arXiv subjects

Stefano Olivares

Publications and source records attributed to Stefano Olivares.

At least 73 records · Page 4Linked to original sources

arXiv2016

Assessing the significance of fidelity as a figure of merit in quantum state reconstruction of discrete and continuous variable systems

We experimentally address the significance of fidelity as a figure of merit in quantum state reconstruction of discrete (DV) and continuous variable (CV) quantum optical systems. In particular, we analyze the use of fidelity in quantum homodyne tomography of CV states and maximum-likelihood polarization tomography of DV ones, focussing attention on nonclassicality, entanglement and quantum discord as a function of fidelity to a target state. Our findings show that high values of fidelity, despite well quantifying geometrical proximity in the Hilbert space, may be obtained for states displaying opposite physical properties, e.g. quantum or semiclassical features. In particular, we analyze in details the quantum-to-classical transition for squeezed thermal states of a single-mode optical system and for Werner states of a two-photon polarization qubit system.

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arXiv2016

Asymmetric Talbot-Lau interferometry for inertial sensing

We study in detail a peculiar configuration of the Talbot-Lau matter wave interferometer, characterised by unequal distances between the two diffraction gratings and the observation plane. We refer to this apparatus as the "asymmetric Talbot-Lau setup". Particular attention is given to its capabilities as an inertial sensor for particle and atomic beams, also in comparison with the classical moiré deflectometer. The present analysis is motivated by possible experimental applications in the context of antimatter wave interferometry, including the measurement of the gravitational acceleration of antimatter particles. To support our findings, we have performed numerical simulations of realistic particle beams with varying speed distributions.

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arXiv2015

Quantum state transfer via Bloch oscillations

The realization of reliable quantum channels, able to transfer a quantum state with high fidelity, is a fundamental step in the construction of scalable quantum devices. In this paper we describe a transmission scheme based on the genuinely quantum effect known as Bloch oscillations. The proposed protocol makes it possible to carry quantum state over different distances with a minimal engineering of the transmission medium and can be implemented and verified on current quantum technology hardware.

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arXiv2015

Full quantum state reconstruction of symmetric two-mode squeezed thermal states via spectral homodyne detection

We suggest and demonstrate a scheme to reconstruct the symmetric two-mode squeezed thermal states of spectral sideband modes from an optical parametric oscillator. The method is based on a single homodyne detector and active stabilization of the cavity. The measurement scheme have been successfully tested on different two-mode squeezed thermal states, ranging from uncorrelated coherent states to entangled states.

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arXiv2015

Gaussian state interferometry with passive and active elements

We address precision of optical interferometers fed by Gaussian states and involving passive and/or active elements, such as beam splitters, photodetectors and optical parametric amplifiers. We first address the ultimate bounds to precision by discussing the behaviour of the quantum Fisher information. We then consider photodetection at the output and calculate the sensitivity of the interferometers taking into account the non unit quantum efficiency of the detectors. Our results show that in the ideal case of photon number detectors with unit quantum efficiency the best configuration is the symmetric one, namely, passive (active) interferometer with passive (active) detection stage: in this case one may achieve Heisenberg scaling of sensitivity by suitably optimizing over Gaussian states at the input. On the other hand, in the realistic case of detectors with non unit quantum efficiency, the performances of passive scheme are unavoidably degraded, whereas detectors involving optical parametric amplifiers allow to fully compensate the presence of loss in the detection stage, thus restoring the Heisenberg scaling.

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arXiv2015

Matter-wave interferometry: towards antimatter interferometers

Starting from an elementary model and refining it to take into account more realistic effects, we discuss the limitations and advantages of matter-wave interferometry in different configurations. We focus on the possibility to apply this approach to scenarios involving antimatter, such as positrons and positronium atoms. In particular, we investigate the Talbot-Lau interferometer with material gratings and discuss in details the results in view of the possible experimental verification.

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arXiv2015

Quantum phase communication channels in the presence of static and dynamical phase diffusion

We address quantum communication channels based on phase modulation of coherent states and analyze in details the effects of static and dynamical (stochastic) phase diffusion. We evaluate mutual information for an ideal phase receiver and for a covariant phase-space-based receiver, and compare their performances by varying the number of symbols in the alphabet and/or the overall energy of the channel. Our results show that phase communication channels are generally robust against phase noise, especially for large alphabets in the low energy regime. In the presence of dynamical (non-Markovian) noise the mutual information is preserved by the time correlation of the environment, and when the noise spectra is detuned with respect to the information carrier, revivals of mutual information appears.

