Down-conversion Process Research Articles

Monogamy relations for bipartite and tripartite entanglement via intracavity spontaneous parametric down-conversion

Quantum entanglement is an important resource for quantum information processing, quantum communication, quantum computation and quantum sensing. Hence, several proposals have focused on the generation and characterization of entanglement. Some of those proposals are based on intracavity nonlinear processes. Here, we investigate bipartite and tripartite continuous variable entanglement generated by three intracavity spontaneous parametric down-conversion processes. In the model, three beams interact with a nonlinear medium inside a resonant cavity, as well as through the nonlinear process, creating three output fields. We use the positive-P phase-space method to analyze this system and to obtain the steady-states solutions. We also calculate the intracavity fluctuation spectra and utilize different continuous-variable entanglement criteria to certify bipartite/tripartite entanglement. Our results show the frequency regime where bipartite and genuine tripartite entanglement are present in the system. For this system, we also analyze the regimes where the monogamy relations hold.

Multimode Luminescent Properties from Tb3+-Yb3+ Codoped CaMoO4 for Solar Cell Spectral Conversion

NIR and visible emitting CaMoO4:Tb3+/Tb3+-Yb3+ phosphors were synthesized to harness the energy losses attribute to wavelength mismatch between response spectrum of incident solar photons and solar cells. The X-ray diffraction (XRD), field emission scanning microscopy (FE-SEM), fourier transform infrared spectroscopy (FTIR), UV–vis diffuse reflectance spectroscopy, photoluminescence (PL), up-conversion and life time measurements were used to characterized the prepared phosphors. XRD study followed by Rietveld refinement confirms the tetragonal crystal structure with space group I41a(88). The phosphors exhibit strong green emission (5D4→7F4) in visible region after an excitation wavelengths 378 nm and 980 nm, respectively through down-shifting (DS) and up-conversion (UC) processes. The UV photons (378 nm) are also split it into two NIR photons (2F7/2→2F5/2) through down-conversion process. Fluorescence quenching was observed by increasing concentration of Tb3+ and Yb3+ ions. Lifetime measurements clearly illustrates the energy transfer from Tb3+ to Yb3+ ions and mechanism involved is co-operative energy transfer. The estimated values of energy transfer efficiency (ETE) and corresponding theoretical internal quantum efficiency are achieved as 69% and 169% respectively.

Magnetic Field Effects in Triplet-Triplet Annihilation Upconversion: Revisiting Atkins and Evans' Theory.

In their seminal description of magnetic field effects on chemiluminescent fluid solutions, Atkins and Evans considered the spin-dependent interactions between two triplets, incorporating the effects of the diffusion of the molecules in the liquid phase. Their results, crucial for the advancement of photochemical upconversion, have received renewed attention due to the increasing interest in triplet-triplet annihilation for photovoltaic and optoelectronic applications. Here we revisit their approach, using a modern formulation of open quantum system dynamics and extend their results. We provide corrections to the theory of the magnetic field response of the fluorescent triplet pair state with singlet multiplicity. These corrections are timely, as improvements in the precision and range of available experimental methods are supported by the determination of quantitatively accurate rotational and interaction model parameters. We then extend Atkins and Evans' theory to obtain the magnetic field response of triplet pair states with triplet and quintet multiplicity. Although these states are not optically active, transitions between them are becoming imperative to study the working mechanism of spin-mediated upconversion and downconversion processes, thanks to advances in electron spin resonance and time-resolved transient absorption spectroscopy.

Compact generation scheme of path–frequency hyperentangled photons using 2D periodical nonlinear photonic crystal

Hyperentanglement is a promising resource for achieving high capacity quantum communication. Here, we propose a compact scheme for the generation of path–frequency hyperentangled photon pairs via spontaneous parametric down-conversion (SPDC) processes, where six different paths and two different frequencies are covered. A two-dimensional periodical χ (2) nonlinear photonic crystal (NPC) is designed to satisfy type-I quasi-phase-matching conditions in the plane perpendicular to the incident pump beam, and a perfect phase match is achieved along the pump beam’s direction to ensure high conversion efficiency, with theoretically estimated photon flux up to 2.068 × 105 pairs⋅s−1⋅mm−2. We theoretically calculate the joint-spectral amplitude (JSA) of the generated photon pair and perform Schmidt decomposition on it, where the resulting entropy S of entanglement and effective Schmidt rank K reach 3.2789 and 6.4675, respectively. Our hyperentangled photon source scheme could provide new avenues for high-dimensional quantum communication and high-speed quantum information processing.

