Spontaneous Down-conversion Research Articles

Improving the Signal-to-Noise Ratio of Photonic Frequency Conversion from 852 nm to 1560 nm Based on a Long-Wavelength Laser-Pumped PPLN Waveguide Module

The storage wavelength of quantum nodes based on atomic systems does not match the wavelength of optical fiber communication, which requires the establishment of an efficient conversion system between flying bits and storage bits. In this paper, based on the nonlinear wavelength conversion technology of the periodically poled lithium niobate waveguide, a low-noise conversion of 852-nm photons to 1560-nm photons was achieved by a 1878-nm pump laser. The generation mechanism and transmission mechanism of noise due to nonlinear process are analyzed theoretically. The noise photons introduced by the spontaneous parameter downconversion and spontaneous Raman scattering process of a strong pump laser are experimentally studied. The noise suppression near 1560 nm is realized by the fiber Bragg grating (FBG) filter. In the experiment, when the FBG bandwidth is reduced from 0.257 nm to 0.130 nm, the signal-to-noise ratio (SNR) increases from 52 to 90. Our results show that the SNR can be greatly improved by using a narrower band filter. Therefore, the quantum node is connected to the fiber channel, and the signal can be transmitted over long distances with low loss and high fidelity.

Remotely preparing optical Schrödinger cat states via homodyne detection in nondegenerate triple-photon spontaneous downconversion

Optical downconversion is a key resource for generating nonclassical states. Very recently, direct nondegenerate triple-photon spontaneous downconversion (NTPSD) with bright photon triplets and strong third-order correlations has been demonstrated in a superconducting device (2020 Phys. Rev. X 10 011011). Besides, linear and nonlinear tripartite entanglement in this process have also been predicted (2018 Phys. Rev. Lett. 120 043601; 2020 Phys. Rev. Lett. 125 020502). In this paper, we consider the generation of nonclassical optical quantum superpositions and investigate nonlinear quantum steering effects in NTPSD. We find that large-size Schrödinger cat states of one downconverted mode can be achieved when the other two modes are subjected to homodyne detection. Also, a two-photon Bell entangled state can be generated when only one mode is homodyned. We further reveal that such ability of remote state steering originates from nonlinear quantum steerable correlations among the triplets. This is specifically embodied by the seeming violation of the Heisenberg uncertainty relation for the inferred variances of two noncommutating higher-order quadratures of downconverted modes, based on the outcomes of homodyne detection on the other mode, i.e., nonlinear quantum steering, compared to original Einstein–Podolsky–Rosen steering. Our results demonstrate non-Gaussian nonclassical features in NTPSD and would be useful for the fundamental tests of quantum physics and implementations of optical quantum technologies.

Manipulating Orbital Angular Momentum Entanglement in Three-Dimensional Spiral Nonlinear Photonic Crystals

We propose and theoretically investigate two-photon orbital angular momentum (OAM) correlation through spontaneous parameter down-conversion (SPDC) processes in three-dimensional (3D) spiral nonlinear photonic crystals (NPCs). By properly designing the NPC structure, one can feasibly modulate the OAM-correlated photon pair, which provides a potential platform to realize high-dimensional entanglement for quantum information processing and quantum communications.

Full-mode characterization of correlated photon pairs generated in spontaneous downconversion.

Spontaneous parametric downconversion is the primary source to generate entangled photon pairs in quantum photonics laboratories. Depending on the experimental design, the generated photon pairs can be correlated in the frequency spectrum, polarization, position-momentum, and spatial modes. Exploring the spatial modes' correlation has hitherto been limited to the polar coordinates' azimuthal angle, and a few attempts to study Walsh mode's radial states. Here, we study the full-mode correlation, on a Laguerre-Gauss basis, between photon pairs generated in a type-I crystal. Furthermore, we explore the effect of a structured pump beam possessing different spatial modes onto bi-photon spatial correlation. Finally, we use the capability to project over arbitrary spatial mode superpositions to perform the bi-photon state's full quantum tomography in a 16-dimensional subspace.

Phase tristability in parametric three-photon down-conversion.

