Spontaneous Parametric Down-conversion Process Research Articles

Effects of multi-photon states in the calibration of single-photon detectors based on a portable bi-photon source.

Single-photon detectors (SPDs) are ubiquitous in many protocols for quantum imaging, sensing, and communications. Many of these protocols critically depend on the precise knowledge of their detection efficiency. A method for the calibration of SPDs based on sources of quantum-correlated photon pairs uses single-photon detection to generate heralded single photons, which can be used as a standard of radiation at the single-photon level. These heralded photons then allow for precise calibration of SPDs in absolute terms. In this work, we investigate the absolute calibration of avalanche photodiodes based on a portable, commercial bi-photon source, and investigate the effects of multi-photon events from the spontaneous parametric down conversion (SPDC) process in these sources. We show that the multi-photon character of the bi-photon source, together with system losses, has a significant impact on the achievable accuracy for the calibration of SPDs. However, modeling the expected photon counting statistics from the squeezed vacuum in the SPDC process allows for accurate estimation of the efficiency of SPDs, assuming that the system losses are known. This study provides essential information for the design and optimization of portable bi-photon sources for their application in on-site calibration of SPDs with high accuracy, without requiring any other reference standard.

Cavity-enhanced induced coherence without induced emission

This paper presents a theoretical study of the enhancement of Zou-Wang-Mandel (ZWM) interferometry through cavity-enhanced spontaneous parametric down-conversion (SPDC) processes producing frequency-entangled biphotons. The ZWM interferometry shows the capability to generate interference effects between single signal photons via indistinguishability between the entangled idler photons. This paper extends the foundational principles of ZWM interferometry by integrating cavity-enhanced SPDCs, aiming to narrow photon bandwidths for improved coherence and photon pair generation efficiency, which is critical for applications in quantum information technologies, quantum encryption, and quantum imaging. This work explores the theoretical implication of employing singly resonant optical parametric oscillators within the ZWM interferometer to produce narrow-band single photons. By combining cavity-enhanced SPDCs with ZWM interferometry, this study fills a gap in current theoretical proposals, offering significant advancements in quantum cryptography and network applications that require reliable, narrow-band single photons.

Multimode squeezed state for reconfigurable quantum networks at telecommunication wavelengths

Continuous variable encoding of quantum information requires the deterministic generation of highly correlated quantum states of light in the form of quantum networks, which, in turn, necessitates the controlled generation of a large number of squeezed modes. In this paper, we present an experimental source of multimode squeezed states of light at telecommunication wavelengths. Generation at such wavelengths is especially important as it can enable quantum information processing, communication, and sensing beyond the laboratory scale. We use a single-pass spontaneous parametric down-conversion process in a nonlinear waveguide pumped with the second harmonic of a femtosecond laser. Our measurements reveal significant squeezing in more than 21 frequency modes, with a maximum squeezing value exceeding 2.5 dB. We demonstrate multiparty entanglement by measuring the state's covariance matrix. Finally, we show the source reconfigurability by preparing few-node cluster states and measure their nullifier squeezing level. These results pave the way for a scalable implementation of continuous variable quantum information protocols at telecommunication wavelengths, particularly for multiparty, entanglement-based quantum communications. Moreover, the source is compatible with additional pulse-by-pulse multiplexing, which can be utilized to construct the necessary three-dimensional entangled structures for quantum computing protocols. Published by the American Physical Society 2024

Chirality-controlled second-order nonlinear frequency conversion in lithium niobate film metasurfaces.

The high-quality factor resonant metasurfaces have extensive applications in enhancing nonlinear frequency conversion efficiency at the subwavelength scale. However, methods for actively modulating the frequency conversion process are limited. We design a chiral lithium niobate film metasurface and investigate the photonic spin as a new degree of freedom to dynamically control the second-order nonlinear frequency conversion, without reconfiguring the structure by using external stimuli. The chiral resonance with circular dichroism (CD) of 0.62 gives rise to a high nonlinear CD of 0.84 in second-harmonic generation efficiency. Interestingly, combining the chiral resonance and an achiral quasi-bound state in the continuum enables us to investigate the photonic-spin-controlled sum-frequency generation and the photon pair generation from the spontaneous parametric downconversion process. Owing to the ultrahigh quality factor exceeding 103 both for two resonances, the second-order nonlinear frequency conversion occurs at a wavelength region of 0.2 nm, suggesting good monochromaticity. Our work opens new, to our knowledge, avenues for practical implementation of dynamically controlled nonlinear optical devices and will find utility in holography, switchable light sources, and information processing.

