The peculiar properties of quantum optical states represent a new resource for innovative imaging schemes, as sub shot noise imaging or quantum illumination. Here we present in detail two works realized in INRIM. The first involves exploiting entanglement to enhance the imaging of a pure phase object in a non-interferometric setting. This wide-field method, based on the "transport of intensity equation", provides the absolute value of the phase without prior knowledge of the object. It does not require spatial and temporal coherence of the incident light. Besides improving image quality at a fixed number of photons, we demonstrate a clear reduction of the uncertainty in the quantitative phase estimation. This research also paves the way for applications at different wavelengths, e.g., x-ray imaging, where reducing the photon dose is of utmost importance. Then, we demonstrate a novel imaging technique, named Light Field Ghost Imaging, which exploits light correlations and light field imaging principles to enable overcome the limitations of ghost imaging in a wide range of applications. Notably, our technique removes the requirement to have prior knowledge of the object distance allowing us to refocus in post-processing and to perform 3D imaging while retaining all the benefits of GI protocols.
In this work we exploit entangled twin-beam to enhance quantitative phase retrieval of an object in a non-interferometric setting, only measuring the propagated intensity pattern after interaction with the object. The scheme achieves full field phase reconstruction with quasi-single-shot acquisition, not requiring time consuming raster-scanning of the sample. In our experiment a 40% improved precision has been achieved with respect to the corresponding classical scheme.
Quantum states of light have been shown to be able to provide an advantage over classical ones in a variety of tasks. Our work shows how practical enhancement can be obtained, using quantum correlations, in the readout of classical information from a digital memory. The quantum advantadge is preserved also in a more complex tasks, namely pattern recognition, highlighting the potential for applications of the techniques proposed.
The protocol of quantum reading refers to the quantum enhanced retrieval of information from an optical memory, whose generic cell stores a bit of information in two possible lossy channels. In the following we analyze the case of a particular class of optical receiver, based on photon counting measurement, since they can be particularly simple in view of real applications. We show that a quantum advantage is achievable when a transmitter based on two-mode squeezed vacuum (TMSV) states is combined with a photon counting receiver, and we experimentally confirm it. In this paper, after introducing some theoretical background, we focus on the experimental realisation, describing the data collection and the data analysis in detail.
The search for Planck scale effects is one of holy grains of physics. At Fermilab, a system of two Michelson interferometers (MIs) was built for this purpose: the holometer. This device operates using classical light, and, therefore, its sensitivity is shot-noise limited. In collaboration with the Danish Technical University, we built a proof of principle experiment devoted to experimentally demonstrate how quantum light could improve the holometer sensitivity below the shot noise limit. It is the first time that quantum light is used in a correlated interferometric system. In particular the injection of two single mode squeezed state (one in each interferometer) and of a twin-beam state is considered, and the system performance compared in the two cases. In this proceeding, after a general introduction to the holometer purposes and to our experimental set-up, we present some characterization measurements concerning the quantum light injection.
In this paper we describe the preliminary results obtained at INRiM laboratories toward realizing a couple of correlated power-recycled Michelson interferometers. This system is the first step toward the realization of a quantum-enhanced holometer.
Quantum Key Distribution, a fundamental component of quantum secure communication that exploits quantum states and resources for communication protocols, can future-proof the security of digital communications, when if advanced quantum computing systems and mathematical advances render current algorithmic cryptography insecure. A QKD system relies on the integration of quantum physical devices, as quantum sources, quantum channels and quantum detectors, in order to generate a true random (unconditionally secure) cryptographic key between two remote parties connected through a quantum channel. The gap between QKD implemented with ideal and real devices can be exploited to attack real systems, unless appropriate countermeasures are implemented. Characterization of real devices and countermeasure is necessary to guarantee security. Free-space QKD systems can provide secure communication to remote parties of the globe, while QKD systems based on entanglement are intrinsically less vulnerable to attack. Metrology to characterize the optical components of these systems is required. Actually, the “Optical metrology for quantum-enhanced secure telecommunication” Project (MIQC2) is steering the metrological effort for Quantum Cryptography in the European region in order to accelerate the development and commercial uptake of Quantum Key Distribution (QKD) technologies. Aim of the project is the development of traceable measurement techniques, apparatus, and protocols that will underpin the characterisation and validation of the performance and quantum-safe security of such systems, essential steps towards standardization and certification of practical implementations of quantum-based technologies.
