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1.
Phys Rev Lett ; 112(14): 140501, 2014 Apr 11.
Artículo en Inglés | MEDLINE | ID: mdl-24765931

RESUMEN

In quantum mechanics, observing is not a passive act. Consider a system of two quantum particles A and B: if a measurement apparatus M is used to make an observation on B, the overall state of the system AB will typically be altered. When this happens, no matter which local measurement is performed, the two objects A and B are revealed to possess peculiar correlations known as quantum discord. Here, we demonstrate experimentally that the very act of local observation gives rise to an activation protocol which converts discord into distillable entanglement, a stronger and more useful form of quantum correlations, between the apparatus M and the composite system AB. We adopt a flexible two-photon setup to realize a three-qubit system (A, B, M) with programmable degrees of initial correlations, measurement interaction, and characterization processes. Our experiment demonstrates the fundamental mechanism underpinning the ubiquitous act of observing the quantum world and establishes the potential of discord in entanglement generation.

2.
Sci Rep ; 3: 2726, 2013 Sep 25.
Artículo en Inglés | MEDLINE | ID: mdl-24067548

RESUMEN

In quantum information, complementarity of quantum mechanical observables plays a key role. The eigenstates of two complementary observables form a pair of mutually unbiased bases (MUBs). More generally, a set of MUBs consists of bases that are all pairwise unbiased. Except for specific dimensions of the Hilbert space, the maximal sets of MUBs are unknown in general. Even for a dimension as low as six, the identification of a maximal set of MUBs remains an open problem, although there is strong numerical evidence that no more than three simultaneous MUBs do exist. Here, by exploiting a newly developed holographic technique, we implement and test different sets of three MUBs for a single photon six-dimensional quantum state (a "qusix"), encoded exploiting polarization and orbital angular momentum of photons. A close agreement is observed between theory and experiments. Our results can find applications in state tomography, quantitative wave-particle duality, quantum key distribution.

3.
Opt Lett ; 37(2): 172-4, 2012 Jan 15.
Artículo en Inglés | MEDLINE | ID: mdl-22854457

RESUMEN

In this work we experimentally implement a deterministic transfer of a generic qubit initially encoded in the orbital angular momentum of a single-photon to its polarization. Such a transfer of quantum information, which is completely reversible, has been implemented adopting an electrically tunable q-plate device and a Sagnac interferometer with a Dove prism. The adopted scheme exhibits high fidelity and low losses.

4.
Nat Commun ; 3: 961, 2012 Jul 17.
Artículo en Inglés | MEDLINE | ID: mdl-22805568

RESUMEN

Quantum communication employs the counter-intuitive features of quantum physics for tasks that are impossible in the classical world. It is crucial for testing the foundations of quantum theory and promises to revolutionize information and communication technologies. However, to execute even the simplest quantum transmission, one must establish, and maintain, a shared reference frame. This introduces a considerable overhead in resources, particularly if the parties are in motion or rotating relative to each other. Here we experimentally show how to circumvent this problem with the transmission of quantum information encoded in rotationally invariant states of single photons. By developing a complete toolbox for the efficient encoding and decoding of quantum information in such photonic qubits, we demonstrate the feasibility of alignment-free quantum key-distribution, and perform proof-of-principle demonstrations of alignment-free entanglement distribution and Bell-inequality violation. The scheme should find applications in fundamental tests of quantum mechanics and satellite-based quantum communication.

5.
Sci Rep ; 2: 443, 2012.
Artículo en Inglés | MEDLINE | ID: mdl-22720131

RESUMEN

The extraction of information from a quantum system unavoidably implies a modification of the measured system itself. In this framework partial measurements can be carried out in order to extract only a portion of the information encoded in a quantum system, at the cost of inducing a limited amount of disturbance. Here we analyze experimentally the dynamics of sequential partial measurements carried out on a quantum system, focusing on the trade-off between the maximal information extractable and the disturbance. In particular we implement two sequential measurements observing that, by exploiting an adaptive strategy, is possible to find an optimal trade-off between the two quantities.


Asunto(s)
Algoritmos , Modelos Teóricos , Teoría Cuántica , Fenómenos Químicos , Fotones
6.
Phys Rev Lett ; 108(9): 090501, 2012 Mar 02.
Artículo en Inglés | MEDLINE | ID: mdl-22463620

RESUMEN

Quantum resources outperform classical ones for certain communication and computational tasks. Remarkably, in some cases, the quantum advantage cannot be improved using hypothetical postquantum resources. A class of tasks with this property can be singled out using graph theory. Here we report the experimental observation of an impossible-to-beat quantum advantage on a four-dimensional quantum system defined by the polarization and orbital angular momentum of a single photon. The results show pristine evidence of the quantum advantage and are compatible with the maximum advantage allowed using postquantum resources.

7.
Opt Express ; 18(17): 18243-8, 2010 Aug 16.
Artículo en Inglés | MEDLINE | ID: mdl-20721215

RESUMEN

Hybrid entangled states exhibit entanglement between different degrees of freedom of a particle pair and thus could be useful for asymmetric optical quantum network where the communication channels are characterized by different properties. We report the first experimental realization of hybrid polarization-orbital angular momentum (OAM) entangled states by adopting a spontaneous parametric down conversion source of polarization entangled states and a polarization-OAM transferrer. The generated quantum states have been characterized through quantum state tomography. Finally, the violation of Bell's inequalities with the hybrid two photon system has been observed.


Asunto(s)
Procesamiento Automatizado de Datos/métodos , Modelos Teóricos , Óptica y Fotónica/métodos , Fotones , Teoría Cuántica , Bario/química , Boratos/química , Cristalografía/instrumentación , Cristalografía/métodos , Procesamiento Automatizado de Datos/instrumentación , Óptica y Fotónica/instrumentación
8.
Phys Rev Lett ; 103(1): 013601, 2009 Jul 03.
Artículo en Inglés | MEDLINE | ID: mdl-19659145

RESUMEN

The optical "spin-orbit" coupling occurring in a suitably patterned nonuniform birefringent plate known as a "q plate" allows entangling the polarization of a single photon with its orbital angular momentum (OAM). This process, in turn, can be exploited for building a bidirectional "spin-OAM interface," capable of transposing the quantum information from the spin to the OAM degree of freedom of photons and vice versa. Here, we experimentally demonstrate this process by single-photon quantum tomographic analysis. Moreover, we show that two-photon quantum correlations such as those resulting from coalescence interference can be successfully transferred into the OAM degree of freedom.

9.
Phys Rev Lett ; 99(19): 193601, 2007 Nov 09.
Artículo en Inglés | MEDLINE | ID: mdl-18233075

RESUMEN

In 1981 N. Herbert proposed a gedanken experiment in order to achieve by the "First Laser-Amplified Superluminal Hookup" (FLASH) a faster-than-light (FTL) communication by quantum nonlocality. The present work reports the first experimental realization of that proposal by the optical parametric amplification of a single photon belonging to an entangled EPR pair into an output field involving N=5 x 10{3} photons. A theoretical and experimental analysis explains in general and conclusive terms the precise reasons for the failure of the FLASH program as well as of any similar FTL proposals.

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