14. Photon Counting and Interferometry

Revista, Zen, ASTRO, 10

Optical astronomy has continuously pushed the boundaries of observational capabilities, revealing profound insights into the cosmos. However, a fundamental limitation persists: the sensitivity wall imposed primarily by photon shot noise, especially when observing faint or distant objects. This paper introduces and thoroughly explores the concept of Coherent Photon-Counting Interferometry (CPCI) as a revolutionary technique poised to overcome this sensitivity barrier. By combining the quantum efficiency of photon counting with the phase sensitivity of coherent detection in an interferometric setup, CPCI offers a pathway to unprecedented signal-to-noise ratios, even in extremely low light conditions. We delve into the theoretical underpinnings of CPCI, detailing how it leverages the wave-particle duality of light to extract maximal information from incident photons. The methodology outlines a proposed experimental architecture, emphasizing the critical roles of highly stable local oscillators, advanced photodetectors, and sophisticated signal processing algorithms. A comprehensive literature review contextualizes CPCI within the broader landscape of quantum optics and astronomical instrumentation, drawing parallels with advancements in gravitational wave detection that utilize quantum noise reduction techniques. The potential results demonstrate that CPCI can achieve sensitivities significantly beyond the standard quantum limit for classical interferometry, enabling the detection and characterization of previously inaccessible astronomical phenomena, such as exoplanet atmospheres, faint galactic halos, and distant active galactic nuclei. We discuss the inherent challenges, including maintaining phase coherence over long baselines and mitigating atmospheric turbulence, alongside potential mitigation strategies like adaptive optics and advanced phase-locking schemes. Ultimately, CPCI represents a paradigm shift for optical astronomy, promising to unlock new observational windows and foster a deeper understanding of the universe's most enigmatic components.

Ricardo Asch Campos

Hiroshi Matsuo, H. Ezawa, Masahiro Ukibe +2 more

Photon counting detectors in terahertz frequencies are developed for future astronomical intensity interferometry. Superconducting tunnel junction detectors with extremely low leakage current can be used for fast photon counting, more than a GHz bandwidth, in terahertz frequencies. We plan to demonstrate intensity correlation using antenna coupled SIS terahertz photon counters to measure the statistical behavior of thermal photons. This could bring a new measurement opportunity in the field of terahertz quantum optics.

P. Hariharan

Fabien Boitier, Antoine Godard, N. Dubreuil +3 more

Two-photon-counting interferometry has been realized by measuring the electrical current due to two-photon absorption in the space-charge layer of a semiconductor detector located at the output port of an interferometer. We apply this technique to study the correlation properties of twin beams issued from parametric fluorescence. We describe in detail how the different second-order correlation functions (interbeam, intrabeam) can be extracted at the femtosecond time scale from raw data. The values of these correlation functions determined by our experiments are in excellent agreement with theory. More precisely, extrabunching in twin beams is unambiguously demonstrated and theoretically described using two models: a comprehensive multimode quantum optics model and a simpler classical stochastic approach. Given the high brightness of our twin-beam source, both theories yield similar results. Finally, convenient analytical expressions of the correlation functions were derived from both theories, expressions in which we have been able to relate specific terms to accidental and exact coincidences between photons. Two-photon interferometry thus determines to which extent twin photons are twin. This technique should become a useful tool for future quantum optics developments.