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arXiv2015

Single- and two-mode quantumness at a beam splitter

In the context of bipartite bosonic systems, two notions of classicality of correlations can be defined: P-classicality, based on the properties of the Glauber-Sudarshan P-function; and C-classicality, based on the entropic quantum discord. It has been shown that these two notions are maximally inequivalent in a static (metric) sense -- as they coincide only on a set of states of zero measure. We extend and reinforce quantitatively this inequivalence by addressing the dynamical relation between these types of non-classicality in a paradigmatic quantum-optical setting: the linear mixing at a beam splitter of a single-mode Gaussian state with a thermal reference state. Specifically, we show that almost all P-classical input states generate outputs that are not C-classical. Indeed, for the case of zero thermal reference photons, the more P-classical resources at the input the less C-classicality at the output. In addition, we show that the P-classicality at the input -- as quantified by the non-classical depth -- does instead determine quantitatively the potential of generating output entanglement. This endows the non-classical depth with a new operational interpretation: it gives the maximum number of thermal reference photons that can be mixed at a beam splitter without destroying the output entanglement.

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arXiv2015

Squeezing of mechanical motion via qubit-assisted control

We propose a feedback control mechanism for the squeezing of the phononic mode of a mechanical oscillator. We show how, under appropriate working conditions, a simple adiabatic approach is able to induce mechanical squeezing. We then go beyond the limitations of such a working point and demonstrate the stationary squeezing induced by using repeated measurements and re-initialisation of the state of a two-level system ancilla coupled to the oscillator. Our non-adaptive feedback loop offers interesting possibilities for quantum state engineering and steering in open-system scenarios.

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arXiv2014

Soft-metric based channel decoding for photon counting receivers

We address photon-number-assisted, polarization- based, binary communication systems equipped with photon counting receivers. In these channels information is encoded in the value of polarization phase-shift but the carrier has and additional degree of freedom, i.e. its photon distribution, which may be exploited to implement binary input-multiple output (BIMO) channels also in the presence of a phase-diffusion noise affecting the polarization. Here we analyze the performances of these channels, which approach capacity by means of iteratively decoded error correcting codes. In this paper we use soft-metric- based low density parity check (LDPC) codes for this purpose. In order to take full advantage of all the information available at the output of a photon counting receiver, soft information is generated in the form of log-likelihood ratios, leading to improved frame error rate (FER) and bit error rate (BER) compared to binary symmetric channels (BSC). We evaluate the classical capacity of the considered BIMO channel and show the potential gains that may be provided by photon counting detectors in realistic implementations.

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arXiv2014

Two-step procedure to discriminate discordant from classical correlated or factorized states

We devise and experimentally realize a procedure capable of detecting and distinguishing quantum discord and classical correlations as well the presence of factorized states in a joint system-environment setting. Our scheme builds on recent theoretical results showing how the distinguishability between two reduced states of a quantum system in a bipartite setting can convey important information about the correlations present in the bipartite state and the interaction between the subsystems. The two addressed subsystems are the polarization and spatial degrees of freedom of the signal beam generated by parametric down-conversion, which are suitably prepared by the idler beam. Different global and local operations allow for the detection of different correlations by studying via state tomography the trace distance behavior between suitable polarization subsystem states.

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arXiv2014

Entanglement generation in the ultra-strongly coupled Rabi model

We analyze the dynamics of the quantum Rabi model for two qubits interacting through a common bosonic field mode (resonator), focusing on the generation and detection of maximally entangled Bell states. We obtain analytical results for the unitary dynamics of this system in the slow-qubit (or degenerate) regime, considering ultra-strong coupling between qubits and resonator mode, for which the rotating wave approximation is no longer applicable. We also numerically investigate the dynamics beyond the slow-qubit condition in order to study the validity of the model in the presence of less strict conditions.

