Quantum imaging with sub-Poissonian light: challenges and perspectives in optical metrology

Abstract

Non-classical correlations in optical beams offer the unprecedented opportunity of surpassing conventional limits of sensitivity and resolution in optical measurements and imaging, especially but not only, when a low photon flux down to the single photons are measured. We review the principles of quantum imaging and sensing techniques which exploit sub-Poissonian photon statistics and non-classical photon number correlation, presenting some state-of-the-art achievements in the field. These quantum photonics protocols have the potential to trigger major steps in many applications, such as microscopy and biophotonics and represent an important opportunity for a new deal in radiometry and photometry.Quantum imaging with sub-Poissonian light Limits of Classical (conventional) imagingMeasuring changes in intensity or in phase of an electromagnetic field, after interacting with matter, is the most simple way to extract relevant information on the properties of a system under investigation, whether a biological sample [18] or a digital memory disc [19].The term "imaging" usually (not always) refers to the reconstruction of the whole spatial properties of the sample, namely in 2D or 3D, and it can be achieved in two different modalities, wide field or point-by-point scanning. Wide field imaging is preferable in many cases, since it provides a more compete dynamic pictures but, for static sample, the point by point scanning offers advantages, for example the better z-resolution in confocal microscopy.In any case, the two parameters quantifying the quality (the amount of information) of the image are the resolution, i.e. the minimum distance at which two points can be distinguished, and the sensitivity, i.e. the minimum measurable variation of the physical quantity in a certain point.The quality of an image is always affected by several limitations, some of them avoidable by a careful design of the experiment (aberration, background, artifacts), others imposed by technical limitations of the available actual technology (for instance unavoidable noise or low efficiency of the detector), and others related to more fundamental reasons. In particular diffraction limit and shot-noise limit represent "fundamental" bounds, to resolution and sensitivity, at least when classical states of light are considered. A possibility to overcome these limitations is offered by peculiar properties of quantum light. Diffraction Limit (DL)The diffraction limit, R 0.61λ/N A (with λ being the wavelength of the light and N A the numerical aperture of the imaging system), represents the maximum obtainable imaging resolution in classical far-field imaging/microscopy. Depending on the kind of light radiation involved, namely incoherent or coherent, it takes the name of Abbe or Rayliegh diffraction limit, respectively.

Resolving starlight: a quantum perspective

“…For example, Davis and Tango reported an amplitude interferometer that obtained similar results to those from the intensity interferometer, using only ∼2% of the observation time [111]. For microscopy, the use of multiphoton coincidence has recently been demonstrated in some heroic experiments [112][113][114][115], but again its statistical performance needs to be studied more carefully. SPADE, on the other hand, is a g (1) measurement that relies on the much more abundant onephoton events without the need for coincidence detection and its statistical performance has been proved rigorously.…”

Spectrally resolved Hong–Ou–Mandel interferometry for quantum-optical coherence tomography

“…Current advances in photodetection technologies and the development of highly efficient SPDC sources have revived the idea of developing efficient QOCT systems [12][13][14]. A Michelson version of QOCT has been shown to represent a functional and robust configuration, which can benefit from the incorporation of novel photon-number-resolving detectors [15], and which is amenable to miniaturization [16,17].…”

Quantum differential ghost microscopy

“…In order to demonstrate the non-classicality of the detected intensity correlation, we evaluate the NRF parameter defined as NRF = ∆ 2 (n 1 −n 2 ) / n 1 +n 2 , that is the variance of the photon number difference between a pair of correlated pixels normalized by their sum (the shot noise bound). Only non-classical correlation allows to have 0 < NRF < 1 [3,12]. In our case we estimated NRF = 0.77 ± 0.02.…”