Properties of quantum states have disclosed new technologies, ranging from quantum information to quantum metrology. Among them a recent research field is quantum imaging, addressed to overcome limits of classical imaging by exploiting spatial properties of quantum states of light . In particular quantum correlations between twin beams represent a fundamental resource for these studies. One of the most interesting proposed scheme exploits spatial quantum correlations between parametric down conversion light beams for realizing sub-shot-noise imaging of the weak absorbing objects, leading ideally to a noise-free imaging. Here we present the first experimental realisation of this scheme, showing its capability to reach a larger signal to noise ratio (SNR) with respect to classical imaging methods. This work represents the starting point of this quantum technology that can have relevant applications, especially whenever there is a need of a low photon flux illumination (e.g. as with certain biological samples)

We present the first experimental realization of the quantum illumination protocol proposed by Lloyd [ Science 321 1463 (2008)] and S. Tan et al. [ Phys. Rev. Lett. 101 253601 (2008)], achieved in a simple feasible experimental scheme based on photon-number correlations. A main achievement of our result is the demonstration of a strong robustness of the quantum protocol to noise and losses that challenges some widespread wisdom about quantum technologies

Experimental reconstructions of photon number distributions of both continuous-wave and pulsed light beams are reported. Our scheme is based on on/off avalanche photodetection assisted by maximum-likelihood estimation and does not involve photon counting. Reconstructions of the distribution for both semiclassical and quantum states of light are reported for single-mode as well as for multimode beams.

In the last years several theoretical papers discussed if time can be an emergent propertiy deriving from quantum correlations. Here, to provide an insight into how this phenomenon can occur, we present an experiment that illustrates Page and Wootters' mechanism of "static" time, and Gambini et al. subsequent refinements. A static, entangled state between a clock system and the rest of the universe is perceived as evolving by internal observers that test the correlations between the two subsystems. We implement this mechanism using an entangled state of the polarization of two photons, one of which is used as a clock to gauge the evolution of the second: an "internal" observer that becomes correlated with the clock photon sees the other system evolve, while an "external" observer that only observes global properties of the two photons can prove it is static."Quid est ergo tempus? si nemo ex me quaerat, scio; si quaerenti explicare velim, nescio." [1] The "problem of time" [2][3][4][5][6] in essence stems from the fact that a canonical quantization of general relativity yields the Wheeler-De Witt equation [7,8] predicting a static state of the universe, contrary to obvious everyday evidence. A solution was proposed by Page and Wootters [9, 10]: thanks to quantum entanglement, a static system may describe an evolving "universe" from the point of view of the internal observers. Energy-entanglement between a "clock" system and the rest of the universe can yield a stationary state for an (hypothetical) external observer that is able to test the entanglement vs. abstract coordinate time. The same state will be, instead, evolving for internal observers that test the correlations between the clock and the rest [9][10][11][12][13][14]. Thus, time would be an emergent property of subsystems of the universe deriving from their entangled nature: an extremely elegant but controversial idea [2,15]. Here we want to demystify it by showing experimentally that it can be naturally embedded into (small) subsystems of the universe, where Page and Wootters' mechanism (and Gambini et al. subsequent refinements [12,16]) can be easily studied. We show how a static, entangled state of two photons can be seen as evolving by an observer that uses one of the two photons as a clock to gauge the time-evolution of the other photon. However, an external observer can show that the global entangled state does not evolve.Even though it revolutionizes our ideas on time, Page and Wootters' (PaW) mechanism is quite simple [9-11]: they provide a static entangled state |Ψ whose subsystems evolve according to the Schrödinger equation for an observer that uses one of the subsystems as a clock system C to gauge the time evolution of the rest R. While the division into subsystems is largely arbitrary, the PaW model assumes the possibility of neglecting interaction among them writing the Hamiltonian of the global system as H = H c ⊗ 1 1 r + 1 1 c ⊗ H r , where H c , H r are the local terms associated with C and R, respectively and Uc(t) = e −iHct/ are the ...

We experimentally demonstrate quantum enhanced resolution in confocal fluorescence microscopy exploiting the nonclassical photon statistics of single nitrogen-vacancy color centers in diamond. By developing a general model of superresolution 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. DOI: 10.1103/PhysRevLett.113.143602 PACS numbers: 42.50.-p, 42.30.Va, 42.50.Ar, 42.50.St In the last decade, measurement techniques enhanced by using peculiar properties of quantum light [1,2] have been successfully demonstrated in several remarkable real application scenarios, for example, interferometric measurements aimed to reveal gravitational waves and the quantum gravity effect [3,4], biological particle tracking [5], phase contrast microscopy [6], and imaging [7,8]. Very recently, a novel technique to beat the diffraction limit in microscopy that relies on the antibunching behavior of photons emitted by single fluophores has been proposed [9], and realized in wide field microscopy [10] by using an EMCCD camera.The maximum obtainable imaging resolution in classical far-field fluorescence microscopy, according to the Abbe diffraction limit, is R ≃ 0.61λ=NA, where λ is the wavelength of the light and NA is the numerical aperture of the objective. This restricts the current capability of precisely measuring the position of very small objects such as single photon emitters (color centers, quantum dots, etc.) [11][12][13][14][15][16][17][18][19], limiting their potential exploitation in the frame quantum technology [20,21]. In general, the research of methods to obtain a microscopy resolution below the diffraction limit is a topic of the utmost interest [22][23][24][25][26][27][28][29] that could provide dramatic improvement in the observation of several systems spanning from quantum dots [30] to living cells [31][32][33][34]. As a notable example, in several entanglement-related experiments using strongly coupled single photon emitters it is of the utmost importance to measure their positions with the highest spatial resolution [35]. In principle, this limitation can be overcome by recently developed microscopy techniques such as stimulated emission depletion (STED) and ground state depletion (GSD) [36,37]. Nevertheless, even if they have been demonstrated effectively able to provide superresolved imaging in many specific applications, among which are color centers in diamond [38], they are characterized by rather specific experimental requirements (dual laser excitation system, availability of luminescence quenching mechanisms by stimulated emission, nontrivial shaping of the quenching beam, high power). Furthermore, these techniques are not suitable in applications in which the fluorescence is not optically induced [39,40], so that new methods are required for those applications.Inspired by the works in [9], in this Letter we develop a comprehensive theory of superresolution ima...