Feature-enhanced X-ray imaging using fused neural network strategy with designable metasurface

Abstract Scintillation-based X-ray imaging can provide convenient visual observation of absorption contrast by standard digital cameras, which is critical in a variety of science and engineering disciplines. More efficient scintillators and electronic postprocessing derived from neural networks are usually used to improve the quality of obtained images from the perspective of optical imaging and machine vision, respectively. Here, we propose to overcome the intrinsic separation of optical transmission process and electronic calculation process, integrating the imaging and postprocessing into one fused optical–electronic convolutional autoencoder network by affixing a designable optical convolutional metasurface to the scintillator. In this way, the convolutional autoencoder was directly connected to down-conversion process, and the optical information loss and training cost can be decreased simultaneously. We demonstrate that feature-specific enhancement of incoherent images is realized, which can apply to multi-class samples without additional data precollection. Hard X-ray experimental validations reveal the enhancement of textural features and regional features achieved by adjusting the optical metasurface, indicating a signal-to-noise ratio improvement of up to 11.2 dB. We anticipate that our framework will advance the fundamental understanding of X-ray imaging and prove to be useful for number recognition and bioimaging applications.

Pump-tailored alternative Bell state generation in the first-order Hermite–Gaussian basis

We demonstrate entangled-state swapping, within the Hermite–Gaussian (HG) basis of first-order modes, directly from the process of spontaneous parametric downconversion within a nonlinear crystal. The method works by explicitly tailoring the spatial structure of the pump photon such that it resembles the product of the desired entangled spatial modes exiting the crystal. Importantly, the result is an entangled state of balanced HG modes, which may be beneficial in applications that depend on symmetric accumulations of geometric phase through optics or in applications of quantum sensing and imaging with azimuthal sensitivity. Furthermore, the methods are readily adaptable to other spatial mode bases.

Down- and up-conversion luminescence processes in olivine-type ceramic phosphors Li2AGeO4:Er3+ (A = Zn, Mg)

Luminescence properties of olivine-type germanate phosphors with general formula Li2AGeO4:0.5Er3+ (A = Zn, Mg) have been studied. Olivine-type materials prepared via conventional solid-state reaction method were examined for down- and up-conversion luminescence processes. In the down-conversion process after 381 nm excitation, two strong green emission bands due to 2H11/2/4S3/2 → 4I15/2 transitions of Er3+ were registered, with a change in the intensity ratio depending on the ceramic matrix. Weak red emission bands related to 4F9/2 → 4I15/2 transition were also recorded. In contrast, luminescence bands behaved completely differently for the up-conversion process after infrared excitation at λexc = 980 nm. Red emission bands were much more significant than green emission bands. These effects are stronger for Li2MgGeO4:0.5Er3+ than Li2ZnGeO4:0.5Er3+. In the red and green spectral range, slope values for both matrices Li2AGeO4:0.5Er3+ (A = Zn, Mg) confirmed two-photon process.

A cascaded dual-wavelength laser based on an Er3+-doped fluorozirconate glass microsphere resonator

Cascaded dual-wavelength lasing at 0.85 μm and 1.55 μm from an Er3+-doped ZBYA fluorozirconate glass microsphere is demonstrated for the first time. Two wavelengths corresponding to the transitions of Er3+: 4S3/2 → 4I13/2 and 4I13/2 → 4I15/2 are generated due to up-conversion and down-conversion processes respectively. Cascade lasing from ZBYA microsphere resonators with various Er3+ doping concentrations and diameters are experimentally studied by utilizing a 980 nm laser diode as the pump source. The influence of concentration and diameter on up-conversion and down-conversion lasers are investigated. The results indicate that Er3+-doped ZBYA fluorozirconate glass microspheres are promising candidates for practical applications requiring multi-wavelength micro-lasers and in optical communications.

Bipartite and tripartite steering by a nonlinear medium in a cavity

Nonlocal quantum correlations are important for potential applications in quantum optics and quantum information. Multipartite quantum correlations are relevant to access to high-dimensional systems such as qudits. There are also different proposals for the generation of quantum steering based on nonlinear systems and cavities. Here, we consider the Hamiltonian of a triple photon parametric down-conversion process to investigate the regions where bipartite and tripartite steering are generated. In this model, three beams interact with a nonlinear medium inside a cavity through the process of parametric down-conversion, creating three output fields. The positive P representation is used to analyze this system and steady-state solutions are obtained. We calculated the intracavity fluctuation spectra to analyze steering in the frequency domain and used different criteria to certify this quantum correlation. We certified bipartite steering for the three output fields from the cavity. In the case of full tripartite two-way steering inseparability, it is also present for all the bipartitions under consideration. Our results show regimes where bipartite and tripartite steering is certified for this model.

