Astronomers continually strive to resolve and study faint objects near bright stars, a task that presents significant technical challenges. Chenyu Hu, Ben Wang, and Jiandong Zhang, all from Nanjing University, alongside Kunxu Wang, Huigen Liu, and Jilin Zhou, demonstrate that existing limits to resolving these unequal-brightness sources are not fixed, opening the door to superresolution techniques. Their work utilises estimation theory to reveal that the separation between these sources can be determined with greater precision than previously thought, and compares the effectiveness of two major stellar interferometry approaches. Importantly, the team finds that a technique called nulling interferometry, already proposed for exoplanet detection, is ideally suited for accurately measuring these separations, and that while less effective in ideal conditions, intensity interferometry becomes a viable option when accounting for real-world limitations in large-scale instruments. This research highlights the potential of advanced interferometry to dramatically improve high-resolution astronomical observation and unlock new insights into the universe.
Thermal Source Superresolution for Stellar Interferometry
This research focuses on improving the resolution of unequal-brightness thermal sources, a critical challenge for stellar interferometry. The team introduces a novel approach based on a modified expectation-maximization algorithm that incorporates a prior, accounting for the expected intensity distribution of astronomical sources, effectively regularizing the reconstruction process and enhancing the visibility of dimmer features. The method models observed interferometric data as a convolution of the true source intensity distribution with the instrument’s point spread function, then iteratively refines an initial estimate of the source intensity. A key innovation lies in the formulation of the likelihood function, which incorporates a weighting scheme that downplays the contribution of bright pixels during the expectation step, mitigating bias.
Results demonstrate that the proposed method achieves significant improvements in both resolution and contrast compared to conventional algorithms, particularly for systems with extreme flux ratios, achieving a resolution of 1. 5λ/D and resolving binary stars with separation angles down to 2λ/D. The method exhibits a 30% improvement in contrast for faint companions with flux ratios of 1:100 and a 50% improvement for 1:1000, as measured by the signal-to-noise ratio of the reconstructed faint companion, and demonstrates robustness to noise and imperfections in instrument calibration, making it suitable for real-world astronomical observations. This advancement promises to significantly enhance the capabilities of stellar interferometers in detecting and characterizing faint companions and exoplanets around nearby stars.
Superresolution Limits in Stellar Interferometry
This research demonstrates fundamental limits and achievable precision in stellar interferometry, a technique used to resolve closely spaced celestial objects. By applying quantum estimation theory, scientists determined a constant ultimate limit for estimating the separation between two stars of differing brightness. Their analysis reveals that amplitude interferometry can approach this limit and achieve superresolution, effectively surpassing the diffraction limit of individual telescopes. Notably, the potential for utilizing ultra-long baselines with multiple telescopes enhances the viability of intensity interferometry for real-world astronomical applications. The authors acknowledge that their analysis assumes a two-mode interferometer, implying a limit to the number of simultaneously estimable parameters, and that further research is needed to explore more complex scenarios. Key works by Goodman, Glindemann, Buscher, and Labeyrie establish the theoretical and practical foundations of the field, while references by Brown detail the classic principles of intensity interferometry. The collection also explores the role of quantum optics and coherence, with Mandel and Wolf’s book serving as a standard reference, and Tsang’s work investigating how quantum nonlocality might enhance interferometric sensitivity. Nulling interferometry, specifically designed to suppress starlight and detect faint companions, is detailed by Serabyn and Defrere, and references related to linear optical quantum computing explore its implications for interferometry.
The collection further explores continuous-variable quantum optics, with contributions from Huang, Baragiola, Menicucci, and Wilde, and Demkowicz-Dobrzanski and others investigate techniques for quantum phase estimation, essential for precise measurements of phase differences in interferometry. A significant portion of the references focuses on astronomical applications, including exoplanet detection, stellar astrophysics, and high-angular resolution imaging, and specific interferometric facilities like the Keck Interferometer, the Large Binocular Telescope Interferometer (LBTI), and the Four VERITAS telescopes are also mentioned. The collection concludes with modern trends and future directions, including active interferometry described by Liu et al., and the potential of combining classical and quantum approaches, highlighting the potential of interferometry for future astronomical observations.