Detecting the faint ripples of gravitational waves from the very earliest moments of the universe requires extraordinarily sensitive instruments, and the planned DECi-hertz Interferometer Gravitational-wave Observatory, or DECIGO, aims to meet this challenge. Kenji Tsuji, Tomohiro Ishikawa, and Kentaro Komori, along with their colleagues, now demonstrate a significant advance in reducing noise within DECIGO’s critical observation band. The team addresses a long-standing problem, the degradation of light quality due to diffraction losses in the instrument’s long optical cavities, by developing a new model that accurately accounts for the mixing of light and vacuum states. This rigorous approach reveals that carefully designed optical springs and homodyne detection schemes can, in fact, achieve high sensitivity even with these losses, representing a crucial step towards detecting primordial gravitational waves and unlocking secrets of cosmic inflation.
DECIGO Sensitivity Improvement For Early Universe Signals
Scientists are refining the design of DECIGO, a planned space-based gravitational wave observatory, to enhance its ability to detect faint ripples in spacetime from the earliest moments of the universe and compact objects like black holes. A key challenge is minimizing noise, particularly quantum noise, within the detector. This research explores techniques to reduce this noise and improve DECIGO’s sensitivity by investigating the use of ‘optical springs’, created by carefully controlling light, to stabilize mirrors against vibrations, and ‘quantum locking’ to further enhance stability. A crucial component of DECIGO is the use of Fabry-Perot cavities, which amplify the light signal, but a significant hurdle is ‘diffraction loss’, the unavoidable loss of light as it travels through these cavities.
The study demonstrates that diffraction loss significantly impacts instrument sensitivity and must be carefully managed. Researchers developed a detailed mathematical model of the interferometer, incorporating quantum noise, diffraction loss, and the optical spring, allowing them to simulate performance under various conditions and optimize parameters. This work focuses on optimizing sensitivity within a specific frequency range by employing spring systems and a technique called homodyne detection. Researchers demonstrated that high sensitivity can be achieved despite light loss within the instrument’s long cavities through a new, rigorous model of light behavior, accounting for the mixing of quantum states resulting from this loss. The team established a detailed treatment of quantum fluctuations to accurately model noise limitations.
An input laser beam is split, with one portion directed into a long cavity and the other serving as a local oscillator for the homodyne detection system, which interferes the reflected light with the local oscillator. Scientists mapped the paths of quantum fluctuations through this configuration, calculating the cumulative effect of each component. To account for light loss, the study adopted a factor characterizing these losses, and rigorously treated quantum noise by replacing lost light with equivalent vacuum fluctuations, preserving total quantum fluctuations. This work focuses on minimizing noise by employing spring mechanisms and a technique called homodyne detection to achieve unprecedented precision. Researchers demonstrated that high sensitivity can be maintained even with light loss within the instrument’s arm cavities, previously considered detrimental. The team formulated a rigorous model accounting for the mixing of quantum states resulting from light loss, revealing that optimal configurations of springs and homodyne detection schemes can effectively mitigate these effects.
Measurements confirm that carefully managing light loss can achieve substantial improvements in sensitivity. Experiments demonstrate that circulating light within the cavities exerts a force on the mirrors, known as radiation pressure, which can be harnessed as an ‘optical spring’. By maintaining the cavity length slightly offset from resonance, any displacement of the mirrors alters the light within the cavity, creating a restoring force that counteracts the displacement. Calculations show that this optical spring, coupled with the homodyne detection scheme, allows for selective readout of quantum noise, enhancing the detector’s ability to isolate faint gravitational wave signals. Scientists have demonstrated that incorporating optical springs and a technique called homodyne detection can improve the instrument’s sensitivity, even when accounting for signal degradation caused by light loss within the detector’s arm cavities. Through a rigorous model of quantum noise, the team calculated that sensitivity can be improved by approximately a factor of 1. 5, and potentially further enhanced with reductions in acceleration noise.
However, the researchers acknowledge that achieving the necessary sensitivity to detect primordial gravitational waves remains a challenge. While the proposed improvements represent a step forward, they are currently insufficient to reach the required detection threshold. The team notes that creating an overly narrow sensitivity profile could inadvertently reduce overall performance due to other limiting noise sources. Future work should focus on combining this approach with techniques unaffected by light loss, such as optical-spring quantum locking, to further enhance detection capabilities and broaden the scope of scientific inquiry. This research contributes valuable insights towards the ongoing development of advanced gravitational wave detectors and the pursuit of a deeper understanding of the universe.
👉 More information
🗞 Quantum Noise Reduction in the Space-based Gravitational Wave Antenna DECIGO Using Optical Springs and Homodyne Detection scheme
🧠 ArXiv: https://arxiv.org/abs/2509.17372