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M. G. A. Paris

Publications and source records attributed to M. G. A. Paris.

At least 37 records · Page 2Linked to original sources
arXiv2010

Quantum state reconstruction using binary data from on/off photodetection

The knowledge of the density matrix of a quantum state plays a fundamental role in several fields ranging from quantum information processing to experiments on foundations of quantum mechanics and quantum optics. Recently, a method has been suggested and implemented in order to obtain the reconstruction of the diagonal elements of the density matrix exploiting the information achievable with realistic on/off detectors, e.g. silicon avalanche photo-diodes, only able to discriminate the presence or the absence of light. The purpose of this paper is to provide an overview of the theoretical and experimental developments of the on/off method, including its extension to the reconstruction of the whole density matrix.

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arXiv2010

Conditional measurements on multimode pairwise entangled states from spontaneous parametric downconversion

We address the intrinsic multimode nature of the quantum state of light obtained by pulsed spontaneous parametric downconversion and develop a theoretical model based only on experimentally accessible quantities. We exploit the pairwise entanglement as a resource for conditional multimode measurements and derive closed formulas for the detection probability and the density matrix of the conditional states. We present a set of experiments performed to validate our model in different conditions that are in excellent agreement with experimental data. Finally, we evaluate nonGaussianity of the conditional states obtained from our source with the aim of discussing the effects of the different experimental parameters on the efficacy of this type of conditional state preparation.

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arXiv2010

Quantum characterization of bipartite Gaussian states

Gaussian bipartite states are basic tools for the realization of quantum information protocols with continuous variables. Their complete characterization is obtained by the reconstruction of the corresponding covariance matrix. Here we describe in details and experimentally demonstrate a robust and reliable method to fully characterize bipartite optical Gaussian states by means of a single homodyne detector. We have successfully applied our method to the bipartite states generated by a sub-threshold type-II optical parametric oscillator which produces a pair of thermal cross-polarized entangled CW frequency degenerate beams. The method provide a reliable reconstruction of the covariance matrix and allows to retrieve all the physical information about the state under investigation. These includes observable quantities, as energy and squeezing, as well as non observable ones as purity, entropy and entanglement. Our procedure also includes advanced tests for Gaussianity of the state and, overall, represents a powerful tool to study bipartite Gaussian state from the generation stage to the detection one.

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arXiv2010

Non-classical correlations in non-Markovian continuous variable systems

We consider two identical and non-interacting harmonic oscillators coupled to either two independent bosonic baths or to a common bosonic bath. Under the only assumption of weak coupling, we analyze in details the non-Markovian short time-scale evolution of intensity correlations, entanglement and quantum discord for initial two-mode squeezed-thermal vacuum states. In the independent reservoirs case we observe the detrimental effect of the environment for all these quantities and we establish a hierarchy for their robustness against the environmental noise. In the common reservoir case, for initial uncorrelated states, we find that only quantum discord can be created via interaction with the bath, while entanglement and sub shot noise intensity correlations remain absent.

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arXiv2009

Experimental estimation of one-parameter qubit gates in the presence of phase diffusion

We address estimation of one-parameter qubit gates in the presence of phase diffusion. We evaluate the ultimate quantum limits to precision, seek for optimal probes and measurements, and demonstrate an optimal estimation scheme for polarization qubits. An adaptive method to achieve optimal estimation in any working regime is also analyzed in details and experimentally implemented.

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arXiv2009

Continuous variable entanglement dynamics in structured reservoirs

We address the evolution of entanglement in bimodal continuous variable quantum systems interacting with two independent structured reservoirs. We derive an analytic expression for the entanglement of formation without performing the Markov and the secular approximations and study in details the entanglement dynamics for various types of structured reservoirs and for different reservoir temperatures, assuming the two modes initially excited in a twin-beam state. Our analytic solution allows us to identify three dynamical regimes characterized by different behaviors of the entanglement: the entanglement sudden death, the non-Markovian revival and the non-secular revival regimes. Remarkably, we find that, contrarily to the Markovian case, the short-time system-reservoir correlations in some cases destroy quickly the initial entanglement even at zero temperature.

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arXiv2009

State mapping and discontinuous entanglement transfer in a multipartite open system

We describe the transfer of quantum information and correlations from an entangled tripartite bosonic system to three separate qubits through their local environments also in the presence of various dissipative effects. Optimal state mapping and entanglement transfer are shown in the framework of optical cavity quantum electrodynamics involving qubit-like radiation states and two-level atoms via the mediation of cavity modes. For an input GHZ state mixed with white noise we show the occurrence of sudden death and birth of entanglement that is discontinuously exchanged among the tripartite subsystems.

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arXiv2009

On the discrimination between classical and quantum states

With the purpose to introduce a useful tool for researches concerning foundations of quantum mechanics and applications to quantum technologies, here we study three quantumness quantifiers for bipartite optical systems: one based on sub-shot-noise correlations, one related to antibunching and one springing from entanglement determination. The specific cases of parametric down conversion seeded by thermal, coherent and squeezed states are discussed in detail.

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arXiv2009

State reconstruction by on/off measurements

We demonstrate a state reconstruction technique which provides either the Wigner function or the density matrix of a field mode and requires only avalanche photodetectors, without any phase or amplitude discrimination power. It represents an alternative, of simpler implementation, to quantum homodyne tomography.

