quantum systems especially for the quantification of the entanglement of pure states. However, the Schmidt decomposition is only unique for bipartite pure states, and some multipartite pure states. Here a generalized Schmidt decomposition is given for a class of mixed quantum states. It is shown that it shares some desirable properties with its pure-state counterpart, but lacks some properties which make the pure-state decomposition so important. Experimental methods for the identification of this class of mixed states are provided and some examples are discussed which show the utility of this description.

Hyperentanglement-assisted dense coding: Beating the channelcapacity "limit" JULIO T. BARREIRO, TZU-CHIEH WEI, PAUL G. KWIAT, University of Illinois at Urbana-Champaign -Dense coding was the very first quantum information protocol to be proposed and experimentally realized more than a decade ago. Today, however, the achieved channel capacity (CC) remains fundamentally limited as conceived for photons using linear optics: Alice can only decode three of four potential messages sent by Bob, due to the impossibility to deterministically resolve all four Bell states using conventional techniques; the maximum CC is thus limited to log 2 3 ≈ 1.585 bits. However, when the particles encoding the Bell pair are entangled in an additional degree of freedom, the complete and deterministic discrimination of all Bell states is possible. Via the process of spontanteous parametric down conversion, we produce photon pairs simultaneously entangled in polarization and orbital angular momentum. Using the auxiliary entanglement and a robust intermode coupling scheme, our experiment achieves CC=1.624(7) bits, the first to surpass the CC "limit" for linear photonic superdense coding. Our encoding is suited for quantum communication without alignment and satellite to satellite communication. Additionally, our scheme also enables the remote preparation of single-photon highly-entangled states.

In 1920’s on photon polarization research, scientists faced famous measurement problem and needed to answer a multiple choice question. The question was like that:

    Which one of the postulates below is wrong?
a) The individuality of the photon should be preserved. Photons does not break into pieces while passing an obliquely oriented linear polarizing filter.
b) A beam that is plane-polarized in a certain direction is to be pictured as made up of photons each plane-polarized in that direction.
c) No local hidden variables exist that predetermine the measurement result.
d) In physics (including quantum physics) every occurrence is bound to a reason. So, causality is most basic principle that should be preserved.

    For the quantum case, in order to be compatible with experimental  (or mathematically reached) results, one of the choices above had to be wrong. There is no choice as “none”. Otherwise classical physics and quantum physics could not reach same results.
    Some scientists have chosen “d” and created Copenhagen Interpretation. Superposition born and valuable determinacy is abandoned. Some others have chosen “c” but falsified scientifically with Bell’s Theorem. No one has chosen “a”. Because this was the very basic postulate that created  quantum physics after Max Planck’s Quantization of Light. No one has chosen “b” too, until today.
    This paper proposes a new interpretation by selecting “b”, presenting Uoidal Quantum Interpretation (UQI) as an alternative interpretation to Quantum Physics and an updated geometric representation for the Quantum State that can be a solution to the measurement problem and lead better understanding of probabilistic nature of the universe in quantum level.
While falsifying the ancient postulate at choice “b”, UQI keeps up with logical consistency, being definitely compatible with all the experimental results without abandoning determinacy. So, after almost 100 years  it can explain many phenomena that other interpretations can’t.
For the ease of explanation, a two dimensional experimental optical system on the linear photon polarization was presented to test this compatibility and a basic computer simulation is prepared to show the idea visually. Here, the reader will be introduced to the UQI’s main graphical element called “UOID”. Next, the approach will be generalized to three dimensional spin-½ particle transformations and measurements.
Later, as a demonstration of scientific usefulness, how a spin-½ particle’s three dimensional uoid is used for Quantum Computation will be shown.

