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Coherent Photon-Counting Interferometry: Breaking the Sensitivity Wall for Optical Astronomy

Revista, Zen, ASTRO, 10

Published Nov 28, 2025• 0 citations
Publisher

Abstract

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.

DOI
10.5281/zenodo.17754869

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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.
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The first detections of gravitational waves have opened an exciting new field of astronomy. One of the most fundamental limitations for the sensitivity of current and future interferometric gravitational-wave detectors is imposed by the quantum nature of light: Quantum vacuum fluctuations entering the interferometer through the readout port will contribute to the detection noise, at high frequencies in the form of shot noise and at low frequencies by radiation pressure noise. A promising way to reduce this quantum noise is the injection of squeezed states of light that have a lower uncertainty in one quadrature than the vacuum state. The GEO 600 gravitational-wave detector demonstrated the use of squeezed light in 2010 and it is now the first detector to routinely apply squeezing to improve its sensitivity beyond the limits set by classical quantum shot noise. This thesis details the practical aspects of long-term stable and efficient squeezed-light integration in a large-scale gravitational-wave detector. Imperfections that can limit the amount of observable non-classical noise improvement, such as optical losses and phase fluctuations, were studied in detail and methods for their mitigation were developed. Novel control schemes for the active stabilisation of the squeezed light field's phase and alignment were one main focus of the investigations. At the same time, important experience was gathered in the operation of the squeezed light source over long timescales. Over the course of the thesis work, improvements were implemented that significantly increased the performance of the squeezed-light application. Squeezing was injected with an overall duty cycle of 88%, reaching a noise reduction of up to 4.4 dB, corresponding to a 40% lowered shot-noise level. This work has firmly established the practical application of squeezing as a mature technology. The gained knowledge will directly inform the implementation of squeezed light for all future gravitational-wave detectors.
One important problem in quantum optics is to resolve an extremely small change of phase shift. The complementarity between photon number and phase sets an ultimate limit, the so-called Heisenberg limit, on the phase measurement sensitivity. The precise phase estimation has many technological applications, such as optical gyroscopes, gravitational wave detection, quantum imaging and sensing. In this thesis I show that the utilization of the parity measurement in the optical interferometry is actually applicable to a wide range of quantum entangled input states. Comparison of the performance of the various quantum states then can be made within such a unified output measurement scheme. Based on such a universal detection scheme, we present a comparison of the phase sensitivity reduction for various quantum states of light in the presence of photon loss. I also provide a simple condition that could be used to check whether an arbitrary state can achieve the Heisenberg limit independently of the detection scheme. It implies that the fidelity between the two output states with zero phase and minimal detectable phase applied respectively should significantly different from unity as the minimal phase shift scaling as 1/N, whereas the average number of input photons N goes to infinity. Next I give several measures to characterize the which-way information in the interference experiment. We define a new distinguishability associated with the fidelity between two density matrices to measure the which-way information. We demonstrate that the changes of mutual entropy as well as the entanglement of formation of the whole system, i.e. the physical system plus the which-way detector can also be used to describe the which-way information. With such quantities, we show that as the fringe visibility of the interference pattern gets larger, the less which-way information is obtained. Finally, I show that coherent light coupled with photon number resolving detectors can provide a super-resolution much below the Rayleigh diffraction limit, with sensitivity no worse than shot-noise in terms of the detected photon power. This scheme would have applications to laser radar, given the difficulty in making entangled states of light, as well as their susceptibility to atmospheric absorption.
Quantum technologies may revolutionize optical interferometry through new means to distribute and preserve electric-field amplitude and phase correlations between widely separated telescopes. After reviewing existing techniques, I survey proposed quantum-sensing and quantum-networking applications and highlight recent laboratory demonstrations of key building blocks. By easing field-transport demands for baselines beyond $\sim$1 km, quantum-enhanced interferometry could enable sub-milliarcsecond imaging and microarcsecond-class differential astrometry. That said, "going quantum" is not a magic shortcut to sensitivity because many of the most compelling science cases remain photon- and turbulence-limited. I therefore emphasize the practical constraints -- loss, bandwidth, coherence time, synchronization, and wavelength limits of quantum interfaces -- that must also be overcome to make these capabilities useful for astronomy, whether deployed on the ground or from space.