137 research outputs found
Quantum Correlations and Quantum Non-Locality: A Review and a Few New Ideas
In this paper we make an extensive description of quantum non-locality, one
of the most intriguing and fascinating facets of quantum mechanics. After a
general presentation of several studies on this subject, we consider if quantum
non-locality, and the friction it carries with special relativity, can
eventually find a "solution" by considering higher dimensional spaces.Comment: 1
Quantum Time: experimental multi-time correlations
In this paper we provide an experimental illustration of Page and Wootters'
quantum time mechanism that is able to describe two-time quantum correlation
functions. This allows us to test a Leggett-Garg inequality, showing a
violation from the "internal" observer point of view. The "external" observer
sees a time-independent global state. Indeed, the scheme is implemented using a
narrow-band single photon where the clock degree of freedom is encoded in the
photon's position. Hence, the internal observer that measures the position can
track the flow of time, while the external observer sees a delocalized photon
that has no time evolution in the experiment time-scale.Comment: 5 pages, 4 figure
Experimental realization of a measurement conditional unitary operation at single photon level and application to detector characterization
Our last experimental results on the realization of a measurement-conditional
unitary operation at single photon level are presented. This gate operates by
rotating by the polarization of a photon produced by means of Type-II
Parametric Down Conversion conditional to a polarization measurement on the
correlated photon. We then propose a new scheme for measuring the quantum
efficiency of a single photon detection apparatus by using this set-up. We
present experimental results obtained with this scheme compared with {\it
traditional} biphoton calibration. Our results show the interesting
potentiality of the suggested scheme.Comment: to appear in Proc. of SPIE meeting, Denver august 200
Optimal estimation of entanglement and discord in two-qubit states
Recently, the fast development of quantum technologies led to the need for
tools allowing the characterization of quantum resources. In particular, the
ability to estimate non-classical aspects, e.g. entanglement and quantum
discord, in two-qubit systems, is relevant to optimise the performance of
quantum information processes. Here we present an experiment in which the
amount of entanglement and discord are measured exploiting different
estimators. Among them, some will prove to be optimal, i.e., able to reach the
ultimate precision bound allowed by quantum mechanics. These estimation
techniques have been tested with a specific family of states ranging from
nearly pure Bell states to completely mixed states. This work represents a
significant step in the development of reliable metrological tools for quantum
technologies
Experimental quantum cryptography scheme based on orthogonal states
Since, in general, non-orthogonal states cannot be cloned, any eavesdropping
attempt in a Quantum Communication scheme using non-orthogonal states as
carriers of information introduces some errors in the transmission, leading to
the possibility of detecting the spy. Usually, orthogonal states are not used
in Quantum Cryptography schemes since they can be faithfully cloned without
altering the transmitted data. Nevertheless, L. Goldberg and L. Vaidman [\prl
75 (1995) 1239] proposed a protocol in which, even if the data exchange is
realized using two orthogonal states, any attempt to eavesdrop is detectable by
the legal users. In this scheme the orthogonal states are superpositions of two
localized wave packets travelling along separate channels. Here we present an
experiment realizing this scheme
Anomalous Weak Values and the Violation of a Multiple-measurement Leggett-Garg Inequality
Quantum mechanics presents peculiar properties that, on the one hand, have
been the subject of several theoretical and experimental studies about its very
foundations and, on the other hand, provide tools for developing new
technologies, the so-called quantum technologies. The nonclassicality pointed
out by Leggett-Garg inequalities has represented, with Bell inequalities, one
of the most investigated subject. In this letter we study the connection of
Leggett-Garg inequalities with a new emerging field of quantum measurement, the
weak values. In particular, we perform an experimental study of the four-time
correlators Legget-Garg test, by exploiting single and sequential weak
measurements performed on heralded single photons. We show violation of a
four-parameters Leggett-Garg inequality in different experimental conditions,
demonstrating an interesting connection between Leggett-Garg inequality
violation and anomalous weak values
Exploring Quantum Correlations from Discord to Entanglement
Quantum correlations represent a fundamental tool for studies ranging from basic science to quantum technologies. Different non-classical correlations have been identified and studied, as entanglement and discord. In this Paper we explore experimentally the rich geometry of polarization Bell-diagonal states. By taking advantage of the statistical method of generation, the values of entanglement and discord along different trajectories in the space of the parameters of density matrix have been measured. The effects of sudden death of entanglement and complete "freeze" of discord were investigated in order to detect the domains with different domination of one type of quantum correlation against to other. A geometric interpretation for each considered phenomena is addressed. The observed good agreement between experiment and theory for all investigated trajectories ensures the reliability of this method
On the reconstruction of diagonal elements of density matrix of quantum optical states by on/off detectors
We discuss a scheme for reconstructing experimentally the diagonal elements
of the density matrix of quantum optical states. Applications to PDC heralded
photons, multi-thermal and attenuated coherent states are illustrated and
discussed in some details.Comment: 10 pages, presented at Palermo "TQMFA2005" Conference. To appear on
"Open Systems & Information Dynamics" (2006
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