Waveguide Designs Improve Squeezed Light Sensing

Squeezed states of light are a key technology for advancing precision metrology and quantum-enhanced measurements. Erik Anders Torsten Svanberg and colleagues at KTH Royal Institute of Technology performed a thorough analysis of waveguide-based squeezed-light sources, identifying key limitations imposed by fabrication-induced losses, phase noise, and light leakage. Their research details the behaviour of these sources and explores a cascaded architecture designed to counteract out-coupling and detection losses. The analysis highlights the potential of strong and easily integrated waveguide technology as a viable pathway towards quantum noise reduction, particularly for next-generation gravitational wave detectors like the Einstein Telescope.

Cascaded waveguide system replicates high levels of squeezed light for gravitational wave detection

Achieving 15 dB of squeezing, a benchmark for current cavity-based systems, was previously unattainable with waveguide technology. A cascaded squeezer architecture now provides a pathway towards replicating these levels of quantum reduction. This advance overcomes limitations imposed by fabrication-induced losses, phase noise, and light leakage, all of which have historically restricted the performance of waveguide-based sources. Detailed modelling reveals that deliberately layering optical parametric amplifiers, or OPAs, mitigates out-coupling and detection losses. These OPAs amplify the ‘squeezed light’, reducing quantum noise. The principle behind squeezing relies on the Heisenberg uncertainty principle, which dictates a fundamental limit to the precision with which conjugate variables, such as the amplitude and phase of light, can be simultaneously known. Squeezed light manipulates this uncertainty, reducing noise in one variable at the expense of increased noise in the other, thereby enhancing sensitivity for specific measurements. This is particularly relevant for detecting the incredibly faint ripples in spacetime caused by gravitational waves.

Waveguides now position themselves as a strong and scalable alternative for future gravitational wave detectors, potentially simplifying integration and improving operational stability compared to complex optical cavities. Detailed analysis reveals that layering optical parametric amplifiers effectively mitigates signal loss during transmission and detection. Modelling demonstrates insensitivity to phase noise when operating at high gain, a key factor for stable operation, unlike current cavity-based systems susceptible to fluctuations in cavity length and pump stability. The Einstein Telescope, a proposed third-generation gravitational wave observatory, demands significantly improved sensitivity compared to existing detectors like LIGO and Virgo. Waveguide-based squeezed light sources offer a potential pathway to meet these demands due to their compact size and potential for increased reliability. Furthermore, these waveguides exhibit greater durability to diffraction and backscatter, potentially improving the duty cycle of the squeezer and simplifying operational complexity. Fabricating perfect waveguides remains a considerable hurdle, with imperfections inevitably causing signal loss and hindering the attainment of substantial squeezing, necessitating further refinement of fabrication techniques. Current fabrication methods, such as electron beam lithography and reactive ion etching, introduce imperfections at the nanoscale, leading to scattering and absorption of light. Reducing these imperfections requires advancements in materials science and nanofabrication processes.

Cascaded waveguide design overcomes fabrication losses for enhanced quantum squeezing

Precision measurement demands ever-decreasing noise, driving new innovation in technologies like squeezed light, where quantum fluctuations are redistributed to enhance sensitivity. Current systems rely on carefully crafted optical cavities, which are notoriously difficult to scale and maintain. These cavities typically employ highly reflective mirrors to trap light and enhance the interaction necessary for squeezing. However, maintaining the precise alignment and stability of these mirrors is a significant engineering challenge. Scientists at KTH Royal Institute of Technology are now exploring waveguide technology as a potentially simpler alternative. However, achieving comparable squeezing levels with waveguides necessitates overcoming inherent fabrication imperfections and signal losses, a challenge the team addresses with a cascaded design.

This layered approach amplifies the squeezed light, with performance currently limited by fabrication-induced losses. It establishes a clear pathway towards sources delivering squeezing levels comparable to the 10 dB currently achieved with cavity-based technologies, offering ease of integration and improved resilience to high pump powers and low intrinsic phase noise. The cascaded architecture functions by sequentially amplifying the squeezed light signal through multiple OPAs. Each OPA introduces some loss, but the overall effect is to increase the signal strength while maintaining a high degree of squeezing. The choice of materials for the waveguides is crucial, with materials exhibiting low optical loss and high nonlinear coefficients being preferred. Materials like silicon nitride and lithium niobate are commonly used in integrated photonics due to their favourable properties. These channels manipulating light offer a potentially scalable alternative to optical cavities currently used in precision measurement technologies. Analysis confirms the potential of waveguide-based squeezed-light sources as alternatives to cavity systems, particularly for demanding applications like gravitational wave detection, such as the Einstein Telescope. The cascaded design allows for increased gain and reduced sensitivity to imperfections, offering a promising route to improved performance in future quantum technologies. The theoretical limit of squeezing is determined by the pump power and the efficiency of the nonlinear process within the OPA. Increasing the pump power can enhance squeezing, but it also introduces challenges related to thermal management and nonlinear absorption. Optimising the OPA design and operating conditions is therefore essential to maximise squeezing performance.

The researchers demonstrated a cascaded waveguide design capable of amplifying squeezed light, a technique used to reduce noise in precision measurements. This matters because it offers a potentially more robust and integrable alternative to current squeezing technologies reliant on optical cavities, which are difficult to manufacture and maintain. Achieving squeezing levels comparable to the 10 dB currently available could significantly improve the sensitivity of instruments like the Einstein Telescope, used to detect gravitational waves. Further work will focus on optimising waveguide materials, such as silicon nitride and lithium niobate, and fabrication processes to minimise signal loss and maximise squeezing performance.