Nou mètode per detectar estats "exòtics" de la matèria condensada

El grup de computació quàntica de la UAB ha participat en una recerca internacional que proposa un mètode nou, superior als que existeixen fins ara, per detectar estats "exòtics" de la matèria condensada, de tal manera que la mostra (àtoms ultrafreds) no es destrueixi en ser observada. Aquest mètode s'ha anomenat Quantum non demolition, i és la mesura menys destructiva possible que permeten les lleis de la mecànica quàntica. Aquest treball, publicat a Nature, s'ha dut a terme en col·laboració amb l'Institut de Ciències Fotòniques (ICFO) i el Niels Bohr Institute de Dinamarca.El grupo de computación cuántica de la UAB ha liderado una investigación que propone un método nuevo, superior a los que existen hasta ahora, para detectar estados "exóticos" de la materia condensada de tal manera que la muestra (átomos ultrafríos) no se destruya al ser observada. Este método se ha llamado Quantum non demolition, y es la medida menos destructiva posible que permiten las leyes de la mecánica cuántica. Este trabajo, publicado en Nature, se ha llevado a cabo en colaboración con el Instituto de Ciencias Fotónicas (ICFO) y el Niels Bohr Institute de Dinamarca

Quanta Quàntica

Conferència realitzada el Dissabte, 3 de Febrer de 2018La Física Quàntica que descriu, ara per ara, força bé el nostre món microscòpic ja no és només un recull de fenòmens exòtics que ens fa ballar el cap. En els darrers anys hem après com fer servir les lleis de la física quàntica per fer noves coses que eren impensables fa només vint anys. Avui parlarem dels simuladors quàntics (uns ordinadors especials que estem fent) que permeten resoldre problemes nous i que esdevindran eines rutinàries en la revolució tecnológica que viurem en un no res

Mesurant la temperatura més freda de l'Univers

Conferència realitzada el Dissabte, 5 de Març de 2022La física quàntica ens dona eines per explicar el món a escala atòmica i també per mesurar coses de manera molt precisa. Es pot mesurar el batec del cor d'un mosquit o l'activitat de les nostres neurones al nostre cervell ? Quina és la temperatura més freda de l'Univers? I com es mesura la temperatura d'una cèl·lula? El rellotge més precís que podem construir, quin error té? Quin és el metre més acurat? Es pot detectar l'activitat dels volcans marins encara que no els veiem? i el vol d'un sol ocell pel mig de l'oceà? La física quàntica ens dona una resposta sorprenent i fascinant a algunes de les nostres preguntes

Lattice solitons in quasicondensates

We analyze finite temperature effects in the generation of bright solitons in condensates in optical lattices. We show that even in the presence of strong phase fluctuations solitonic structures with a well defined phase profile can be created. We propose a novel family of variational functions which describe well the properties of these solitons and account for the nonlinear effects in the band structure. We discuss also the mobility and collisions of these localized wave packets

Atomic Fermi-Bose mixtures in inhomogeneous and random lattices: From Fermi glass to quantum spin glass and quantum percolation

We investigate atomic Fermi-Bose mixtures in inhomogeneous and random optical lattices in the limit of strong atom-atom interactions. We derive the effective Hamiltonian describing the dynamics of the system and discuss its low temperature physics. We demonstrate possibility of controlling the interactions at local level in inhomogeneous but regular lattices. Such a control leads to the achievement of Fermi glass, quantum Fermi spin glass, and quantum percolation regimes involving bare and/or composite fermions in random lattices.Comment: minor changes; Physical Review Letters 93, 040401 (2004

Key distillation from Gaussian states by Gaussian operations

We study the secrecy properties of Gaussian states under Gaussian operations. Although such operations are useless for quantum distillation, we prove that it is possible to distill a secret key secure against any attack from sufficiently entangled Gaussian states with non-positive partial transposition. Moreover, all such states allow for key distillation, when Eve is assumed to perform finite-size coherent attacks before the reconciliation process.Comment: 2 figures, REVTEX

Witnessing multipartite entanglement

We present the experimental detection of genuine multipartite entanglement using entanglement witness operators. To this aim we introduce a canonical way of constructing and decomposing witness operators so that they can be directly implemented with present technology. We apply this method to three- and four-qubit entangled states of polarized photons, giving experimental evidence that the considered states contain true multipartite entanglement.Comment: 4 pages, 2 figure

Lattice solitons in quasicondensates

We analyze finite temperature effects in the generation of bright solitons in condensates in optical lattices. We show that even in the presence of strong phase fluctuations solitonic structures with a well defined phase profile can be created. We propose a novel family of variational functions which describe well the properties of these solitons and account for the nonlinear effects in the band structure. We discuss also the mobility and collisions of these localized wave packets

Local description of quantum inseparability

We show how to decompose any density matrix of the simplest binary composite systems, whether separable or not, in terms of only product vectors. We determine for all cases the minimal number of product vectors needed for such a decomposition. Separable states correspond to mixing from one to four pure product states. Inseparable states can be described as pseudomixtures of four or five pure product states, and can be made separable by mixing them with one or two pure product states

Thermometry precision in strongly correlated ultracold lattice gases

The precise knowledge of the temperature of an ultracold lattice gas simulating a strongly correlated system is a question of both fundamental and technological importance. Here, we address such question by combining tools from quantum metrology together with the study of the quantum correlations embedded in the system at finite temperatures. Within this frame we examine the spin- XY chain, first estimating, by means of the quantum Fisher information, the lowest attainable bound on the temperature precision. We then address the estimation of the temperature of the sample from the analysis of correlations using a quantum non demolishing Faraday spectroscopy method. Remarkably, our results show that the collective quantum correlations can become optimal observables to accurately estimate the temperature of our model in a given range of temperatures