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arXiv2014

Quantifying the source of enhancement in experimental continuous variable quantum illumination

A quantum illumination protocol exploits correlated light beams to enhance the probability of detection of a partially reflecting object lying in a very noisy background. Recently a simple photon-number-detection based implementation of a quantum illumination-like scheme has been provided in [Lopaeva {\it et al,}, Phys. Rev. Lett. {\bf 101}, 153603 (2013)] where the enhancement is preserved despite the loss of non-classicality. In the present paper we investigate the source for quantum advantage in that realization. We introduce an effective two-mode description of the light sources and analyze the mutual information as quantifier of total correlations in the effective two-mode picture. In the relevant regime of a highly thermalized background, we find that the improvement in the signal-to-noise ratio achieved by the entangled sources over the unentangled thermal ones amounts exactly to the ratio of the effective mutual informations of the corresponding sources. More precisely, both quantities tend to a common limit specified by the squared ratio of the respective cross-correlations. A thorough analysis of the experimental data confirms this theoretical result.

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arXiv2014

Real-time phase-reference monitoring of a quasi-optimal coherent-state receiver

The Kennedy-like receiver is a quasi-optimal receiver employed in binary phase-shift-keyed communication schemes with coherent states. It is based on the interference of the two signals encoding the message with a reference local oscillator and on/off photodetection. We show both theoretically and experimentally that it is possible to extract useful information about the phase reference by Bayesian processing of the very same data sample used to discriminate the signals shot by shot. We demonstrate that the minimum uncertainty in phase estimation, given by the inverse of the Fisher information associated with the statistics of the collected data, can be achieved. We also numerically and experimentally investigate the performances of our phase-estimation method in the presence of phase noise, when either on/off or photon-number resolving detectors are employed.

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arXiv2014

About the use of fidelity in continuous variable systems

We present examples of continuous variable (CV) states having high fidelity to a given target, say $F > 0.9$ or $F > 0.99$, and still showing striking differences in their physical properties, including classical and quantum states within the set, separable and entangled ones, or nearly Gaussian and strongly non-Gaussian ones. We also show that the phenomenon persists also when one imposes additional constraints on the energy or the squeezing fraction of the states, thus generally questioning the use of fidelity to assess properties of CV systems.

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arXiv2014

Detecting quantum non-Gaussianity of noisy Schrödinger cat states

Highly quantum non-linear interactions between different bosonic modes lead to the generation of quantum non-Gaussian states, i.e. states that cannot be written as mixtures of Gaussian states. A paradigmatic example is given by Schroedinger's cat states, that is coherent superpositions of coherent states with opposite amplitude. We here consider a novel quantum non-Gaussianity criterion recently proposed in the literature and prove its effectiveness on Schroedinger cat states evolving in a lossy bosonic channel. We prove that quantum non-Gaussianity can be effectively detected for high values of losses and for large coherent amplitudes of the cat states.

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arXiv2014

Exact and approximate solutions for the quantum minimum-Kullback-entropy estimation problem

The minimum Kullback entropy principle (mKE) is a useful tool to estimate quantum states and operations from incomplete data and prior information. In general, the solution of a mKE problem is analytically challenging and an approximate solution has been proposed and employed in different context. Recently, the form and a way to compute the exact solution for finite dimensional systems has been found, and a question naturally arises on whether the approximate solution could be an effective substitute for the exact solution, and in which regimes this substitution can be performed. Here, we provide a systematic comparison between the exact and the approximate mKE solutions for a qubit system when average data from a single observable are available. We address both mKE estimation of states and weak Hamiltonians, and compare the two solutions in terms of state fidelity and operator distance. We find that the approximate solution is generally close to the exact one unless the initial state is near an eigenstate of the measured observable. Our results provide a rigorous justification for the use of the approximate solution whenever the above condition does not occur, and extend its range of application beyond those situations satisfying the assumptions used for its derivation.

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arXiv2014

Bracket states for communication protocols with coherent states

We present the generation and characterization of the class of bracket states, namely phase-sensitive mixtures of coherent states exhibiting symmetry properties in the phase-space description. A bracket state can be seen as the statistical ensemble arriving at a receiver in a typical coherent-state-based communication channel. We show that when a bracket state is mixed at a beam splitter with a local oscillator, both the emerging beams exhibit a Fano factor larger than 1 and dependent on the relative phase between the input state and the local oscillator. We discuss the possibility to exploit this dependence to monitor the phase difference for the enhancement of the performances of a simple communication scheme based on direct detection. Our experimental setup involves linear optical elements and a pair of photon-number-resolving detectors operated in the mesoscopic photon-number domain.

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