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Physical Review A

On-chip engineering of high-dimensional path-entangled states in a quadratic coupled-waveguide system

Xiang-Wen Luo, Qun-Yong Zhang, Ping Xu, Rong Zhang, Huaying Liu, Changwei Sun, Yan-Xiao Gong, Zhenda Xie, Shining N. Zhu

Published Jun 24, 2019• 10 citations
Publisher

Abstract

High-dimensional quantum entangled states are a highly desired resource for their extended possibilities in quantum information processing. In this paper, we introduce on-chip engineering of a variety of path-entangled states based on the simultaneous enabling of spontaneous parametric down-conversion and efficient coupling between waveguides in a quadratic coupled-waveguide system. By varying the properties of pump beams, the phase-matching conditions, and the coupling coefficients in a three-coupled-waveguide system, it is possible to generate the deterministic and robust three-path biphoton entangled states without any postprocessing, as well as to transform the output photons among different types of path entanglement. This integrated implementation of the entanglement source will become an important building block for practical applications of on-chip quantum technologies.

DOI
10.1103/physreva.99.063833

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High-dimensional quantum entangled states are a highly desired resource for their extended possibilities in quantum information processing. In this paper, we introduce on-chip engineering of a variety of path-entangled states based on the simultaneous enabling of spontaneous parametric down-conversion and efficient coupling between waveguides in a quadratic coupled-waveguide system. By varying the properties of pump beams, the phase-matching conditions, and the coupling coefficients in a three-coupled-waveguide system, it is possible to generate the deterministic and robust three-path biphoton entangled states without any postprocessing, as well as to transform the output photons among different types of path entanglement. This integrated implementation of the entanglement source will become an important building block for practical applications of on-chip quantum technologies.
Harnessing high-dimensional entangled states of light presents a frontier for advancing quantum information technologies, from fundamental tests of quantum mechanics to enhanced computation and communication protocols. In this context, the spatial degree of freedom stands out as particularly suited for on-chip integration. But while traditional demonstrations produce and manipulate path-entangled states sequentially with discrete optical elements, continuously coupled nonlinear waveguide systems offer a promising alternative where photons can be generated and interfere along the entire propagation length, unveiling novel capabilities within a reduced footprint. Here we exploit this concept to implement a compact and reconfigurable source of path-entangled photon pairs based on parametric down-conversion in semiconductor nonlinear waveguide arrays. We use a double-pump configuration to engineer the output quantum state and implement various types of spatial correlations, exploiting a quantum interference effect between the biphoton state generated in each pumped waveguide. This demonstration, at room temperature and telecom wavelength, illustrates the potential of continuously coupled systems as a promising alternative to discrete multicomponent quantum circuits for leveraging the high-dimensional spatial degree of freedom of photons.
© 2016 WILEY-VCH Verlag GmbH & Co. KGaA. The on-chip integration of quantum light sources has enabled the realization of complex quantum photonic circuits. However, for the practical implementation of such circuits in quantum information applications, it is crucial to develop sources delivering entangled quantum photon states with on-demand tunability. Here we propose and experimentally demonstrate the concept of a widely tunable quantum light source based on spontaneous parametric down-conversion in a simple nonlinear directional coupler. We show that spatial photon-pair correlations and entanglement can be reconfigured on-demand by tuning the phase difference between the pump beams and the phase mismatch inside the structure. We experimentally demonstrate the generation of split states, robust N00N states, various intermediate regimes and biphoton steering on a single chip. Furthermore we theoretically investigate other regimes allowing all-optically tunable generation of all Bell states and flexible control of path-energy entanglement. Such wide-range capabilities of a structure comprised of just two coupled nonlinear waveguides are attributed to the intricate interplay between linear coupling and nonlinear phase matching. This scheme provides an important advance towards the realization of reconfigurable quantum circuitry. An integrated all-optically tunable source of two-photon quantum states based on spontaneous parametric down-conversion is experimentally demonstrated in a directional coupler consisting of two lithium niobate waveguides. The degree of entanglement and shape of two-photon spatial correlations is widely tunable by varying the phase mismatch and relative phases of the two pump beams. The operating principle is suitable for practical implementation of reconfigurable photon sources in on-chip quantum circuits.
Realizing an on-chip reconfigurable source of path-entangled photons is of critical importance for the advancement of quantum information processing and networking. Achieving this goal has proven challenging to date. We present an on-chip scheme for the deterministic creation of co-propagating or counter-propagating path-entangled photon pairs that can be routed in multiple configurations by tuning a classical parameter. The simplest manifestation of this approach makes use of two \textit{coupled} waveguides: a \textit{nonlinear} waveguide that produces photon pairs via spontaneous parametric downconversion from an externally incident unguided optical pump, and an auxiliary \textit{linear} waveguide. Although the photon pairs are born in only one waveguide, which alone cannot create path-entanglement, linear coupling over an extended length to the passive waveguide introduces unexpected indistinguishability that induces path-entanglement. Tuning of the classical pump spatial profile allows routing the photon pairs over all possible configurations. The proposed device is a building block for future quantum-optical networks.
Photon pairs generated by spontaneous parametric downconversion are essential for optical quantum information processing, in which the quality of biphoton states is crucial for the performance. To engineer the biphoton wave function (BWF) on-chip, the pump envelope function and the phase matching function are commonly adjusted, while the modal field overlap has been considered as a constant in the frequency range of interest. In this work, by using modal coupling in a system of coupled waveguides, we explore the modal field overlap as a new degree of freedom for biphoton engineering. We provide design examples for on-chip generations of polarization entangled photons and heralded single photons. This strategy can be applied to waveguides of different materials and structures, offering new possibilities for photonic quantum state engineering.
Abstract The generation of three‐photon states with spatial entanglement in cubic nonlinear coupled waveguides is theoretically investigated using third‐order spontaneous parametric down‐conversion and quantum walks. This approach involves independently pumping two coupled waveguides to generate a path‐encoded three‐photon Greenberger–Horne–Zeilinger (GHZ) state, which then evolves with complex spatial dynamics governed by coupling coefficients and phase mismatch. By appropriate parameter tuning, the generation of robust heralded Bell states is demonstrated, uniform states, and GHZ‐like states at the chip output. This work demonstrates an integrated source of three‐photon spatial entanglement on a simple chip, offering additional reconfigurability for advanced multiphoton quantum applications.
Harnessing high-dimensional entangled states of light presents a frontier for advancing quantum information technologies, from fundamental tests of quantum mechanics to enhanced computation and communication protocols. In this context, the spatial degree of freedom stands out as particularly suited for on-chip integration. But while traditional demonstrations produce and manipulate path-entangled states sequentially with discrete optical elements, continuously-coupled nonlinear waveguide systems offer a promising alternative where photons can be generated and interfere along the entire propagation length, unveiling novel capabilities within a reduced footprint. Here we exploit this concept to implement a compact and reconfigurable source of path-entangled photon pairs based on parametric down-conversion in semiconductor AlGaAs nonlinear waveguides arrays.