This Letter shows that the parametric processes of spontaneous three-photon down-conversion is accompanied by phase tristability of the sub-harmonic signal. The oscillations of the signal in a resonant cavity are modeled through an analytically solvable second-order nonlinear oscillator. Self-sustained oscillations of the signal at a finite amplitude are found to be equally probable in three states with uniform phase contrasts. The onset of oscillations is a case of bifurcation from infinity. The stability of the ternary states is proven through an energy landscape function that identifies the attractor basins of the three states. An analogy is drawn between the oscillation threshold of a three-photon down-conversion oscillator and a first-order phase transition. The investigated phase-tristable oscillator can serve as a classical ternary bit for unconventional computing applications.

Optical Potts machine through networks of three-photon down-conversion oscillators

AbstractIn recent years, there has been a growing interest in optical simulation of lattice spin models for applications in unconventional computing. Here, we propose optical implementation of a three-state Potts spin model by using networks of coupled parametric oscillators with phase tristability. We first show that the cubic nonlinear process of spontaneous three-photon down-conversion is accompanied by a tristability in the phase of the subharmonic signal between three states with 2π/3 phase contrast. The phase of such a parametric oscillator behaves like a three-state spin system. Next, we show that a network of dissipatively coupled three-photon down-conversion oscillators emulates the three-state planar Potts model. We discuss potential applications of the proposed system for all-optical optimization of combinatorial problems such as graph 3-COL and MAX 3-CUT.

Multi-Wavelength Emission from Er-Implanted YbVO4 Crystal

The implantation of Z-cut YbVO4 single crystal is carried out with a sequence of kiloelectronvolts Er ions at room temperature, and the successful incorporation of Er3+ ions in YbVO4 matrix is supported by photoluminescence (PL) analysis under the laser excitation at 980 nm. The detailed spectroscopic analysis on the spontaneous down-conversion (DC) and up-conversion (UC) luminescence in Er-implanted YbVO4 is carried out in a wide range of excitation power. The UC luminescence is composed of blue (475 nm), green (525 and 553 nm), red (650 and 660 nm), and near-infrared (798 nm) emission bands. The spectral results reveal that the intensity of blue emission is enhanced dramatically with excitation power increase, and finally dominates the UC emissions spectrum. The quantitative studies are performed on the pumping power dependence, as well as the relative intensity ratio between representative UC emissions. The obtained results suggest that different energy transfer excitation (ET) pathways are responsible for the generations of UC luminescence signal. Herein, the possible mechanisms for the up-conversion are also proposed.

Frequency doubling and parametric fluorescence in a four-port aluminum gallium arsenide photonic chip.

In this Letter, we report on the fabrication and characterization of a monolithic III-V semiconductor photonic chip, designed to perform nonlinear parametric optical processes for frequency conversion and non-classical state generation. This chip co-integrates an AlGaAs microdisk that is evanescently coupled to two distinct suspended waveguides designed for light injection and collection around 1600 nm and 800 nm, respectively. Quasi-phase matching provided by the resonator geometry and material symmetry, resonant field enhancement, and confinement ensure efficient nonlinear interactions. We demonstrate second-harmonic generation efficiency of 5%W-1 and a biphoton generation rate of 1.2 kHz/µW through spontaneous down-conversion.

Improved quantum key distribution based on Lucas-valued orbital angular momentum states

We propose an improved quantum key distribution (QKD) protocol based on orbital angular momentum (OAM) entangled states. Three Lucas-valued OAM entangled photons are generated by two cascaded spontaneous parameter down-conversion processes. Two photons are detained and detected by Alice, while the other is transmitted to Bob and then detected. A new scheme is designed to sift the original key bits. Compared to existing protocols, we believe our protocol greatly improves coding efficiency and is verifiable. The analysis results show that our protocol is immune to the photon-number-splitting attack, the intercept-resend attack and the classical information leakage. This work provides an efficient path to high-dimensional QKD systems.

Simple experimental setup for producing polarization-entangled photons

Quantum entanglement is an important action in quantum mechanics. Basically, quantum entanglement of correlated photon pairs can be produced by spontaneous down-conversion process inside the two birefringence crystals. This work aims to create a pair of photons, i.e. signal and idler, that are entangled and to assay the relation between these photons by using polarization-entangled photon pairs to demonstrate quantum non-locality by comparing with the Malus’s law. From the experiments, the coincidence counts as a function of relative angle (α - β) between transmission axis of the polarizer and analyzer were obtained and the polarization entanglement curve was demonstrated. This result corresponding with polarization entanglement prediction term of ½ cos2 (α - β) which confirmed the entanglement of photons.