Plenoptic imaging (PI) is an optical technique to perform three-dimensional imaging in a single shot. It is enabled by the simultaneous measurement of both the location and the propagation direction of light in a given scene. Despite being very useful for extending the depth of field, such technique entails a strong trade- off between spatial and angular resolution. This makes the resolution and the maximum achievable depth of focus inversely proportional; hence, resolution cannot be diffraction-limited. We have recently proposed a new procedure, called Correlation Plenoptic Imaging (CPI), to overcome such fundamental limits by collecting plenoptic information through intensity correlation measurement. Using two correlated beams, from either a chaotic or an entangled photon source, we perform imaging in one arm and simultaneously obtain the angular information in the other arm. In this paper, we discuss the case in which the two correlated beams of light are generated by spontaneous parametric down-conversion. We review the principles of CPI with entangled photons and discuss its resolution and depth-of-field limits.
KEYWORDS: Microscopes, Absorption, Image resolution, Real time imaging, Photodetectors, Microscopy, Metrology, Fluctuations and noise, Signal to noise ratio
Quantum technologies promise to overcome by far the limits of the classical schemes. However, the present challenge is to overpass the limits of proof of principle demonstrations to approach real applications. In this paper, we present an experiment which aims to bridge this gap in the field of quantum enhanced imaging. In particular, we realize a sub-shot noise wide field microscope based on spatially multi-mode non-classical photon number correlations in twin beams. The microscope produces real time images of 8000 pixels at full resolution, with noise reduced to the 80% of the shot noise level (for each pixel), hence able to image faint samples at low illumination level. The noise can be further reduced (less than 30% of the shot noise level) turning down the resolution. It demonstrates the best sensitivity per incident photon ever achieved in absorption microscopy.
Properties of quantum light represent a tool for overcoming limits of classical optics. Several experiments have demonstrated this advantage ranging from quantum enhanced imaging to quantum illumination. In this work, experimental demonstration of quantum-enhanced resolution in confocal fluorescence microscopy will be presented. This is achieved by exploiting the non-classical photon statistics of fluorescence emission of single nitrogen-vacancy (NV) color centers in diamond. By developing a general model of super-resolution based on the direct sampling of the kth-order autocorrelation function of the photoluminescence signal, we show the possibility to resolve, in principle, arbitrarily close emitting centers. Finally, possible applications of NV-based fluorescent nanodiamonds in biosensing and future developments will be presented.
We analyze in detail a system of two interferometers aimed at the detection of extremely faint phase fluctuations. The idea behind is that a correlated phase-signal like the one predicted by some phenomenological theory of Quantum Gravity (QG) could emerge by correlating the output ports of the interferometers, even when in the single interferometer it confounds with the background. We demonstrated that injecting quantum light in the free ports of the interferometers can reduce the photon noise of the system beyond the shot-noise, enhancing the resolution in the phase-correlation estimation. Our results confirm the benefit of using squeezed beams together with strong coherent beams in interferometry, even in this correlated case. On the other hand, our results concerning the possible use of photon number entanglement in twin beam state pave the way to interesting and probably unexplored areas of application of bipartite entanglement and, in particular, the possibility of reaching surprising uncertainty reduction exploiting new interferometric configurations, as in the case of the system described here.