Studying the structural and optical properties of Er3+ doped TiO2 powders synthesized by the sol-gel process

A series of TiO2:xEr powders (with x from 0% to 1% at.) was synthesised by a simple sol–gel process. The structural and optical properties were studied in detail using diffusion reflection spectroscopy, x-ray diffraction, Raman spectroscopy, high-resolution transmission electron microscopy, and energy-dispersive x-ray spectroscopy, to systematically determine the band edge, morphology, and structural properties of both undoped and Er-doped TiO2 samples. Notably, the introduction of Er dopants in the TiO2 matrix results in a decrease in both the band gap and crystallite size as well as the anatase phase of host matrix TiO2 remained in the anatase phase even at 800 °C. The heat treatment temperature has an insignificant effect on the green/red emission ratio of Er ions. However, this ratio strongly depends on excitation wavelengths corresponding to down-conversion and up-conversion processes. The colour emission of TiO2:Er is easily tunable from green to yellow under different excitations. The emission mechanism of TiO2:Er is proposed based on spectroscopy techniques. These findings reveal that the TiO2:Er powders are potential materials for labelling and photocatalyst applications.

Unfolding the Hong–Ou–Mandel interference between heralded photons from narrowband twin beams

The Hong–Ou–Mandel (HOM) interference is one of the most intriguing quantum optical phenomena and crucial in performing quantum optical communication and computation tasks. Lately, twin beam emitters such as those relying on the process of parametric down-conversion (PDC) have become confident sources of heralded single photons. However, if the pump power is high enough, the pairs produced via PDC—often called signal and idler—incorporate multiphoton contributions that usually distort the investigated quantum features. Here, we derive the temporal characteristics of the HOM interference between heralded states from two independent narrowband PDC sources. Apart from the PDC multiphoton content, our treatment also takes into account effects arriving from an unbalanced beam splitter ratio and optical losses. We perform a simulation in the telecommunication wavelength range and provide a useful tool for finding the optimal choice for PDC process parameters. Our results offer insight in the properties of narrowband PDC sources and turn useful when driving quantum optical applications with them.

On the role of optical materials in the realization of schemes for secure quantum communication

We introduce a prescription using optical materials to perform a quantum sealed-bid auction using an n-partite cluster state, produced using an optical parametric oscillator (OPO) through the process of spontaneous parametric down-conversion (SPDC) and an array of Beam splitters whose properties define the amount of correlation in the corresponding state. We further establish the role of optical materials in performing quantum communication protocols and analyse the effect of noise on the state.

Green emission characteristics of Er3+ -doped TeO2-WO3-GeO2 glasses through up and down conversion processes

Different concentrations of Er3+ions doped TeO2-WO3-GeO2 transparent oxyfluoride glassy materials were fabricated by conventional melt quench method and analyzed through X-ray diffraction, Fourier Transform infrared, scanning electron microscope and energy dispersive X-ray, photoluminescence-excitation, visible emission and luminescence decay studies. Various physical, structural, spectroscopic and laser characteristic parameters were evaluated following the Judd-Ofelt theory. The studied glasses were capable of emitting intense green luminescence through (2H11/2,4S3/2) → 4I15/2 transitions. The emission of green luminescence was studied through down-conversion process at 377 nm excitation and up-conversion process at 980 nm excitation. The color coordinates were found to fall in the green region of chromaticity diagram and close to the European Broadcosting Union green illuminant. The evaluated branching ratio, stimulated emission cross-section, gain linewidth, figure of merit and the quantum efficiency corresponding to 4S3/2 → 4I15/2 transition suggests that the studied glasses find wide suitability to design green emitting solid state lighting devices.

Experimental generation and characterization of partially spatially coherent qubits

Partially spatially coherent qubits are more immune to turbulent atmospheric conditions than coherent qubits, which makes them excellent candidates for free-space quantum communication. In this article, we report the generation of partially spatially coherent qubits in a spontaneous parametric down-conversion (SPDC) process using a Gaussian Schell model (GSM) pump beam. For this non-linear process, we demonstrate experimentally for the first time, the transfer of spatial coherence features of the pump (classical) to the biphotons (quantum) field. Also, the spatial profiles of partially coherent qubits generated in type-I and type-II non-collinear SPDC process are experimentally observed and multi-mode nature of partially coherent photons (qubit) is ascertained. These investigations pave the way toward the efficient generation of partially spatially coherent qubits with a tunable degree of spatial coherence, which lead to wide range of applications in frontier areas such as quantum cryptography, teleportation, imaging, and lithography.

