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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Generation of Spatially Entangled States of Light in Nonlinear Waveguide Arrays

Arnault Raymond, Saverio Francesconi, José M. Palomo, Pascal G. Filloux, Martina Morassi, Aristide Lemaître, Fabrice Raineri, Maria I. Amanti, S. Ducci, F. Baboux

Published Jun 26, 2023• 2 citations
Publisher

Abstract

High-dimensional degrees of photons – such as orbital angular momentum, spatial modes of frequency – provide a powerful mean to increase the density and security of quantum communication, and to enhance flexibility in quantum computing. In particular, the spatial degree of freedom is particularly suited to on-chip integration. Rapid progress has been made in recent years to develop integrated circuits achieving on-chip quantum interference, entanglement and gate operations on spatially encoded states, culminating in the demonstration of the Boson Sampling task on-chip [1]. While many of these demonstrations have relied on external sources to generate quantum states of light, which were then fed into a passive circuitry, a next step has been taken recently with the demonstration of active chips combining the generation and manipulation of spatially encoded states [2].

DOI
10.1109/cleo/europe-eqec57999.2023.10231817

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High-dimensional degrees of photons – such as orbital angular momentum, spatial modes of frequency – provide a powerful mean to increase the density and security of quantum communication, and to enhance flexibility in quantum computing. In particular, the spatial degree of freedom is particularly suited to on-chip integration. Rapid progress has been made in recent years to develop integrated circuits achieving on-chip quantum interference, entanglement and gate operations on spatially encoded states, culminating in the demonstration of the Boson Sampling task on-chip [1]. While many of these demonstrations have relied on external sources to generate quantum states of light, which were then fed into a passive circuitry, a next step has been taken recently with the demonstration of active chips combining the generation and manipulation of spatially encoded states [2].
In the last years the increasing complexity of quantum information protocols triggered the development of integrated waveguide technology for quantum applications [1-3] such that stability is intrinsically obtained thanks to the combination of many optical elements on a monolithic chip and scalability is potentially achievable. In this context the realization of integrated sources of quantum light is of fundamental interest. Some experimental implementations using parametric down conversion in nonlinear materials [4], as well as in semiconductors [5] have been reported. However, in the landscape of integrated devices, the realization of one important source of entangled states is still missing: a source of spectrally indistinguishable photon pairs emitted on two different spatial modes, which share entanglement in the polarization degree of freedom without the need of post-selection. This source is widely adopted in bulk setups [6], indeed entanglement is a fundamental requirement for quantum information purposes and the polarization of photons is the most used degree of freedom for quantum information encoding. However the implementation of such a scheme in integrated waveguides is a challenging task due to the combination of orthogonal polarizations and the need of two different spatial modes.
Abstract High-dimensional quantum entanglement is an important resource for emerging quantum technologies such as quantum communication and quantum computation. The scalability of metres-long experimental setups limits high-dimensional entanglement in bulk optics. Advancements in quantum technology hinge on reproducible, and reconfigurable quantum devices—including photon sources, which are challenging to achieve in a scalable manner using bulk optics. Advances in nanotechnology and CMOS-compatible integration techniques have enabled the generation of entangled photons on millimeter-scale chips, significantly enhancing scalability, stability, replicability, and miniaturization for real-world quantum applications. In recent years we have seen several chip-scale demonstrations with different discrete degrees of freedom including path, frequency-bin, time-bin, and transverse modes, on many material platforms. A complete quantum photonic integrated circuit requires the generation, manipulation, and detection of quantum states, involving various active and passive quantum photonic components which further increase the degree of complexity. Here, we focus on the high-dimensional versions of qubits—qudits—and review the nonlinear optical processes that facilitate on-chip high-dimensional entangled photon sources, and the currently used material platforms. We discuss a range of current implementations of on-chip high-dimensional entangled photon sources and demonstrated applications. We comment on the current challenges due to the limitations of individual material platforms and present future opportunities in hybrid and heterogeneous integration strategies for the next generation of integrated quantum photonic chips.
Photonic quantum technologies rely on the deterministic preparation of quantum states encoded in single-photons degrees of freedom. While polarisation, orbital angular momentum and path have been routinely used since the early days of photonic quantum information, the last few years have seen an increasing interest in the frequency-time encoding [1, 2]. Indeed, the possibility of generating high-dimensional states, together with the compatibility of frequency modes and standard optical components, makes the spectral degree of freedom suitable for enhancing the efficiency of quantum protocols, spanning from quantum communication to optical quantum computation.
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.
As quantum optics grows and develops, new sophisticated optical schemes require an increasing number of optical elements. Integrated photonic circuits have the capacity to provide a solution to this challenge, as they are intrinsically scalable and interferometrically stable. Integrated realization of multi-photon entanglement, quantum factoring algorithms, and polarization entanglement has already been demonstrated. One of particularly interesting structures in integrated photonics is the waveguide array, which enables flexible shaping of optical beam and pulses in space. Recently waveguide arrays have been shown to generate unusual and strongly non-classical spatial correlations of photon pairs propagating in the regime of quantum walks [1].
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.