A quantitative comparison of amplitude versus intensity interferometry for astronomy

TL;DR

This paper compares amplitude and intensity interferometry for astronomical imaging, analyzing their performance limits and practical applications, especially for resolving closely spaced stars with very large baselines.

Contribution

It provides a detailed performance comparison of the two interferometry methods and explores practical implementation options for Very Large Baseline Intensity Interferometry.

Findings

Intensity interferometry can outperform amplitude interferometry at very small angular separations due to larger baselines.

Amplitude interferometry is limited by coherence length, while intensity interferometry benefits from larger baselines.

Practical strategies for implementing Very Large Baseline Intensity Interferometry are discussed.

Abstract

Astronomical imaging can be broadly classified into two types. The first type is amplitude interferometry, which includes conventional optical telescopes and Very Large Baseline Interferometry (VLBI). The second type is intensity interferometry, which relies on Hanbury Brown and Twiss-type measurements. At optical frequencies, where direct phase measurements are impossible, amplitude interferometry has an effective numerical aperture that is limited by the distance from which photons can coherently interfere. Intensity interferometry, on the other hand, correlates only photon fluxes and can thus support much larger numerical apertures, but suffers from a reduced signal due to the low average photon number per mode in thermal light. It has hitherto not been clear which method is superior under realistic conditions. Here, we give a comparative analysis of the performance of amplitude and intensity interferometry, and we relate this to the fundamental resolution limit that can be achieved in any physical measurement. Using the benchmark problem of determining the separation between two distant thermal point sources, e.g., two adjacent stars, we give a short tutorial on optimal estimation theory and apply it to stellar interferometry. We find that for very small angular separations the large baseline achievable in intensity interferometry can more than compensate for the reduced signal strength. We also explore options for practical implementations of Very Large Baseline Intensity Interferometry (VLBII).