Lithium Niobate Waveguides Enable Mode-Selective Single-Photon Subtraction and Addition

Precise control over the spectral and temporal properties of light represents a significant hurdle in developing scalable quantum technologies, and researchers are now addressing this challenge with innovative waveguide designs. Peter Namdar from Laboratoire Kastler Brossel, alongside Patrick Folge from Paderborn University, and colleagues, present a new framework for creating high-fidelity, non-Gaussian quantum operations using thin-film lithium niobate waveguides. The team develops an inverse-design optimisation scheme that models how light interacts with these waveguides, allowing them to identify parameters that maximise the purity and selectivity of quantum states created through processes like single-photon subtraction and addition. This work, which builds on previous demonstrations in the near-infrared, demonstrates how tailored nonlinear interactions within these waveguides can support the complex quantum operations essential for building next-generation photonic networks and advancing quantum information processing.

This work addresses the challenge of implementing spectro-temporal mode-selective non-Gaussian quantum operations, specifically single-photon subtraction (SPS) and addition (SPA), within the telecommunications wavelengths essential for practical quantum networks. Building on previous demonstrations of near-infrared SPS, the researchers present a new design framework for achieving mode-selective SPA and SPS using thin-film lithium niobate nonlinear waveguide platforms, introducing a novel approach to manipulating quantum states of light with greater precision and control.

Mode Selectivity via Joint Spectral Analysis

This research details a comprehensive approach to optimizing single-photon addition (SPA) and subtraction (SPS) processes for quantum optics applications. The team aimed to achieve highly mode-selective SPA and SPS, ensuring the process primarily affects a single spatial mode of light, crucial for maintaining quantum information integrity. They modeled SPS as a weak beam-splitter interaction and SPA as a parametric down-conversion (PDC) process, simulating these processes to calculate the Joint Spectral Amplitude (JSA) or Transfer Function (TF). These calculations determined the Schmidt number (K), where a lower value, ideally K=1, indicates higher mode selectivity.

To optimize these processes, the researchers employed a genetic algorithm (GA). The GA created a population of potential solutions, selecting the best candidates based on their fitness, how closely their Schmidt number approached 1. Through crossover and mutation, the GA combined and modified these solutions over many generations, gradually improving the population’s performance. Each solution’s fitness was evaluated by calculating its Schmidt number, with values closer to 1 receiving higher scores. This iterative process refined the designs, maximizing mode selectivity.

The team evaluated the purity of the SPA and SPS processes, directly relating it to the Schmidt number. Simulations using different Hermite-Gauss pump modes confirmed the robustness of the designs. This sophisticated computational framework designs and optimizes quantum optical processes to achieve high mode selectivity, critical for quantum communication and computation.

Single Photon Addition and Subtraction Demonstrated

Researchers have significantly advanced the control of light for quantum information processing by demonstrating a new design framework for manipulating the spectral and temporal properties of single photons. This work focuses on implementing complex operations, specifically single-photon addition and subtraction, within the telecommunications wavelengths crucial for building practical photonic networks. The team successfully designed and modeled waveguides capable of performing these operations with high precision, paving the way for more sophisticated quantum communication and computation. The research introduces a novel optimization scheme that leverages light confinement and dispersion engineering within thin-film lithium niobate waveguides.

By carefully tailoring the geometry of these waveguides, researchers precisely control how light interacts with the material, maximizing the efficiency and purity of the photon addition and subtraction processes. This approach overcomes limitations of previous designs, which often struggled with maintaining the desired characteristics of the photons involved. The team demonstrated that tailored nonlinear processes, including parametric down-conversion and frequency up-conversion, are essential for next-generation photonic networks. A key breakthrough lies in achieving high fidelity in these operations, meaning the output photons closely match the intended quantum state.

The team’s designs minimize unwanted modes and imperfections, resulting in a significantly purer output signal. They quantify this performance using a metric related to the Schmidt number, where a value of one indicates a perfect operation, and their designs approach this ideal. This level of control is critical for maintaining the integrity of quantum information during processing and transmission. The thin-film waveguides, with their ability to tailor dispersion properties, offer substantial performance improvements, enabling the creation of highly selective and efficient photon manipulation processes. This advancement represents a crucial step towards building scalable and reliable quantum photonic systems.

Waveguide Designs for High-Fidelity Quantum States

This research presents a new design framework for generating specific quantum states of light, known as non-Gaussian states, crucial for advanced photonic technologies. The team successfully demonstrated a method for both adding and subtracting single photons in a controlled manner, achieving mode-selective operations using two distinct waveguide platforms: metallic waveguides and thin-film lithium niobate waveguides. By carefully optimizing the design of these waveguides and the parameters of the light used to interact with them, researchers identified configurations capable of generating the desired quantum states with high fidelity. The findings demonstrate the potential of both waveguide types for creating non-Gaussian states, with the thin-film lithium niobate platform offering particularly promising capabilities due to its ability to engineer light dispersion.

Importantly, the developed method allows for the selective generation of these states within a specific mode of light, using a static device configuration, and relies on efficient heralding techniques based on photon detection. This approach preserves coherence between multiple modes, essential for scalability and exploiting the interplay between non-Gaussianity and quantum entanglement. Future work will likely focus on experimental implementation and further optimisation of these designs to enhance performance and explore potential applications in quantum communication and computation.