Structured light beams are essential in optical communications, quantum information processing, and light-matter interactions. The generation and conversion of high-purity structured light beams are crucial for these applications. In this study, we propose a bidirectional high-purity structured light beam transformation approach based on the multi-plane light conversion (MPLC) method. This approach enables mutual conversion of arbitrary orthogonal modes and precise control of system loss and mode purity. Using the proposed method, we demonstrate the conversion of 7-modes Gaussian beams array (GBA) and perfect vortex beams (PVB), achieving a bidirectional output modes signal-to-noise ratio (SNR) exceeding 30 dB and a loss of 5.23 dB. For generation of 13-modes, an SNR above 25 dB and an insertion loss below 5.45 dB are obtained. Furthermore, the validity of methods is confirmed by generating multi-wavelength and structured light beams with more complex field distribution. The work paves the way toward structured-light-beam-based optical communications, high-dimensional quantum communications, and advancements in MPLC.
Structured light beams are essential in optical communications, quantum information processing, and light-matter interactions. The generation and conversion of high-purity structured light beams are crucial for these applications. In this study, we propose a bidirectional high-purity structured light beam transformation approach based on the multi-plane light conversion (MPLC) method. This approach enables mutual conversion of arbitrary orthogonal modes and precise control of system loss and mode purity. Using the proposed method, we demonstrate the conversion of 7-modes Gaussian beams array (GBA) and perfect vortex beams (PVB), achieving a bidirectional output modes signal-to-noise ratio (SNR) exceeding 30 dB and a loss of 5.23 dB. For generation of 13-modes, an SNR above 25 dB and an insertion loss below 5.45 dB are obtained. Furthermore, the validity of methods is confirmed by generating multi-wavelength and structured light beams with more complex field distribution. The work paves the way toward structured-light-beam-based optical communications, high-dimensional quantum communications, and advancements in MPLC.
Light with complex structures in polarization, phase and amplitude, has attracted a lot of attention in a broad range of applications and fundamental studies in classical and quantum optics. Along with the increased interest in structured light comes a need for efficient modulation platforms operating simultaneously for many modes. Multi plane light conversions (MPLC), i.e., multiple consecutive phase modulations in combination with free space propagation, have enabled such unitary transformations, which are usually built by bulky optical components, limited to scalar modulation, or rely on advanced nanofabrication techniques. Here, we demonstrate an efficient, monolithic MPLC architecture through direct laser writing in standard fused silica glass, resulting in a device with a compact form factor of only a few cubic millimeters. Our scheme is based on volumetric engineering of the glass's birefringence through laser-written nanogratings, which enables spatial control over full vectorial light structures. To showcase the approach's potential for integrated multimode-multipath optical networks, we demonstrate multi-mode unitary transformations, mode conversions, and complex beam-splitting for scalar light. We further extend the MPLC operation to vectorial light and implement various polarization-controlled spatial mode operations as well as the transformation of the topology of an optical Skyrmion. Finally, we highlight our scheme's promise for optical communications and implement a miniaturized multiplexer for spatial modes and polarization operating at telecom wavelength.
Spatial transformations of light are ubiquitous in optics, with examples ranging from simple imaging with a lens to quantum and classical information processing in waveguide meshes. Multi-plane light converter (MPLC) systems have emerged as a platform that promises completely general spatial transformations, i.e., a universal unitary. However, until now, MPLC systems have demonstrated transformations that are far from general, e.g., converting from a Gaussian to Laguerre-Gauss mode. Here, we demonstrate the promise of an MLPC, the ability to impose an arbitrary unitary transformation that can be reconfigured dynamically. Specifically, we consider transformations on superpositions of parallel free-space beams arranged in an array, which is a common information encoding in photonics. We experimentally test the full gamut of unitary transformations for a system of two parallel beams and make a map of their fidelity. We obtain an average transformation fidelity of 0.85 ± 0.03. This high-fidelity suggests that MPLCs are a useful tool for implementing the unitary transformations that comprise quantum and classical information processing.