Noise analysis of a quasi-phase-matched quantum frequency converter and higher-order counter-propagating SPDC

Quantum frequency conversion (QFC) will be an indispensable ingredient in future quantum technologies. For example, large-scale fibre-based quantum networks will require QFC to interconnect heterogeneous building blocks like emitters, channels, memories, and detectors. The performance of existing QFC devices – typically realized in periodically poled nonlinear crystals – is often severely limited by parasitic noise that arises when the pump wavelength lies between the inter-converted wavelengths. Here we comprehensively investigate the noise spectrum of a QFC device pumped by a CW 1064 nm laser. The converter was realized as a bulk periodically poled potassium titanyl phosphate (ppKTP) crystal quasi-phase-matched for conversion between 637 nm and 1587 nm, which was also polished and coated to resonantly enhance the pump field by a factor of 50. While Raman scattering dominates the noise background from 1140 nm to 1330 nm, at larger energy shifts (beyond 60 THz), parasitic spontaneous parametric down-conversion (SPDC) is the strongest noise source. Further, the noise spectrum was contaminated by a regular succession of narrow-band peaks, which we attribute to a heretofore unidentified higher-order counter-propagating SPDC processes – with quasi-phase-matching orders up to 44 evident in our measurements. This work provides a comprehensive overview of the limiting noise sources in QFC devices that use quasi-phase-matched crystals and will prove an invaluable resource in guiding their future development.

Efficient photon-pair generation in layer-poled lithium niobate nanophotonic waveguides

Integrated photon-pair sources are crucial for scalable photonic quantum systems. Thin-film lithium niobate is a promising platform for on-chip photon-pair generation through spontaneous parametric down-conversion (SPDC). However, the device implementation faces practical challenges. Periodically poled lithium niobate (PPLN), despite enabling flexible quasi-phase matching, suffers from poor fabrication reliability and device repeatability, while conventional modal phase matching (MPM) methods yield limited efficiencies due to inadequate mode overlaps. Here, we introduce a layer-poled lithium niobate (LPLN) nanophotonic waveguide for efficient photon-pair generation. It leverages layer-wise polarity inversion through electrical poling to break spatial symmetry and significantly enhance nonlinear interactions for MPM, achieving a notable normalized second-harmonic generation (SHG) conversion efficiency of 4615% W−1cm−2. Through a cascaded SHG and SPDC process, we demonstrate photon-pair generation with a normalized brightness of 3.1 × 106 Hz nm−1 mW−2 in a 3.3 mm long LPLN waveguide, surpassing existing on-chip sources under similar operating configurations. Crucially, our LPLN waveguides offer enhanced fabrication reliability and reduced sensitivity to geometric variations and temperature fluctuations compared to PPLN devices. We expect LPLN to become a promising solution for on-chip nonlinear wavelength conversion and non-classical light generation, with immediate applications in quantum communication, networking, and on-chip photonic quantum information processing.

Optimizing biphoton generation via reconfigurable nonlinear waveguide arrays based on scattering tensor.

Spontaneous parametric down-conversion (SPDC) plays a pivotal role in the field of quantum optics, especially in generating quantum entangled states. This study delves into utilizing the scattering tensor of nonlinear waveguide arrays to characterize the SPDC process and optimize its efficiency. By computing the pseudo-inverse of the third-order scattering tensor describing the nonlinear arrays for biphoton generation, we determine the optimal pump profile and array conditions that best approximate a desired output biphoton distribution. Counterintuitively, due to the diffraction of the pump beam, we find that the optimal biphoton correlation does not occur at positions corresponding to the maximum pumping in real space within the coupled nonlinear arrays. Our finding demonstrates that the scattering tensor method combined with universal beam-splitter meshes provides a promising route toward reconfigurable integrated quantum light sources.

Avoiding lateral mode leakage in thin film lithium niobate waveguides for the generation of spectrally pure photons at telecom wavelengths

Photonic integrated optical components, notably straight waveguides, serve as pivotal elements for on-chip generation and manipulation of quantum states of light. In this work, we focus on optimizing waveguides based on lithium niobate on insulator (LNOI) to generate photon pairs at telecom wavelengths using spontaneous parametric down-conversion (SPDC). Specifically, we investigate lateral leakage for all possible SPDC processes involving type 0, type I, and type II phase matching conditions in an X-cut lithium niobate waveguide and provide a recipe to avoid leakage loss for the interacting photons. Furthermore, focusing on type II phase matching, we engineer the waveguide in the single-mode regime such that it also satisfies group index matching for generating spectrally pure single photons with high purity (99.33%). We also address fabrication imperfections of the optimized design and find that the spectral purity of the generated photons is robust to fabrication errors. This work offers guidance for the suitable selection of morphological parameters to obtain lossless, single-mode LNOI waveguides for building linear optical circuits and photon pair generation at telecom wavelengths using desired phase-matching conditions.