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arXiv2009

An effective method to estimate multidimensional Gaussian states

A simple and efficient method for characterization of multidimensional Gaussian states is suggested and experimentally demonstrated. Our scheme shows analogies with tomography of finite dimensional quantum states, with the covariance matrix playing the role of the density matrix and homodyne detection providing Stern-Gerlach-like projections. The major difference stems from a different character of relevant noises: while the statistics of Stern-Gerlach-like measurements is governed by binomial statistics, the detection of quadrature variances correspond to chi-square statistics. For Gaussian and near Gaussian states the suggested method provides, compared to standard tomography techniques, more stable and reliable reconstructions. In addition, by putting together reconstruction methods for Gaussian and arbitrary states, we obtain a tool to detect the non-Gaussian character of optical signals.

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arXiv2009

Full characterization of Gaussian bipartite entangled states by a single homodyne detector

We present the full experimental reconstruction of Gaussian entangled states generated by a type--II optical parametric oscillator (OPO) below threshold. Our scheme provides the entire covariance matrix using a single homodyne detector and allows for the complete characterization of bipartite Gaussian states, including the evaluation of purity, entanglement and nonclassical photon correlations, without a priori assumptions on the state under investigation. Our results show that single homodyne schemes are convenient and robust setups for the full characterization of OPO signals and represent a tool for quantum technology based on continuous variable entanglement.

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arXiv2008

Constrained MaxLik reconstruction of multimode photon distributions

We address the reconstruction of the full photon distribution of multimode fields generated by seeded parametric down-conversion (PDC). Our scheme is based on on/off avalanche photodetection assisted by maximum-likelihood (MaxLik) estimation and does not involve photon counting. We present a novel constrained MaxLik method that incorporates the request of finite energy to improve the rate of convergence and, in turn, the overall accuracy of the reconstruction.

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arXiv2006

Reconstruction of photon statistics using low performance photon counters

The output of a photodetector consists of a current pulse whose charge has the statistical distribution of the actual photon numbers convolved with a Bernoulli distribution. Photodetectors are characterized by a nonunit quantum efficiency, i.e. not all the photons lead to a charge, and by a finite resolution, i.e. a different number of detected photons leads to a discriminable values of the charge only up to a maximum value. We present a detailed comparison, based on Monte Carlo simulated experiments and real data, among the performances of detectors with different upper limits of counting capability. In our scheme the inversion of Bernoulli convolution is performed by maximum-likelihood methods assisted by measurements taken at different quantum efficiencies. We show that detectors that are only able to discriminate between zero, one and more than one detected photons are generally enough to provide a reliable reconstruction of the photon statistics for single-peaked distributions, while detectors with higher resolution limits do not lead to further improvements. In addition, we demonstrate that, for semiclassical states, even on/off detectors are enough to provide a good reconstruction. Finally, we show that a reliable reconstruction of multi-peaked distributions requires either higher quantum efficiency or better capability in discriminating high number of detected photons.

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arXiv2006

Joint multipartite photon statistics by on/off detection

We demonstrate a method to reconstruct the joint photon statistics of two or more modes of radiation using on/off photodetection performed at different quantum efficiencies. The two-mode case is discussed in details and experimental results are presented for the bipartite states obtained after a beam-splitter fed by a single photon state or a thermal state.

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arXiv2006

Measuring the photon distribution by ON/OFF photodectors

Reconstruction of photon statistics of optical states provide fundamental information on the nature of any optical field and find various relevant applications. Nevertheless, no detector that can reliably discriminate the number of incident photons is available. On the other hand the alternative of reconstructing density matrix by quantum tomography leads to various technical difficulties that are particular severe in the pulsed regime (where mode matching between signal an local oscillator is very challenging). Even if on/off detectors, as usual avalanche PhotoDiodes operating in Geiger mode, seem useless as photocounters, recently it was shown how reconstruction of photon statistics is possible by considering a variable quantum efficiency. Here we present experimental reconstructions of photon number distributions of both continuous-wave and pulsed light beams in a scheme based on on/off avalanche photodetection assisted by maximum-likelihood estimation. Reconstructions of the distribution for both semiclassical and quantum states of light (as single photon, coherent, pseudothermal and multithermal states) are reported for single-mode as well as for multimode beams. The stability and good accuracy obtained in the reconstruction of these states clearly demonstrate the interesting potentialities of this simple technique.

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arXiv2006

Transmittivity measurements by means of squeezed vacuum light

A method for measuring the transmittivity of optical samples by using squeezed--vacuum radiation is illustrated. A squeezed vacuum field generated by a below--threshold optical parametric oscillator is propagated through a nondispersive medium and detected by a homodyne apparatus. The variance of the detected quadrature is used for measuring the transmittivity. With this method it is drastically reduced the number of photons passing through the sample during the measurement interval. The results of some tests are reported.

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arXiv2005

Quantum and classical correlations of intense beams of light via joint photodetection

We address joint photodetection as a method to discriminate between the classical correlations of a thermal beam divided by a beam splitter and the quantum entanglement of a twin-beam obtained by parametric downconversion. We show that for intense beams of light the detection of the difference photocurrent may be used, in principle, in order to reveal entanglement, while the simple measurement of the correlation coefficient is not sufficient. We have experimentally measured the correlation coefficient and the variance of the difference photocurrent on several classical and quantum states. Results are in good agreement with theoretical predictions taking into account the extra noise in the generated fields that is due to the pump-laser fluctuations.

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