We present results of imaging artificial stellar sources below the diffraction limit using quantum-optimal measurements enabled by spatial mode demultiplexers, and demonstrate progress towards on-sky tests of quantum imaging for practical astronomy.
Classically, the angular resolution of a telescope is limited by its diffraction limit: the ratio of its observing wavelength to its diameter. Developments in quantum information theory over the past decade have uncovered quantum-optimal measurement methods that enable super-resolution imaging beyond the diffraction limit, and many laboratory-scale demonstrations of these methods have been performed over the past decade, culminating in the recent first on-sky demonstration of quantum-enhanced astronomical imaging. In this Review, we detail the current progress and achievements of these theoretical and experimental results, discuss the astronomical science cases motivating the development of higher resolution telescopes, summarise the technological challenges facing the practical application of quantum imaging technologies to astronomy, and present a vision for the realization of \emph{quantum telescopes} with imaging resolutions orders of magnitude better than current observatories.
Recent advances in quantum information theory have led to the development of quantum imaging methods that would allow optical interferometers to achieve resolutions surpassing the conventional diffraction limit. The use of quantum technologies also has the potential to create optical interferometer telescopes with baselines of hundreds of kilometers, achieving unprecedented imaging resolution. We present results of laboratory experiments advancing quantum interferometer techniques for astronomy. We demonstrate quantum-optimal imaging of an artificial star-like source using a phase-stabilized 170 km-span optical interferometer, pointing the way to astronomical interferometers with resolution orders of magnitude better than any current telescope, with outstanding applications for studying exoplanets, the formation of star systems, and tests of General Relativity near the event horizon of a black hole.
Improving the resolution of telescope systems will provide the opportunity to study new physical phenomena in previously unobserved environments. Spatial mode de-multiplexing (SPADE) based imaging is a promising and rapidly evolving technique for pushing the resolution of optical telescopes beyond the diffraction limit. A key application of this technique is for near-optimal hypothesis testing for the presence of secondary and extended sources in the sub-diffraction regime. We present the first demonstration of a binary-SPADE based hypothesis testing instrument deployed on-sky. In our proof-of-principle experiment, based on mode demultiplexing with a double clad fiber coupler, we demonstrate detection of a binary star system separated below the diffraction limit. We perform measurements in the photon-starved regime where no image can be formed by traditional direct imaging. We find the scaling of the system's type II error rate (the ``binary source miss" chance) was heavily limited by unbalanced loss in our double-clad fiber coupler when compared to the idealized quantum limits. Despite this the evaluated type II error is always lower than a perfect direct imaging measurement. We expect that if this instrument is scaled to larger aperture telescope systems the effects of atmospheric turbulence will further degrade this system's performance.
I propose a spatial-mode demultiplexing (SPADE) measurement scheme for the far-field imaging of spatially incoherent optical sources. For any object too small to be resolved by direct imaging under the diffraction limit, I show that SPADE can estimate its second or higher moments much more precisely than direct imaging can fundamentally do in the presence of photon shot noise. I also prove that SPADE can approach the optimal precision allowed by quantum mechanics in estimating the location and scale parameters of a subdiffraction object. Realizable with far-field linear optics and photon counting, SPADE is expected to find applications in both fluorescence microscopy and astronomy.
Resolving light sources below the diffraction limit is a fundamental task both for astronomy and microscopy. Several recent works analysed this problem through the lens of quantum parameter estimation theory and proved that the separation between two point sources can be estimated at the quantum limit using intensity measurements after spatial-mode demultiplexing. We implement this technique and provide an optimal estimator based on a linear combination of demultiplexed intensity measurements [1–3]. Our experimental setup allows for the generation of the images of two sources, with tunable mutual coherence, as well as for spatial mode demultiplexing to estimate their separation [4].