One of the key issues in QKD is the rather limited data rate at which the secret key can be generated. This paper explores the use of quantum correlation associated with twin beams in Parametric Down Conversion (PDC) to in effect create a number of parallel channels for generation of secret keys, thus significantly boosting the achievable key rate. Such quantum correlations have been effectively used as a tool for many applications, including calibration of single photon detectors and QKD applications.1 Within QKD applications, the natural setup of quantization of Charge Coupled Device (CCD) detection areas and subsequent measurement of the correlation statistic needed to detect the presence of the eavesdropper Eve, leads to a set of QKD parallel channel models that are either binary or multilevel Discrete Memoryless Channels (DMC). This work explores the derivation of proper channel models for this application starting from measured data and the optimization of the secret key rate. Analytical results based on measurements performed on a 30 pixel image suggest that nearly an 8-fold increase in secret key rate may be achievable using this technique.
We present two recent results achieved in INRIM laboratories paving the way for next future commercial use of quantum imaging techniques. The first exploits non-classical photon statistics of single nitrogen-vacancy color centers in diamond for realising super-resolution. A little more in detail we demonstrate that the measurement of high order correlation functions allows overcoming Abbe limit. The second exploits ghost imaging in a specific case of practical interest, i.e. in measuring magnetic structures in garnets.
We present some Quantum Tomography related results recently obtained in the Quantum Optics labs of the National Institute of Metrological Research (INRIM). Initially we describe the first experimental implementation of a new protocol for the reconstruction of a photon-number-resolving (PNR) detector’s POVM (Positive Operator-Valued Measure): such a protocol, exploiting the strong quantum correlations of an ancillary state, results more robust and efficient than its classical counterparts. The second part of the paper focuses on the quantum characterization of a transition-edge sensor (TES) based PNR detector, i.e. the experimental tomography of the POVM of a TES, with a method based on a quorum of coherent probes: we show the reconstruction of the POVM elements up to 11 detected photons and 100 incoming photons, demonstrating the linearity of such a device. Finally, we present a method for the experimental reconstruction of the modal structure of multimode optical fields exploiting a single measurement of higher-order photon number autocorrelation functions. We show our reconstructions of up to three different modes per optical state, demonstrating the excellent agreement with the theoretical predictions and the robustness of our method itself.
Quantum properties of the optical field represent a resource of the utmost relevance for the development of quantum
technologies, allowing unprecedented results in disciplines ranging from quantum information and metrology to quantum
imaging. Spatial quantum correlations generated by of parametric down conversion (PDC) represents a tool for quantum
imaging because they are intrinsically multimode, a requirement for obtaining large degree of correlation over small
portions of the beams, allowing to register the spatial structure of an object. In particular a very interesting example is
provided by the detection of weak objects, a result that could have important practical applications. The principle of this
technique is to take advantage of the correlation in the noise of two conjugated branches of PDC emission: in fact,
subtracting the noise measured on one branch from the image of a weak object obtained in the other branch, the image of
the object, eventually previously hidden in the noise, could be restored.
Here, after a general summary of quantum imaging techniques, firstly we will show how we have reached a sub shot
noise regime and then improved this result up to reach a regime where it was possible to achieve the first experimental
realisation of sub shot noise imaging of a weak absorbing object.
Speckle structure1 of Parametric Down Conversion (PDC) has recently received a large attention due to the
relevance in view of applications to quantum imaging. The possibility of tailoring the speckle size by acting
on the pump intensity and dimensions is an interesting tool for the applications to quantum imaging and in
particular to the detection of weak objects under shot-noise limit. In this paper we report a systematic detailed
study of the speckle structure, in particular of the one in type II PDC, with attention to its variation with pump
beam characteristics (power and radius).
In this paper we prove both theoretically and experimentally the insecurity of the Ekert’s protocol for quantum-key distribution when based on the Wigner inequality when entangled photon pairs are used as source. The security proof of the Eckert’s protocol based on Wigner’s inequality consists in verifying the violation of W ≥ 0 (which corresponds to local-realistic theories). We highlight that a violation of the Wigner’s inequality occurs when Eve controls the two quantum channels. We give the theoretical motivation for this result and we perform an experiment proving this weakness, by realizing the condition of Eve gaining total control of the source of photon pairs. In addition we calculate the modified version of this inequality which guarantees safe quantum-key distribution, thus proving it experimentally.
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