This thesis explores the development and application of multi-plane light conversion (MPLC) technol-ogy, focusing on its integration into experimental setups that push the boundaries of quantum opticsand spatial state tomography. Central to this study is a custom-built, research-grade MPLC devicedesigned around a programmable spatial light modulator (SLM), which facilitates two groundbreak-ing experiments: the manipulation of high-dimensional qudits for quantum gate operations, and thedevelopment of a high-dimensional Stokes-space spatial beam analyser for single-shot spatial statetomography.The first experiment involved implementing quantum gates — specifically the X-gate, Z-gate, andDFT-gate — for a 17-dimensional qudit, represented through the first 17 azimuthal Laguerre-Gaussianspatial modes on a MPLC device. This experiment not only demonstrated MPLC’s capability tohandle complex, high-dimensional quantum states but also underscored its potential to enhance thescalability and efficiency of quantum computing architectures. By leveraging the precise control overspatial modes afforded by MPLC and the programmability of SLM-based MPLC, the experimentdemonstrated high-dimensional gate operations, paving the way for more complex quantum computingoperations for qudits.The second project focused on using MPLC to develop a device capable of performing high-dimensional Stokes-space spatial beam analysis. This innovative instrument marks a significantbreakthrough in optical beam characterisation, providing a reference-free, single-shot technique forconducting spatial state tomography on beams characterisable by a six-mode or six-dimensional system.The device’s capacity to swiftly and precisely characterise the wavefront and coherence of opticalfields renders it an invaluable asset for advancing optical communication technologies and enhancingoptical sensing methodologies.Throughout this thesis, the theoretical foundations of MPLC technology are presented, with adetailed account of the MPLC device design and operational principles. Experimental results arepresented to validate the effectiveness of the MPLC device in both quantum gate operations andspatial state tomography. Comparisons with existing technologies highlight the MPLC’s enhancedperformance and versatility.This work not only expands the understanding and application of MPLC in quantum and opticalsciences but also sets the stage for future innovations in these fields. The continuous refinement andadaptation of MPLC technology promise to unlock new possibilities in both fundamental research andpractical applications.
Spatial transformations of light are ubiquitous in optics, with examples ranging from simple imaging with a lens to quantum and classical information processing in waveguide meshes. Multi-plane light converter (MPLC) systems have emerged as a platform that promises completely general spatial transformations, i.e., a universal unitary. However until now, MPLC systems have demonstrated transformations that are far from general, e.g., converting from a Gaussian to Laguerre-Gauss mode. Here, we demonstrate the promise of an MLPC, the ability to impose an arbitrary unitary transformation that can be reconfigured dynamically. Specifically, we consider transformations on superpositions of parallel free-space beams arranged in an array, which is a common information encoding in photonics. We experimentally test the full gamut of unitary transformations for a system of two parallel beams and make a map of their fidelity. We obtain an average transformation fidelity of 0.85±0.03. This high-fidelity suggests MPLCs are a useful tool implementing the unitary transformations that comprise quantum and classical information processing.
High-dimensional entangled photons are a key resource for advanced quantum information processing. Efficient processing of high-dimensional entangled photons requires the ability to synthesize their state using general unitary transformations. The leading technology for processing photons in high-dimensions is integrated multiport interferometers. However, such devices are incompatible with free-space and fiber-based systems, and their architecture poses significant scaling challenges. Here we unlock these limitations by demonstrating a reconfigurable processor of entangled photons that is based on multi-plane light conversion (MPLC), a technology that was recently developed for multiplexing hundreds of spatial modes for classical free-space and fiber communication. To demonstrate the flexibility of MPLC, we perform four key tasks of quantum information processing using the same MPLC hardware: entanglement certification, tailored two-photon interference, arbitrary state transformations, and mode conversion. Based on the high degree of control we obtain, we expect MPLC will become a leading platform for future quantum technologies.
We present a compact and efficient realization of Multi-Plane Light Conversion (MPLC) for full vectorial light modulation, enabling control over polarization, phase, and amplitude. Conventional MPLC implementations based on phase-only modulation suffer from losses and limited polarization control, while metasurface-based solutions require complex nanofabrication processes. Our approach employs laser-written birefringent waveplates in silica slides, each with fixed retardance and a spatially varying optical axis orientation. The structures are fabricated using direct laser writing of nanogratings in silica, achieving ~90% transmission and ~1–2 µm spatial resolution. We demonstrate high-dimensional quantum gates and a polarization-controlled single-photon CNOT gate using spatial and polarization degrees of freedom. This cleanroom-free platform shows strong potential for high-dimensional quantum processing, optical communication, and beam shaping, with future prospects for extending modulation into the frequency domain.