Quantum enhanced optical measurement protocols aim at reducing the uncertainty in the estimation of some physical quantities of a system below the shot-noise limit, classically unavoidable. In particular when small number of photons is used the shot noise can be the main source of uncertainty, in these cases the use of quantum light is of great interest. Note that there are several situations where the number of photons in the probe can not be increased arbitrarily, as when fragile biological samples are under investigation. Two different imaging protocols are discussed in the following.
Quantum enhanced optical measurement protocols aim at reducing the uncertainty in the estimation of some physical quantities of a system below the shot-noise limit, classically unavoidable. In particular when small number of photons is used the shot noise can be the main source of uncertainty, in these cases the use of quantum light is of great interest. Note that there are several situations where the number of photons in the probe can not be increased arbitrarily, as when fragile biological samples are under investigation. Two different imaging protocols are discussed in the following.
Without light, there would be no sight. Light acts as a probe in various measurement techniques ranging from imaging to spectroscopy, from small scale cantilever displace- ment measurement in atomic force microscopy to large scale mirror motion in interfer- ometry, light detection and ranging (LIDAR) and many other fields. As the light probe intensity rises, it not only allows reducing the background noise effect, but it also aids precision to the measurement by reducing the photon noise (shot-noise) contribution. However, increasing intensity beyond certain threshold level is not always advantageous for ultra sensitive measurements. For example in the detection of gravitational waves, the current power circulating in the large scale interferometers can not be increased further without introducing other noise sources like thermal effects on the mirrors, un- wanted scattered photons, and back action due to radiation pressure. In the imaging of delicate photo sensitive sample, high power can causes cell damage or it may lead to disturb the regular process under investigation, viz. favouring certain biochemical reaction, which do not correspond to natural in vitro behaviour. At low intensity, pho- ton noise is an important concern and quantum states of light with correlated photon fluctuation can ideally represent a fruitful way to build specific measurement strategies to surpass the limitation of standard approach based on classical light sources, offering an avenue of solutions for ultra sensitive measurements. This thesis work focuses on two application of quantum enhanced measurement strate- gies. The first part of the thesis work has been dedicated to the realization of the first wide-field microscope with Sub Shot Noise (SSN) sensitivity. This is based on the ex- ploitation of quantum correlations in the Squeezed Vacuum state. In the second part, the investigations have made on the role of the quantum correlated beams in an unusual interferometric scheme, in which two identical optical interferometers are subject to the same phase fluctuation. The scheme, in the classical regime, has been already imple- mented in a large scale experiment devoted to the search of possible quantum gravity (QG) effects at Fermilab.
Quantum-enhanced metrology techniques offer ultra-precise timekeeping, allowing for breakthroughs in navigation systems, global positioning, medical and Biological imaging. Quantum metrology, a core application of quantum technologies, leverages non-classical states of light (such as entanglement and squeezing) to enhance measurement precision, sensitivity, and resolution beyond the classical limits, particularly the shot-noise limit. Its application to bio imaging and microscopy offers significant advantages for studying delicate biological specimens, which are often susceptible to damage from the intense light typically required by conventional high-resolution methods. The abstract highlights the potential of quantum properties of light to revolutionize bio imaging by providing a non-invasive, highly sensitive, and high-resolution platform for observing biological processes at fundamental scales.
Quantum correlations become formidable tools for beating classical capacities of measurement. Preserving these advantages in practical systems, where experimental imperfections are unavoidable, is a challenge of the utmost importance. Here we propose and realize a quantum ghost imaging protocol stemming from the differential ghost imaging, a scheme elaborated so far in the limit of bright thermal light, particularly suitable in the relevant case of faint or sparse objects. The extension toward the quantum regime represents an important step as quantum correlations allow low-brightness imaging, desirable for reducing the absorption dose. Furthermore, we optimize the protocol in terms of signal-to-noise ratio, to compensate for the detrimental effects of detection noise and losses. We perform the experiment using spontaneous parametric down conversion light in a microscope configuration. The image is reconstructed exploiting nonclassical intensity correlation in the low photon flux regime, rather than photon pairs detection coincidences. On the one side, we validate the theoretical model and on the other we show the applicability of this technique by imaging biological samples.
Nonlinear optical microscopy techniques have emerged as a set of successful tools for biological imaging. Stimulated emission microscopy belongs to a small subset of pump-probe techniques which can image non-fluorescent samples without requiring fluorescent labelling. However, its sensitivity has been shown to be ultimately limited by the quantum fluctuations in the probe beam. We propose and experimentally implement sub-shot-noise limited stimulated emission microscopy by preparing the probe pulse in an intensity-squeezed state. This technique paves the way for imaging delicate biological samples that have no detectable fluorescence with sensitivity beyond standard quantum fluctuations.
Summary form only given. In the last years several proof of principle experiments have demonstrated the advantages of quantum technologies respect to classical schemes in imaging, metrology and sensing. The present challenge is to overcome the limits of proof of principle demonstrations to approach real applications. Here, we present the such an achievement in the field of quantum enhanced imaging. In particular, we describe the realization of a sub-shot-noise (SSN) wide field microscope (see Ref. [1]), based on spatially multi-mode non-classical photon number correlations [2]. The microscope produces real-time images of 8000 pixels at full resolution, for (500μm)2field-of-view, with noise reduced at 80% of the shot noise level in each pixel. This is suitable for absorption imaging of complex structures, like biological samples. By simple post-elaboration, specifically applying a “quantum enhanced median filter”, the noise is further reduced (less than 30% of the shot noise level) by setting a trade-off with the resolution. It realizes the best sensitivity per incident photon ever achieved in absorption microscopy.
For absorption imaging, optical noise is a limitation [1]. Photon counting experiments using correlated photon pair sources can achieve sub-shot noise imaging but are not scalable to high brightness [2]. While lowering the noise floor increases the signal-to-noise ratio without increasing the signal's power, enhancing the precision reduces the variance of estimates. In imaging, improved precision can lead to higher contrast images, allowing regions with similar values to be more easily distinguished due to smaller error bars [2].