Texas-We have reported 81.5% efficiency in converting˜600 mW of input IR to 486 nm output using a periodically poled Lithium Tantalate (PPSLT) crystal. A resonant IR enhancement cavity was used and conversion efficiency was high enough that impedance matching was possible with a 10% input coupler. The cavity had a total parasitic loss of 0.63%. Improving these results by identifying and minimizing various sources of loss can make this approach more widely applicable: for instance in the efficient conversion of input powers as low as a few mW or in the efficient conversion of wavelengths down to perhaps 300 nm. To this end we investigate the sources of our cavity loss. We find a 0.25% loss from polarization misalignment caused by the crystal. Also a 0.15% loss due to reflection from the periodically poled boundaries was measured. This refection also causes feedback to the IR source, leading to instability and mode hoping. We report on results using new PPSLT crystals with slightly tilted periodically poled slabs designed to eliminate the reflection loss mechanism and the feedback. We report on the polarization stability of these crystals, which have undergone a more careful annealing process. We have developed a sensitive technique to measure the total scattering and absorption loss within the crystals. Significantly, measurements on previous crystals indicated these losses to be negligible. We discuss this work and update the overall conversion efficiency.

Quantum cryptography or quantum key distribution (QKD) applies fundamental laws of quantum physics to guarantee secure communication. The security of quantum cryptography was proven in the last decade. Many security analyses are based on the assumption that QKD system components are idealized. In practice, inevitable device imperfections may compromise security unless these imperfections are well investigated. A highly attenuated laser pulse which gives a weak coherent state is widely used in QKD experiments. A weak coherent state has multi-photon components, which opens up a security loophole to the sophisticated eavesdropper. With a small adjustment of the hardware, we will prove that the decoy state method can close this loophole and substantially improve the QKD performance. We also propose a few practical decoy state protocols, study statistical fluctuations and perform experimental demonstrations. Moreover, we will apply the methods from entanglement distillation protocols based on two-way classical communication to improve the decoy state QKD performance. Furthermore, we study the decoy state methods for other single photon sources, such as triggering parametric down-conversion (PDC) source. Note that our work, decoy state protocol, has attracted a lot of scientific and media interest. The decoy state QKD becomes a standard technique for prepare-and-measure QKD schemes. Aside from single-photon-based QKD schemes, there is another type of scheme based on entangled photon sources. A PDC source is commonly used as an entangled photon source. We propose a model and post-processing scheme for the entanglement-based QKD with a PDC source. Although the model is proposed to study the entanglementbased QKD, we emphasize that our generic model may also be useful for other non-QKD experiments involving a PDC source. By simulating a real PDC experiment, we show that the entanglement-based QKD can achieve longer maximal secure distance than the single-photon-based QKD schemes.

We demonstrate a new source of frequency-tunable THz wave packets based on parametric down-conversion process in orientationpatterned GaAs (OP-GaAs) that produces μW-level THz average powers at the repetition rate of 100 MHz. The OP-GaAs crystal is pumped by a compact all-fiber femtosecond laser operating at the wavelength of 2 μm. Such combination of fiber laser and OP-GaAs technologies promises a practical source of THz radiation which should be suitable for many applications including imaging and spectroscopy.

We propose an optical parametric down conversion (PDC) scheme that does not suffer a trade-off between the state-purity of single-photon wave-packets and the rate of packet production. This is accomplished by modifying the PDC process by using a microcavity to engineer the density of states of the optical field at the PDC frequencies. The high-finesse cavity mode occupies a spectral interval much narrower than the bandwidth of the pulsed pump laser field, suppressing the spectral correlation, or entanglement, between signal and idler photons. Spectral filtering of the field occurs prior to photon creation rather than afterward as in most other schemes. Operator-Maxwell equations are solved to find the Schmidt-mode decomposition of the two-photon states produced. Greater than 99% pure-state packet production is predicted to be achievable.

Joint signal-idler photoelectron distributions of twin beams containing several tens of photons per mode have been measured recently. Exploiting a microscopic quantum theory for joint quasidistributions in parametric down-conversion developed earlier we characterize properties of twin beams in terms of quasidistributions using experimental data. Negative values as well as oscillating behavior in the quantum region are characteristic for the subsequently determined joint signal-idler quasidistributions of integrated intensities. Also the conditional and difference photon-number distributions are shown to be sub-Poissonian and sub-shot-noise, respectively.