Spontaneous Parametric Down-conversion Process Research Articles

Experimental realization of more quantum randomness generation based on non-projective measurement

Quantum random numbers are fundamental resources (Rev. Mod. Phys. 89 015004) for the fields of communications and information security. Usually, projective measurements can be applied on a two-qubit entangled state to generate quantum random numbers. However, it cannot certify more than one bit of randomness for one set of measurements. On contrast, non-projective measurements in-principle may be able to generate more random bits, and therefore show some merits among present methods. Here, we report the implementation of a quantum random number generator based on a symmetric informationally complete positive operator-valued measure. In our experiment, we prepare a maximally entangled state of two-qubits through spontaneous parametric down-conversion processes, and observe the violation of Gisin’s elegant Bell inequality with the value of 6.8138 ± 0.0822, exceeding the classical bound of 6. Through randomness extractors, including a Toeplitz extractor and a parallel linear feedback shift register based extractor, we finally extract a random string with an autocorrelation coefficient below 10−3 and the string can pass all randomness tests of the National Institute of Science and Technology (NIST).

Interference effects in quantum-optical coherence tomography using spectrally engineered photon pairs

Optical-coherence tomography (OCT) is a technique that employs light in order to measure the internal structure of semitransparent, e.g. biological, samples. It is based on the interference pattern of low-coherence light. Quantum-OCT (QOCT), instead, employs the correlation properties of entangled photon pairs, for example, generated by the process of spontaneous parametric downconversion (SPDC). The usual QOCT scheme uses photon pairs characterised by a joint-spectral amplitude with strict spectral anti-correlations. It has been shown that, in contrast with its classical counterpart, QOCT provides resolution enhancement and dispersion cancellation. In this paper, we revisit the theory of QOCT and extend the theoretical model so as to include photon pairs with arbitrary spectral correlations. We present experimental results that complement the theory and explain the physical underpinnings appearing in the interference pattern. In our experiment, we utilize a pump for the SPDC process ranging from continuous wave to pulsed in the femtosecond regime, and show that cross-correlation interference effects appearing for each pair of layers may be directly suppressed for a sufficiently large pump bandwidth. Our results provide insights and strategies that could guide practical implementations of QOCT.

Spatially entangled photon-pair generation using a partial spatially coherent pump beam

We demonstrate experimental generation of spatially-entangled photon-pairs by spontaneous parametric down conversion (SPDC) using a partial spatially coherent pump beam. By varying the spatial coherence of the pump, we show its influence on the downconverted photon's spatial correlations and on their degree of entanglement, in excellent agreement with theory. We then exploit this property to produce pairs of photons with a specific degree of entanglement by tailoring of the pump coherence length. This work thus unravels the fundamental transfer of coherence occuring in SPDC processes, and provides a simple experimental scheme to generate photon-pairs with a well-defined degree of spatial entanglement, which may be useful for quantum communication and information processing

Multi-Path Ghost Imaging by Means of an Additional Time Correlation**Supported by the National Key R&D Program of China under Grant Nos 2017YFA0303800 and 2017YFA0303700, the National Natural Science Foundation of China under Grant Nos 11534006, 11774183 and 11674184, and the Collaborative Innovation Center of Extreme Optics.

Ghost imaging functions achieved by means of the spatial correlations between two photons is a new modality in imaging systems. With a small number of photons, ghost imaging is usually realized based on the position correlation of photon pairs produced from the spontaneous parametric down-conversion process. Here we demonstrate a way to realize multi-path ghost imaging by introducing an additional time correlation. Different delays of paths will induce the shift of the coincidence peak, which carries the information about objects. By choosing the suitable coincidence window, we obtain images of three objects simultaneously, with a visibility of 87.2%. This method provides insights and techniques into multi-parameter ghost imaging. It can be applied to other correlated imaging systems, for example, quantum spiral imaging.

Compact generation of a two-photon multipath Dicke state from a single χ(2) nonlinear photonic crystal.

Multipartite quantum entanglement is a powerful resource for enriching the functionality of quantum computation and quantum communication. In this Letter, we propose a new method to generate a two-photon multipath Dicke state with concurrent spontaneous parametric downconversion processes from a single periodically poled nonlinear photonic crystal. We design the poling structure to produce a three-path Dicke state where three quasi-phase-matching conditions are fulfilled simultaneously by a hybrid one- and two-dimensionally poled nonlinear photonic crystal. We use genuine multipartite entanglement concurrence to quantify the entanglement of the Dicke state. Using a more complicated poling configuration like multiple-periodically poled two-dimensional nonlinear photonic crystal, we can also produce four-path, five-path, or multipath Dicke states by a single crystal. The multiple-periodically poled two-dimensional nonlinear photonic crystal provides a new method, to the best of our knowledge, for the integrated generation of multipartite quantum light sources.

Photonic crystal waveguides as sources of counterpropagating factorizable biphoton states.