Recovery of polarization entanglement in partially coherent photonic qubits.

Partially coherent photonic qubits, owing to their robustness in propagation through random media compared to fully coherent qubits, find applications in free-space communication, quantum imaging, and quantum sensing. However, the reduction of spatial coherence degrades entanglement in qubits, adversely affecting entanglement-based applications. We report the recovery of entanglement in the partially coherent photonic qubits generated using a spontaneous parametric downconversion process despite retaining their multimode nature. This study utilizes an electron multiplying charge-coupled device (EMCCD) to perform coincidence measurements, eliminating the need for raster scanning of single-pixel detectors, which simplifies optical alignment, enhances precision, and reduces time consumption. We demonstrate that the size of apertures used to select biphotons substantially impacts the visibility and S-parameter of polarization-entangled partially coherent qubits. The entanglement is recovered with partial spatial coherence properties by choosing small sizes of the apertures in the captured image plane. This study could help in the advancement of free-space quantum communication, quantum imaging, and quantum metrology.

Single-pass generation of widely-tunable frequency-domain entangled photon pairs.

We demonstrate a technique that generates frequency-entangled photon pairs with strong polarization correlation by using a single-period nonlinear crystal and single pass configuration. The technique is based on the simultaneous occurrence of two spontaneous parametric down-conversion processes satisfying independent type-II collinear quasi-phase matching conditions in periodically poled stoichiometric lithium tantalate. The generated photon pairs exhibit non-degenerate Hong-Ou-Mandel interference, indicating the presence of quantum entanglement in the frequency domain. This method provides a light source capable of wide-range quantum sensing and quantum imaging or high-dimensional quantum processing.

Time alignment quantum illumination based on single real-time coincidence counting

We propose and demonstrate an improved quantum illumination protocol based on the time correlation of twin photons, for the high signal-to-noise ratio (SNR) of target detection and signal reconstruction in the strong noise environment. The Hong-Ou-Mandel (HOM) interferometer is applied after the spontaneous parametric down-conversion (SPDC) process to construct a probing twin-beam in which the photon times are precisely aligned between the beams. At the radar receiver, we put forward a single real-time coincidence counting (SRCC) method on a series of time slices to reconstruct the probe signals of pulse radar and calculate the SNR advantages against the conventional pulse radar, as well as the quantum illumination (QI) protocol. Our main achievements in this research are the realization of real-time detection of quantum information while acquiring a higher SNR than QI and classical illumination (CI) protocols, as well as its demonstration of strong robustness to noise and losses, which also proposes what we believe to be a novel way for quantum target detection.

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.

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.

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.

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.

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.

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.

Optical spin–orbit interaction in spontaneous parametric downconversion

Optical spin–orbit interaction (SOI), which can be used to simultaneously control the spin and orbital angular momentum of light, is important for both classical and quantum information applications. In linear and nonlinear optics, the SOI of light has been extensively explored in both artificial structures and conventional optical crystals. However, optical SOI in quantum nonlinear optical processes, such as spontaneous parametric downconversion (SPDC), has not been studied before. Here, we experimentally demonstrate that optical SOI in the SPDC process can be realized through a nonlinear crystal with threefold rotational symmetry. Two-photon quantum states with controlled angular momentum can be generated through the symmetry selection rules in nonlinear optics and the SOI of the pump wave. The proposed methodology may facilitate the generation and control of spin and orbital angular momentum of entangled photons.

Ultrathin quantum light source with van der Waals NbOCl2 crystal.

Interlayer electronic coupling in two-dimensional materials enables tunable and emergent properties by stacking engineering. However, it also results in significant evolution of electronic structures and attenuation of excitonic effects in two-dimensional semiconductors as exemplified by quickly degrading excitonic photoluminescence and optical nonlinearities in transition metal dichalcogenides when monolayers are stacked into van der Waals structures. Here we report a van der Waals crystal, niobium oxide dichloride (NbOCl2), featuring vanishing interlayer electronic coupling and monolayer-like excitonic behaviour in the bulk form, along with a scalable second-harmonic generation intensity of up to three orders higher than that in monolayer WS2. Notably, the strong second-order nonlinearity enables correlated parametric photon pair generation, through a spontaneous parametric down-conversion (SPDC) process, in flakes as thin as about 46 nm. To our knowledge, this is the first SPDC source unambiguously demonstrated in two-dimensional layered materials, and the thinnest SPDC source ever reported. Our work opens an avenue towards developing van der Waals material-based ultracompact on-chip SPDC sources as well as high-performance photon modulators in both classical and quantum optical technologies1-4.