arXiv subjects

Stefano Olivares

Publications and source records attributed to Stefano Olivares.

At least 109 records · Page 6Linked to original sources

arXiv2011

Optical phase estimation in the presence of phase-diffusion

The measurement problem for the optical phase has been traditionally attacked for noiseless schemes or in the presence of amplitude or detection noise. Here we address estimation of phase in the presence of phase diffusion and evaluate the ultimate quantum limits to precision for phase-shifted Gaussian states. We look for the optimal detection scheme and derive approximate scaling laws for the quantum Fisher information and the optimal squeezing fraction in terms of the overall total energy and the amount of noise. We also found that homodyne detection is a nearly optimal detection scheme in the limit of very small and large noise.

quant-ph↗

arXiv2010

Optimal unambiguous comparison of two unknown squeezed vacua

We propose a scheme for unambiguous state comparison (USC) of two unknown squeezed vacuum states of an electromagnetic field. Our setup is based on linear optical elements and photon-number detectors, and achieves optimal USC in an ideal case of unit quantum efficiency. In realistic conditions, i.e., for non-unit quantum efficiency of photodetectors, we evaluate the probability of getting an ambiguous result as well as the reliability of the scheme, thus showing its robustness in comparison to previous proposals.

quant-ph↗

arXiv2010

Continuous variable quantum key distribution in non-Markovian channels

We address continuous variable quantum key distribution (QKD) in non-Markovian lossy channels and show how the non-Markovian features may be exploited to enhance security and/or to detect the presence and the position of an eavesdropper along the transmission line. In particular, we suggest a coherent states QKD protocol which is secure against individual attacks for arbitrarily low values of the overall transmission line. Our scheme relies on specific non-Markovian properties, and cannot be implemented in ordinary Markovian channels characterized by uniform losses.

quant-ph↗

arXiv2009

Non-Gaussian states by conditional measurements

We address realistic schemes for the generation of non-Gaussian states of light based on conditional intensity measurements performed on correlated bipartite states. We consider both quantum and classically correlated states and different kind of detection, comparing the resulting non Gaussianity parameters upon varying the input energy and the detection efficiency. We find that quantum correlations generally lead to higher non Gaussianity, at least in the low energy regime. An experimental implementation feasible with current technology is also suggested.

quant-ph↗

arXiv2009

Entanglement induced transparency and applications

We point out a symmetry exhibited by pairs of entangled states and discuss its possible applications in quantum information. More specifically, we consider quadripartite systems prepared in bipartite product states of the form $|Ψ> = |ψ>_{12} \otimes |ψ>_{34}$ and let the uncorrelated subsystems 14 and 23 interact by a given unitary $U_{14}\otimes U_{23}$: we show that the entanglement between the noninteracting subsystems 12 and 34 may induce transparency, i.e. makes $|Ψ>$ an eigenstate of the unitary. We investigate the occurrence of this phenomenon both in continuous variable and qubit systems, and discuss its possible applications to bath engineering, double swapping and remote inversion.

quant-ph↗

arXiv2009

Bayesian estimation in homodyne interferometry

We address phase-shift estimation by means of squeezed vacuum probe and homodyne detection. We analyze Bayesian estimator, which is known to asymptotically saturate the classical Cramer-Rao bound to the variance, and discuss convergence looking at the a posteriori distribution as the number of measurements increases. We also suggest two feasible adaptive methods, acting on the squeezing parameter and/or the homodyne local oscillator phase, which allow to optimize homodyne detection and approach the ultimate bound to precision imposed by the quantum Cramer-Rao theorem. The performances of our two-step methods are investigated by means of Monte Carlo simulated experiments with a small number of homodyne data, thus giving a quantitative meaning to the notion of asymptotic optimality.