We demonstrate numerically that photonic crystal slab waveguides can generate spectrally unentangled biphoton states, highly desired for heralding of single photons. We achieve this by modally phase matching a counterpropagating spontaneous parametric down-conversion process, in a fully integrated scheme and without the need for periodic poling. Such a configuration is an ideal integrated source of heralded single photons, as it spatially separates the photons of a pair at the source without any extra components, while allowing for generation of spectrally narrow photons on a very short length scale.

Second harmonic generation and spontaneous parametric down-conversion in Mie nanoresonators

The recent active research in the field of nonlinear Mie nanoresonators has led to significant increase of harmonic generation efficiency from subwavelength structures. The progress in the second order nonlinear processes became evident after switching to nanoresonators made of materials that possess a bulk nonlinearity such as semiconductor and perovskite-type materials. Despite of that, the theory of second-harmonic generation from spherical Mie nanoparticles with a bulk nonlinearity has not been developed yet. In this work, we present a theoretical approach based on Mie theory for describing the second order nonlinear processes in all-dielectric nanoresonators. We focus on the second harmonic generation (SHG) process as well as on the inverse one, the process of spontaneous parametric down-conversion (SPDC). We demonstrate the SHG-SPDC correspondence in terms of Mie resonances and identify the selection rules governing these second order nonlinear processes. We show how one can control the spatial correlations of photons in SPDC from a single Mie-nanoparticle by properly choosing the parameters of the system, in particular, allowing for Kerker-type unidirectional emission of photons.

Generation and concentration of two-photon partially entangled Knill–Laflamme–Milburn state

The protocol for the compact generation of the two-photon four-dimensional spatial-mode partially entangled Knill–Laflamme–Milburn (KLM) state is proposed. The two-photon entanglement shared among four spatially distinct optical modes can be generated from three concurrent spontaneous parametric down-conversion processes by using the multiple quasi-phase-matching technique. The scheme for the concentration of the two-photon four-dimensional spatial-mode partially entangled KLM state is also proposed. By using quantum non-destructive detection and local operation, the two-photon maximally entangled KLM state can be obtained and used as an entangled quantum channel in quantum communication.

Counterpropagating path-entangled photon pair sources based on simultaneous spontaneous parametric down-conversion processes of nonlinear photonic crystal.

Ultra-bright source of entangled photons is an essential component in optical quantum information processing. Here we propose a counterpropagating path-entangled photon pair sources using a quasi-periodic modulated lithium niobate crystal. The nonlinear crystal designed by a dual-grid method, simultaneously phase-matched two spontaneous parametric down-conversion processes. Signal and idler modes have opposite propagation directions in the two spontaneous parametric down-conversion processes, which is the key to generating path-entangled photon pairs. Compared to copropagating entangled sources, the counterpropagating path-entangled sources result in a much narrower spectrum. The quantum state of the path-entanglement source is not only suited for quantum coding, but also to allow the implementation of complex quantum algorithms on a photonic chip.

Manipulation of the spontaneous parametric down-conversion process in space and frequency domains via wavefront shaping.

The spontaneous parametric down-conversion (SPDC) source of entangled-photon pairs is important for applications in the quantum information process and quantum communication, but suffers from scattering by sample defect and air impurity. Here, we proposed an alternative scheme to manipulate the scattered SPDC process, where only a spatial light modulator was used to control the incident wavefront. The scheme was experimentally tested and also applied on the manipulation of photon pairs through the SPDC process with spectral control. This work proved the feasibility of manipulating nonlinear signals at quantum level with feedback-based wavefront shaping and also indicated applications in long-distance quantum key distribution, quantum communications, and quantum imaging, especially in complex environments.

On four-photon entanglement from parametric down-conversion process

We propose two schemes to generate four-photon polarization-entangled states from the second-order emission of the spontaneous parametric down-conversion process. By using linear optical elements and the coincidence-detection, the four indistinguishable photons emitted from parametric down-conversion source result in the Greenberger-Horne-Zeilinger (GHZ) state or the superposition of two orthogonal GHZ states. For this superposition state, under particular phase settings we analyze the quantum correlation function and the local hidden variable (LHV) correlation. As a result, the Bell inequality derived from the LHV correlation is violated with the visibility larger than 0.442. It means that the present four-photon entangled state is therefore suitable for testing the LHV theory.

CHSH inequality test via disturbance-free measurement

We propose disturbance-free measurement (DFM) using a ‘weak-value’ scheme, in which a weakly measured quantum system is post-selected (to the initial state) to confirm that there is no disturbance. The probability of obtaining the non-disturbed state is asymptotically close to unity. We theoretically show that outcomes of the DFM for a two-qubit state satisfy the Clauser–Horne–Shimony–Holt inequality. We experimentally demonstrate the test for a typical (maximally entangled) two-qubit state based on a linear optical system. In experiments, polarization-entangled photon-pairs generated by the spontaneous parametric down-conversion process are measured by instruments such as strength-variable polarization-measurement apparatuses and a fiber-based Bell-state analyzer.

Steps toward the experimental realization of surface plasmon polariton enhanced spontaneous parametric down-conversion

The realization of efficient and miniature source of entangled photon pairs stills remains a challenge. In this work, we experimentally studied the possibility to enhance the process of spontaneous parametric down-conversion by surface plasmon polaritons. The details of the experimental setup will be discussed along with theoretical calculations of the enhancement factor.

