"Quid est ergo tempus? si nemo ex me quaerat, scio; si quaerenti explicare velim, nescio.''

[Augustinus Hipponensis]

"...an infinite series of times, in a dizzily growing, ever spreading network of diverging, converging and parallel times. This web of time--the strands of which approach one another, bifurcate, intersect or ignore each other through the centuries--embraces every possibility.''


The "problem of time" in essence stems from the fact that a canonical quantization of general relativity yields the Wheeler-De Witt equation predicting a static state of the universe, contrary to obvious everyday evidence. A solution was proposed by Page and Wootters : 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. Thus, time would be an emergent property of subsystems of the universe deriving from their entangled nature: an extremely elegant but controversial idea. 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) 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.


"Time from quantum entanglement: an experimental illustration"
Ekaterina Moreva, Giorgio Brida, Marco Gramegna, Vittorio Giovannetti, Lorenzo Maccone, Marco Genovese
arXiv:1310.4691 (2013)






We present the first experimental realization of the quantum illumination protocol proposed in Ref.s [S. Lloyd, Science 321, 1463 (2008); 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 realisation of quantum illumination"
E. D. Lopaeva, I. Ruo Berchera, I. P. Degiovanni, S. Olivares, G. Brida, and M. Genovese
Phys. Rev. Lett. 110, 153603 (2013)





The properties of quantum states have led to the development of new technologies, ranging from quantum information to quantum metrology. A recent field of research to emerge is quantum imaging, which aims to overcome the limits of classical imaging by making use of the 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 schemes takes advantage of the spatial quantum correlations between parametric down-conversion light beams to realize sub-shot-noise imaging of weak absorbing objects, leading ideally to noise-free imaging. Here, we present the first experimental realization of this scheme, showing its potential to achieve a larger signal-to-noise ratio than classical imaging methods. This work represents the starting point for this quantum technology, which we anticipate will have applications when there is a requirement for low-photon-flux illumination (for example for use with biological samples).

"Experimental realization of sub-shot-noise quantum imaging"
G. Brida, M. Genovese, I. Ruo Berchera
Nature Photonics 4, 227 (2010).




A quantum measurement can be described by a set of matrices, one for each possible outcome, which represents the probability operator-valued measure (POVM) of the sensor. Ecient protocols of POVM extraction for arbitrary sensors are required. We present the rst experimental POVM reconstruction that takes explicit advantage of a quantum resource, i.e. nonclassical correlations with an ancillary state. POVM of a photon-number-resolving detector is reconstructed by using strong quantum correlations of twin-beams generated by parametric downconversion. Our reconstruction method is more statistically robust than POVM reconstruction methods that use classical input states.


"Ancilla-assisted calibration of a measuring apparatus"
G. Brida, L. Ciavarella, I. P. Degiovanni, M. Genovese, A. Migdall, M. G. Mingolla, M.Paris, F. Piacentini, and S.Polyakov
Phys. Rev. Lett. 108, 253601 (2012)




Entanglement is the central resource of quantum information processing and the precise characterization of entangled states is a crucial issue for the development of quantum technologies. This leads to the necessity of a precise, experimental feasible measure of entanglement. Nevertheless, such measurements are limited both from experimental uncertainties and intrinsic quantum bounds. Here we present an experiment where the amount of entanglement of a family of two-qubit mixed photon states is estimated with the ultimate precision allowed by quantum mechanics.


"Experimental Estimation of Entanglement at the Quantum Limit"
Giorgio Brida, Ivo Pietro Degiovanni, Angela Florio, Marco Genovese, Paolo Giorda, Alice Meda, Matteo G. A. Paris and Alexander Shurupov
Phys. Rev. Lett. 104, 100501 (2010)