Polarization Entanglement from Parametric Down-conversion with an LED Pump

Spontaneous parametric down-conversion (SPDC) is a reliable platform for entanglement generation. Routinely, a coherent laser beam is an essential prerequisite for pumping the nonlinear crystal. Here we break this barrier to generate polarization-entangled photon pairs by using a commercial light-emitting diode (LED) source to serve as the pump beam. This effect is counterintuitive, as the LED source is of extremely low spatial coherence, which is transferred during the down-conversion process to the biphoton wave function. However, the type-II phase matching condition naturally filters the specific frequency and wavelength of LED light exclusively to participate in SPDC such that localized polarization Bell states can be generated, regardless of the global incoherence over the full transverse plane. In our experiment, we characterize the degree of LED light-induced polarization entanglement in the standard framework of the violation of Bell's inequality. We achieve the Bell value $S=2.33\ifmmode\pm\else\textpm\fi{}0.097$, obviously surpassing the classical bound $S=2$ and thus witnessing the quantum entanglement. Our work can be extended to prepare polarization entanglement by using other natural light sources, such as sunlight and biolight, which holds promise for electricity-free quantum communications in outer space.

Generation of hyperentangled photon pairs based on lithium niobate waveguide

Generation of hyperentangled photon pairs is investigated based on the lithium niobate straight waveguide. We propose to use the nonlinear optical process of spontaneous parametric down-conversion (SPDC) and a well-designed lithium niobate waveguide structure to generate a hyperentangled (in the polarization dimension and the energy-time dimension) two-photon state. By performing numerical simulations of the waveguide structure and calculating the possible polarization states, joint spectral amplitudes (JSA), and joint temporal amplitudes (JTA) of the generated photon pair, we show that the generated photon pair is indeed hyperentangled in both the polarization dimension and the energy-time dimension.

Spatial-spectral mapping to prepare frequency entangled qudits.

Entangled qudits, the high-dimensional entangled states, play an important role in the study of quantum information. How to prepare entangled qudits in an efficient and easy-to-operate manner is still a challenge in quantum technology. Here, we demonstrate a method to engineer frequency entangled qudits in a spontaneous parametric downconversion process. The proposal employs an angle-dependent phase-matching condition in a nonlinear crystal, which forms a classical-quantum mapping between the spatial (pump) and spectral (biphotons) degrees of freedom. In particular, the pump profile is separated into several bins in the spatial domain, and thus shapes the down-converted biphotons into discrete frequency modes in the joint spectral space. Our approach provides a feasible and efficient method to prepare a high-dimensional frequency entangled state. As an experimental demonstration, we generate a three-dimensional entangled state by using a homemade variable slit mask.

Passively CEP stable sub-2-cycle source in the mid-infrared by adiabatic difference frequency generation.

We report on the generation of a passive carrier-envelope phase (CEP) stable 1.7-cycle pulse in the mid-infrared by adiabatic difference frequency generation. With sole material-based compression, we achieve a sub-2-cycle 16-fs pulse at a center wavelength of 2.7 µm and measured a CEP stability of <190 mrad root mean square. The CEP stabilization performance of an adiabatic downconversion process is characterized for the first time, to the best of our knowledge.

Continuous variable spin-orbit total angular momentum entanglement on the higher-order Poincaré sphere.

Optical spin-orbit coupling is an important phenomenon and has fruitful applications. Here, we investigate the spin-orbit total angular momentum entanglement in the optical parametric downconversion process. Four pairs of entangled vector vortex modes are experimentally generated directly using a dispersion- and astigmatism-compensated single optical parametric oscillator, and for the first time, to the best of our knowledge, the spin-orbit quantum states are characterized on the quantum higher-order Poincaré sphere, and the relationship of spin-orbit total angular momentum Stokes entanglement is demonstrated. These states have potential applications in high-dimensional quantum communication and multiparameter measurement.

Resonant radiation emitted by solitary waves via cascading in quadratic media.

We present a systematic investigation of the resonant radiation emitted by localized soliton-like wave-packets supported by second-harmonic generation in the cascading regime. We emphasize a general mechanism which allows for the resonant radiation to grow without the need for higher-order dispersion, primarily driven by the second-harmonic component, while radiation is also shed around the fundamental-frequency component through parametric down-conversion processes. The ubiquity of such a mechanism is revealed with reference to different localized waves such as bright solitons (both fundamental and second-order), Akhmediev breathers, and dark solitons. A simple phase matching condition is put forward to account for the frequencies radiated around such solitons, which agrees well with numerical simulations performed against changes of material parameters (say, phase mismatch, dispersion ratio). The results provide explicit understanding of the mechanism of soliton radiation in quadratic nonlinear media.