9 research outputs found
Detecting entanglement with non-hermitian operators
We derive several entanglement conditions employing non-hermitian operators.
We start with two conditions that were derived previously for field mode
operators, and use them to derive conditions that can be used to show the
existence of field-atom entanglement and entanglement between groups of atoms.
The original conditions can be strengthened by making them invariant under
certain sets of local unitary transformations, such as Gaussian operations. We
then apply these conditions to several examples, such as the Dicke model. We
conclude with a short discussion of how local uncertainty relations with
non-hermitian operators can be used to derive entanglement conditions.Comment: Typos correcte
A No-Go Theorem for Gaussian Quantum Error Correction
It is proven that Gaussian operations are of no use for protecting Gaussian
states against Gaussian errors in quantum communication protocols.
Specifically, we introduce a new quantity characterizing any single-mode
Gaussian channel, called entanglement degradation, and show that it cannot
decrease via Gaussian encoding and decoding operations only. The strength of
this no-go theorem is illustrated with some examples of Gaussian channels.Comment: 4 pages, 2 figures, REVTeX
Quantum optical coherence can survive photon losses: a continuous-variable quantum erasure correcting code
A fundamental requirement for enabling fault-tolerant quantum information
processing is an efficient quantum error-correcting code (QECC) that robustly
protects the involved fragile quantum states from their environment. Just as
classical error-correcting codes are indispensible in today's information
technologies, it is believed that QECC will play a similarly crucial role in
tomorrow's quantum information systems. Here, we report on the first
experimental demonstration of a quantum erasure-correcting code that overcomes
the devastating effect of photon losses. Whereas {\it errors} translate, in an
information theoretic language, the noise affecting a transmission line, {\it
erasures} correspond to the in-line probabilistic loss of photons. Our quantum
code protects a four-mode entangled mesoscopic state of light against erasures,
and its associated encoding and decoding operations only require linear optics
and Gaussian resources. Since in-line attenuation is generally the strongest
limitation to quantum communication, much more than noise, such an
erasure-correcting code provides a new tool for establishing quantum optical
coherence over longer distances. We investigate two approaches for
circumventing in-line losses using this code, and demonstrate that both
approaches exhibit transmission fidelities beyond what is possible by classical
means.Comment: 5 pages, 4 figure
Information quantique avec variables continues optiques: nonlocalité, intrication, et correction d'erreur
L'objectif de ce travail de recherche est l'étude des posibilités offertes par une nouvelle approche de l'information quantique basée sur des variables quantiques continues. Lorsque ces variables continues sont portées par le champs éléctromagnétique, un grand nombre de protocoles d'information quantique peuvent être implémentés à l'aide de lasers et d'éléments d'optique linéaire standards. Cette simplicité expérimentale rend cette approche très intéressantes d'un point de vue pratique, en particulier pour le développement des futurs réseaux de communications quantiques.Le travail peut se diviser en deux parties complémentaires. Dans la première partie, plus fondamentale, la relation complexe qui existe entre l'intrication et la nonlocalité de la mécanique quantique est étudiée sur base des variables optiques continues. Ces deux ressources étant essentielles pour l'information quantique, il est nécessaire de bien les comprendre et de bien les caractériser. Dans la seconde partie, orientée vers des applications concrètes, le problème de la correction d'erreur à variables continues est étudié. Pouvoir transmettre et manipuler l'information sans erreurs est nécessaire au bon développemnent de l'information quantique, mais, en pratique, les erreurs sont inévitables. Les codes correcteurs d'erreurs permettent de détecter et corriger ces erreures de manière efficace.Doctorat en Sciences de l'ingénieurinfo:eu-repo/semantics/nonPublishe
Tests of multimode quantum nonlocality with homodyne measurements
We investigate the violation of local realism in Bell tests involving
homodyne measurements performed on multimode continuous-variable states. By
binning the measurement outcomes in an appropriate way, we prove that the
Mermin-Klyshko inequality can be violated by an amount that grows exponentially
with the number of modes. Furthermore, the maximum violation allowed by quantum
mechanics can be attained for any number of modes, albeit requiring a quantum
state that is rather unrealistic. Interestingly, this exponential increase of
the violation holds true even for simpler states, such as multipartite GHZ
states. The resulting benefit of using more modes is shown to be significant in
practical multipartite Bell tests by analyzing the increase of the robustness
to noise with the number of modes. In view of the high efficiency achievable
with homodyne detection, our results thus open a possible way to feasible
loophole-free Bell tests that are robust to experimental imperfections. We
provide an explicit example of a three-mode state (a superposition of coherent
states) which results in a significantly high violation of the Mermin-Klyshko
inequality (around 10%) with homodyne measurements.Comment: 9 pages, 5 figure
Continuous-Variable Quantum Erasure Correcting Code
We experimentally demonstrate a continuous variable quantum erasure-correcting code, which protects coherent states of light against complete erasure. The scheme encodes two coherent states into a bi-party entangled state, and the resulting 4-mode code is conveyed through 4 independent channels that randomly erases the signal. We show experimentally that the transmitted state can be corrected by performing a syndrome measurement followed by a corrective transformation. © 2010 Optical Society of America.SCOPUS: cp.pQuantum Electronics and Laser Science Conference, QELS 2010 & Conference on Lasers and Electro-Optics, CLEO 2010 ;San Jose, CA; United States; 16 May 2010 through 21 May 2010info:eu-repo/semantics/publishe