quant-ph↗

arXiv2008

Bayesian estimation of one-parameter qubit gates

We address estimation of one-parameter unitary gates for qubit systems and seek for optimal probes and measurements. Single- and two-qubit probes are analyzed in details focusing on precision and stability of the estimation procedure. Bayesian inference is employed and compared with the ultimate quantum limits to precision, taking into account the biased nature of Bayes estimator in the non asymptotic regime. Besides, through the evaluation of the asymptotic a posteriori distribution for the gate parameter and the comparison with the results of Monte Carlo simulated experiments, we show that asymptotic optimality of Bayes estimator is actually achieved after a limited number of runs. The robustness of the estimation procedure against fluctuations of the measurement settings is investigated and the use of entanglement to improve the overall stability of the estimation scheme is also analyzed in some details.

quant-ph↗

arXiv2007

Quantum estimation via minimum Kullback entropy principle

We address quantum estimation in situations where one has at disposal data from the measurement of an incomplete set of observables and some a priori information on the state itself. By expressing the a priori information in terms of a bias toward a given state the problem may be faced by minimizing the quantum relative entropy (Kullback entropy) with the constraint of reproducing the data. We exploit the resulting minimum Kullback entropy principle for the estimation of a quantum state from the measurement of a single observable, either from the sole mean value or from the complete probability distribution, and apply it as a tool for the estimation of weak Hamiltonian processes. Qubit and harmonic oscillator systems are analyzed in some details.

quant-ph↗

arXiv2007

Improving information/disturbance and estimation/distortion trade-offs with non universal protocols

We analyze in details a conditional measurement scheme based on linear optical components, feed-forward loop and homodyne detection. The scheme may be used to achieve two different tasks. On the one hand it allows the extraction of information with minimum disturbance about a set of coherent states. On the other hand, it represents a nondemolitive measurement scheme for the annihilation operator, i.e. an indirect measurement of the Q-function. We investigate the information/disturbance trade-off for state inference and introduce the estimation/distortion trade-off to assess estimation of the Q-function. For coherent states chosen from a Gaussian set we evaluate both information/disturbance and estimation/distortion trade-offs and found that non universal protocols may be optimized in order to achieve better performances than universal ones. For Fock number states we prove that universal protocols do not exist and evaluate the estimation/distortion trade-off for a thermal distribution.

quant-ph↗

arXiv2007

Entanglement oscillations in non-Markovian quantum channels

We study the non-Markovian dynamics of a two-mode bosonic system interacting with two uncorrelated thermal bosonic reservoirs. We present the solution to the exact microscopic Master equation in terms of the quantum characteristic function and study in details the dynamics of entanglement for bipartite Gaussian states. In particular, we analyze the effects of short-time system-reservoir correlations on the separability thresholds and show that the relevant parameter is the reservoir spectral density. If the frequencies of the involved modes are within the reservoir spectral density entanglement persists for a longer time than in a Markovian channel. On the other hand, when the reservoir spectrum is out of resonance short-time correlations lead to a faster decoherence and to the appearance of entanglement oscillations.

quant-ph↗

arXiv2007

Reconstruction of the photon distribution in a micromaser

We suggest an iterative, maximum-likelihood-based, method to reconstruct the photon number distribution of the steady state cavity field of a micromaser starting from the statistics of the atoms leaving the cavity after the interaction. The scheme is based on measuring the atomic populations of probe atoms for different interaction times and works effectively using a small number of atoms and a limited sampling of the interaction times. The method has been tested by numerically simulated experiments showing that it may be reliably used in any micromaser regime leading to high-fidelity reconstructions for single-peaked distributions as well as for double-peaked ones and for trapping states.

quant-ph↗

arXiv2007

Selective cloning of Gaussian states by linear optics

We investigate the performances of a selective cloning machine based on linear optical elements and Gaussian measurements, which allows to clone at will one of the two incoming input states. This machine is a complete generalization of a 1 to 2 cloning scheme demonstrated by U. L. Andersen et al. [Phys. Rev. Lett. vol. 94, 240503 (2005)]. The input-output fidelity is studied for generic Gaussian input state and the effect of non-unit quantum efficiency is also taken into account. We show that if the states to be cloned are squeezed states with known squeezing parameter, then the fidelity can be enhanced using a third suitable squeezed state during the final stage of the cloning process. A binary communication protocol based on the selective cloning machne is also discussed.