Parametric resonances and resonant delocalisation in quasi-phase matched photon-pair generation and quantum frequency conversion

The existing widely-accepted theory of photon-pair generation via spontaneous down-conversion (SPDC) in nonlinear optical crystals and waveguides is incomplete, as it fails to account for the important physical phenomenon of parametric resonances. We demonstrate that exponential gain of classical fields in the regime of parametric resonance corresponds to resonant delocalisation in the Glauber–Fock model of quantum SPDC. We propose a quantitative measure of localisation of Floquet eigen-modes as an analogue of classical gain to identify regimes of resonant delocalisation. Using this method, we are able to reconstruct the classical ‘Arnold tongues’ map of domains of instabilities for SPDC. We also predict novel regimes of resonant delocalisation in the two-level model describing quantum frequency conversion processes.

Frequency and phase relations of entangled photons observed by a two-photon interference experiment

An entangled photon experiment has been performed with a large variation of the temperature of the non-linear crystal generating the entangled pair by spontaneous downconversion. The photon pairs are separated by a nonpolarizing beamsplitter, and the polarization modes are mixed by half wave plates. The correlation function of the coincidences is studied as a function of the temperature. In the presence of a narrow interference filter we observe that the correlation changes between -1 and +1 about seven times within a temperature interval of about 30 degrees C. We show that the common simplified single-mode pair representation of entangled photons is insufficient to describe the results, but that the biphoton description that includes frequency and phase details gives close to perfect fit with experimental data for two different choices of interference filters. We explain the main ideas of the underlying physics, and give an interpretation of the two-photon amplitude which provides an intuitive understanding of the effect of changing the temperature and inserting interference filters.

Quantum Enhanced X-ray Detection

We present the first experimental demonstration of quantum-enhanced detection at x-ray wavelengths. We show that x-ray pairs that are generated by spontaneous down-conversion can be used for the generation of heralded x-ray photons and measure directly the sub-Poissonian statistics of the single photons by using photon number resolving detectors. We utilize the properties of the strong time-energy correlations of the down converted photons to demonstrate the ability to improve the visibility and the signal-to-noise ratio of an image with a small number of photons in an environment with a noise level that is higher than the signal by many orders of magnitude. In our work we demonstrate a new protocol for the measurement of quantum effects with x-rays using advantages such as background free measurements that the x-ray regime offers for experiments aiming at testing fundamental concepts in quantum optics.

Quantum Storage of Frequency-Multiplexed Heralded Single Photons.

We report on the quantum storage of a heralded frequency-multiplexed single photon in an integrated laser-written rare-earth doped waveguide. The single photon contains 15 discrete frequency modes separated by 261MHz and spanning across 4GHz. It is obtained from a nondegenerate photon pair created via cavity-enhanced spontaneous down-conversion, where the heralding photon is at telecom wavelength and the heralded photon is at 606nm. The frequency-multimode photon is stored in a praseodymium-doped waveguide using the atomic frequency comb (AFC) scheme, by creating multiple combs within the inhomogeneous broadening of the crystal. Thanks to the intrinsic temporal multimodality of the AFC scheme, each spectral bin includes 9 temporal modes, such that the total number of stored modes is about 130. We demonstrate that the storage preserves the nonclassical properties of the single photon, and its normalized frequency spectrum.

Stimulated and spontaneous down-conversion in layered media

This work continues the study of layered structures, which have proved to be useful for the efficiency of the spontaneous parametric down-conversion similarly as the quasi-phase matching. The effect of pumping is included in the reflection and the transmission of the down-converted fields and is considered this way also within the structure so that the nonlinear structure may be described similarly as a linear layered structure. The generalized complex amplitude reflectances and transmittances are utilized for expression of the input–output relations of quantum fields. These relations are approximately unitary. We express the single photon detection rate and the joint detection rate. We mention the macroscopic approach to quantization which can generate both the time-dependent Maxwell equations.