Tunable entanglement distillation of spatially correlated down-converted photons.

We report on a new technique for entanglement distillation of the bipartite continuous variable state of spatially correlated photons generated in the spontaneous parametric down-conversion process (SPDC), where tunable non-Gaussian operations are implemented and the post-processed entanglement is certified in real-time using a single-photon sensitive electron multiplying CCD (EMCCD) camera. The local operations are performed using non-Gaussian filters modulated into a programmable spatial light modulator and, by using the EMCCD camera for actively recording the probability distributions of the twin-photons, one has fine control of the Schmidt number of the distilled state. We show that even simple non-Gaussian filters can be finely tuned to a ∼67% net gain of the initial entanglement generated in the SPDC process.

Direct generation of hybrid entangled photon pairs in waveguides

In this paper, we propose a scheme for direct generation of hybrid spatial modal–polarization entangled photon pairs using a type-II spontaneous parametric down-conversion process and the electro-optic effect in a domain engineered potassium titanyl phosphate nonlinear waveguide. Such hybrid entangled states must find applications in quantum information processing using integrated photonic circuits.

A graphical method in quantum optics

In this paper we describe in detail a graphical method allowing the computation of field operator transformations in quantum optics (QO). Its applications include beam splitters (BS), Mach-Zehnder interferometers (MZI), optical resonators (Fabry–Perot etc) as well as non-linear crystals featuring the process of spontaneous parametric down-conversion (SPDC). Its main advantage compared to the traditional computation step-by-step method is its visual and intuitive approach, somehow similar to Feynman’s diagrammatic approach in Quantum Field Theory. It also seems adapted to computer-based implementations since calculations mainly consist on complex additions and multiplications, not matrix operations.

Measurement-device-independent quantum key distribution with multiple crystal heralded source with post-selection

The multiple crystal heralded source with post-selection (MHPS), originally introduced to improve the single-photon character of the heralded source, has specific applications for quantum information protocols. In this paper, by combining decoy-state measurement-device-independent quantum key distribution (MDI-QKD) with spontaneous parametric downconversion process, we present a modified MDI-QKD scheme with MHPS where two architectures are proposed corresponding to symmetric scheme and asymmetric scheme. The symmetric scheme, which linked by photon switches in a log-tree structure, is adopted to overcome the limitation of the current low efficiency of m-to-1 optical switches. The asymmetric scheme, which shows a chained structure, is used to cope with the scalability issue with increase in the number of crystals suffered in symmetric scheme. The numerical simulations show that our modified scheme has apparent advances both in transmission distance and key generation rate compared to the original MDI-QKD with weak coherent source and traditional heralded source with post-selection. Furthermore, the recent advances in integrated photonics suggest that if built into a single chip, the MHPS might be a practical alternative source in quantum key distribution tasks requiring single photons to work.

Scalable symmetry detector and its applications by using beam splitters and weak nonlinearities

We describe a method to detect twin-beam multiphoton entanglement based on a beam splitter and weak nonlinearities. For the twin-beam four-photon entanglement, we explore a symmetry detector. It works not only for collecting two-pair entangled states directly from the spontaneous parametric down-conversion process, but also for generating them by cascading these symmetry detectors. Surprisingly, by calculating the iterative coefficient and the success probability we show that with a few iterations the desired two-pair can be obtained from a class of four-photon entangled states. We then generalize the symmetry detector to n-pair emissions and show that it is capable of determining the number of the pairs emitted indistinguishably from the spontaneous parametric down-conversion source, which may contribute to explore multipair entanglement with a large number of photons.

Anisotropic narrowing of biphoton wave packet distribution in spontaneous parametric down-conversion with biaxial crystal

We investigate theoretically the strong anisotropy in the wave packet distribution of biphoton pairs generated via spontaneous parametric down-conversion (SPDC) process with biaxial crystals. From the angle relationship in biaxial crystals, we deduce the wave function of type I SPDC by considering the longitudinal detuning under paraxial approximation. Taking BiB3O6 (BIBO) as an example, we numerically simulate the single-particle and coincidence distributions in two observation planes under the optimum phase-matching scheme. In contrast to uniaxial crystals, a stronger anisotropy effect and a higher entanglement of the photon pair states are obtained, which are attributed to the complex structure of biaxial crystals. These results are important for the generation of highly entangled biphoton pairs, especially for multi-photon pairs generation.

Luminescence in germania-silica fibers in a 1-2μm region.

We analyze the origins of the luminescence in germania-silica fibers with high germanium concentration (about 30mol.% GeO2) in the region of 1-2μm with a laser pump at the wavelength 532nm. We show that such fibers demonstrate the high level of luminescence which unlikely allows the observation of photon triplets, generated in a third-order spontaneous parametric downconversion process in such fibers. The only efficient approach to the luminescence reduction is the hydrogen saturation of fiber samples; however, even in this case, the level of residual luminescence is still too high for three-photon registration.