quant-ph↗

arXiv2006

Optimal cloning of coherent states by linear optics

We describe an optical scheme for optimal Gaussian n to m cloning of coherent states. The scheme, which generalizes a recently demonstrated scheme for 1 to 2 cloning, involves only linear optical components and homodyne detection.

quant-ph↗

arXiv2006

De-Gaussification by inconclusive photon subtraction

We address conditional de-Gaussification of continuous variable states by inconclusive photon subtraction (IPS) and review in details its application to bipartite twin-beam state of radiation. The IPS map in the Fock basis has been derived, as well as its counterpart in the phase-space. Teleportation assisted by IPS states is analyzed and the corresponding fidelity evaluated as a function of the involved parameters. Nonlocality of IPS states is investigated by means of different tests including displaced parity, homodyne detection, pseudospin, and displaced on/off photodetection. Dissipation and thermal noise are taken into account, as well as non unit quantum efficiency in the detection stage. We show that the IPS process, for a suitable choice of the involved parameters, improves teleportation fidelity and enhances nonlocal properties.

quant-ph↗

arXiv2006

Cloning of Gaussian states by linear optics

We analyze in details a scheme for cloning of Gaussian states based on linear optical components and homodyne detection recently demonstrated by U. L. Andersen et al. [PRL 94 240503 (2005)]. The input-output fidelity is evaluated for a generic (pure or mixed) Gaussian state taking into account the effect of non-unit quantum efficiency and unbalanced mode-mixing. In addition, since in most quantum information protocols the covariance matrix of the set of input states is not perfectly known, we evaluate the average cloning fidelity for classes of Gaussian states with the degree of squeezing and the number of thermal photons being only partially known.

quant-ph↗

arXiv2005

Characterization of bipartite states using a single homodyne detector

We suggest a scheme to reconstruct the covariance matrix of a two-mode state using a single homodyne detector plus a polarizing beam splitter and a polarization rotator. It can be used to fully characterize bipartite Gaussian states and to extract relevant informations on generic states.

quant-ph↗

arXiv2005

Effect of noise and enhancement of nonlocality in on/off photodetection

Nonlocality of two-mode states of light is addressed by means of CHSH inequality based on displaced on/off photodetection. Effects due to non-unit quantum efficiency and nonzero dark counts are taken into account. Nonlocality of both balanced and unbalanced superpositions of few photon-number states, as well as that of multiphoton twin beams, is investigated. We find that unbalanced superpositions show larger nonlocality than balanced one when noise affects the photodetection process. De-Gaussification by means of (inconclusive) photon subtraction is shown to enhance nonlocality of twin beams in the low energy regime. We also show that when the measurement is described by a POVM, rather than a set of projectors, the maximum achievable value of the Bell parameter in the CHSH inequality is decreased, and is no longer given by the Cirel'son bound.

quant-ph↗

arXiv2005

Squeezed Fock state by inconclusive photon subtraction

We analyze in details the properties of the conditional state recently obtained by J. Wenger, et al. [Phys. Rev. Lett. {\bf 92}, 153601 (2004)] by means of inconclusive photon subtraction (IPS) on a squeezed vacuum state $S(r)\ket{0}$. The IPS process can be characterized by two parameters: the IPS transmissivity $τ$ and the photodetection quantum efficiency $η$. We found that the conditional state approaches the squeezed Fock state $S(r)\ket{1}$ when $τ,η\to 1$, i.e., in the limit of single-photon subtraction. For non-unit IPS transmissivity and efficiency, the conditioned state remains close to the target state, {\em i.e.} shows a high fidelity for a wide range of experimental parameters. The nonclassicality of the conditional state is also investigated and a nonclassicality threshold on the IPS parameters is derived.

quant-ph↗