Quantum limits to optical point-source localization

TL;DR

This paper derives fundamental quantum limits on the accuracy of localizing optical point sources, providing a theoretical framework that applies to all measurement techniques and highlights the potential of quantum methods for superresolution microscopy.

Contribution

It establishes the first quantum lower bounds on localization error for optical sources, considering nonparaxial and vectorial photon properties, and discusses implications for superresolution microscopy.

Findings

Lower bound on single-source localization error: ~λ₀/√N

Error divergence for overlapping two sources due to nuisance parameters

Quantum techniques can potentially surpass classical resolution limits

Abstract

Motivated by the importance of optical microscopes to science and engineering, scientists have pondered for centuries how to improve their resolution and the existence of fundamental resolution limits. In recent years, a new class of microscopes that overcome a long-held belief about the resolution have revolutionized biological imaging. Termed "superresolution" microscopy, these techniques work by accurately locating optical point sources from far field. To investigate the fundamental localization limits, here I derive quantum lower bounds on the error of locating point sources in free space, taking full account of the quantum, nonparaxial, and vectoral nature of photons. These bounds are valid for any measurement technique, as long as it obeys quantum mechanics, and serve as general no-go theorems for the resolution of microscopes. To arrive at analytic results, I focus mainly on the cases of one and two classical monochromatic sources with an initial vacuum optical state. For one source, a lower bound on the root-mean-square position estimation error is on the order of $\lambda_0/\sqrt{N}$, where $\lambda_0$ is the free-space wavelength and $N$ is the average number of radiated photons. For two sources, owing to the statistical effect of nuisance parameters, the error bound diverges when their radiated fields overlap significantly. The use of squeezed light to enhance further the accuracy of locating one classical point source and the localization limits for partially coherent sources and single-photon sources are also discussed. The presented theory establishes a rigorous quantum statiscal inference framework for the study of superresolution microscopy and points to the possibility of using quantum techniques for true resolution enhancement.