Single-photon frequency conversion via cascaded quadratic nonlinear processes

Frequency conversion of single photons is an important technology for quantum interface and quantum communication networks. Here, single-photon frequency conversion in the telecommunication band is experimentally demonstrated via cascaded quadratic nonlinear processes. Using cascaded quasi-phase-matched sum and difference frequency generation in a periodically poled lithium niobate waveguide, the signal photon of a photon pair from spontaneous down-conversion is precisely shifted to identically match its counterpart, i.e., the idler photon, in frequency to manifest a clear nonclassical dip in the Hong-Ou-Mandel interference. Moreover, quantum entanglement between the photon pair is maintained after the frequency conversion, as is proved in time-energy entanglement measurement. The scheme is used to switch single photons between dense wavelength-division multiplexing channels, which holds great promise in applications in realistic quantum networks.

On the possibility of singlet fission in crystalline quaterrylene.

Singlet fission (SF), the spontaneous down-conversion of a singlet exciton into two triplet excitons residing on neighboring molecules, is a promising route to improve organic photovoltaic (OPV) device efficiencies by harvesting two charge carriers from one photon. However, only a few materials have been discovered that exhibit intermolecular SF in the solid state, most of which are acene derivatives. Recently, there has been a growing interest in rylenes as potential SF materials. We use many-body perturbation theory in the GW approximation and the Bethe-Salpeter equation to investigate the possibility of intermolecular SF in crystalline perylene and quaterrylene. A new method is presented for determining the percent charge transfer (%CT) character of an exciton wave-function from double-Bader analysis. This enables relating exciton probability distributions to crystal packing. Based on comparison to known and predicted SF materials with respect to the energy conservation criterion (ES-2ET) and %CT, crystalline quaterrylene is a promising candidate for intermolecular SF. Furthermore, quaterrylene is attractive for OPV applications, thanks to its high stability and narrow optical gap. Perylene is not expected to exhibit SF; however, it is a promising candidate for harvesting sub-gap photons by triplet-triplet annihilation.

High-brightness single-crystal approach in quantum imaging with undetected photons

Quantum imaging with undetected photons is recently appeared promising brunch of quantum optics. Due to properties of photon pairs (signal and idler), generated via spontaneous down-conversion (SPDC) process and phenomenon of induced coherence, one can obtain phase or intensity image of object without detecting of the photon, interacting with it. The theoretical treatment and limitations of such a system with an only crystal and increased brightness are given.

Entanglement, coherence, and redistribution of quantum resources in double spontaneous down-conversion processes

We study the properties of bi-squeezed tripartite Gaussian states created by two spontaneous parametric down-conversion processes that share a common idler. We give a complete description of the quantum correlations across of all partitions, as well as of the genuine multipartite entanglement, obtaining analytical expressions for most of the quantities of interest. We find that the state contains genuine tripartite entanglement, in addition to the bipartite entanglement among the modes that are directly squeezed. We also investigate the effect of homodyne detection of the photons in the common idler mode, and analyse the final reduced state of the remaining two signal modes. We find that this measurement leads to a conversion of the coherence of the two signal modes into entanglement, a phenomenon that can be regarded as a redistribution of quantum resources between the modes. The applications of these results to quantum optics and circuit quantum electrodynamics platforms are also discussed.

Squeezing ofXwaves with orbital angular momentum

Multi-level quantum protocols may potentially supersede standard quantum optical polarization-encoded protocols in terms of amount of information transmission and security. However, for free space telecomunications, we do not have tools for limiting loss due to diffraction and perturbations, as for example turbulence in air. Here we study propagation invariant quantum X-waves with angular momentum; this representation expresses the electromagnetic field as a quantum gas of weakly interacting bosons. The resulting spatio-temporal quantized light pulses are not subject to diffraction and dispersion, and are intrinsically resilient to disturbances in propagation. We show that spontaneous down-conversion generates squeezed X-waves useful for quantum protocols. Surprisingly the orbital angural momentum affects the squeezing angle, and we predict the existence of a characteristic axicon aperture for maximal squeezing. There results may boost the applications in free space of quantum optical transmission and multi-level quantum protocols, and may also be relevant for novel kinds of interferometers, as satellite-based gravitational wave detectors.